Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial

## Full Query Language Syntax

This section discusses the query language syntax, as defined in the Java Persistence API 2.0 specification available at `http://jcp.org/en/jsr/detail?id=317`. Much of the following material paraphrases or directly quotes the specification.

The following topics are addressed here:

- BNF Symbols

- BNF Grammar of the Java Persistence Query Language

- FROM Clause

- Path Expressions

- WHERE Clause

- SELECT Clause

- ORDER BY Clause

- GROUP BY and HAVING Clauses

### BNF Symbols

Table 42-1 describes the BNF symbols used in this chapter.

Table 42-1 BNF Symbol Summary

Symbol | Description
`::=` | The element to the left of the symbol is defined by the constructs on the right.
`*` | The preceding construct may occur zero or more times.
`{…​}` | The constructs within the braces are grouped together.
`[…​]` | The constructs within the brackets are optional.
` | `
An exclusive `OR`. | `BOLDFACE`
A keyword; although capitalized in the BNF diagram, keywords are not case-sensitive. | White space

### BNF Grammar of the Java Persistence Query Language

Here is the entire BNF diagram for the query language:

    QL_statement ::= select_statement | update_statement | delete_statement
    select_statement ::= select_clause from_clause [where_clause] [groupby_clause]
        [having_clause] [orderby_clause]
    update_statement ::= update_clause [where_clause]
    delete_statement ::= delete_clause [where_clause]
    from_clause ::=
        FROM identification_variable_declaration
            {, {identification_variable_declaration |
                collection_member_declaration}}*
    identification_variable_declaration ::=
            range_variable_declaration { join | fetch_join }*
    range_variable_declaration ::= abstract_schema_name [AS]
            identification_variable
    join ::= join_spec join_association_path_expression [AS]
            identification_variable
    fetch_join ::= join_specFETCH join_association_path_expression
    association_path_expression ::=
            collection_valued_path_expression |
            single_valued_association_path_expression
    join_spec::= [LEFT [OUTER] |INNER] JOIN
    join_association_path_expression ::=
            join_collection_valued_path_expression |
            join_single_valued_association_path_expression
    join_collection_valued_path_expression::=
        identification_variable.collection_valued_association_field
    join_single_valued_association_path_expression::=
            identification_variable.single_valued_association_field
    collection_member_declaration ::=
            IN (collection_valued_path_expression) [AS]
            identification_variable
    single_valued_path_expression ::=
            state_field_path_expression |
            single_valued_association_path_expression
    state_field_path_expression ::=
        {identification_variable |
        single_valued_association_path_expression}.state_field
    single_valued_association_path_expression ::=
        identification_variable.{single_valued_association_field.}*
        single_valued_association_field
    collection_valued_path_expression ::=
        identification_variable.{single_valued_association_field.}*
        collection_valued_association_field
    state_field ::=
        {embedded_class_state_field.}*simple_state_field
    update_clause ::=UPDATE abstract_schema_name [[AS]
        identification_variable] SET update_item {, update_item}*
    update_item ::= [identification_variable.]{state_field |
        single_valued_association_field} = new_value
    new_value ::=
         simple_arithmetic_expression |
        string_primary |
        datetime_primary |
        boolean_primary |
        enum_primary simple_entity_expression |
        NULL
    delete_clause ::= DELETE FROM abstract_schema_name [[AS]
        identification_variable]
    select_clause ::= SELECT [DISTINCT] select_expression {,
        select_expression}*
    select_expression ::=
        single_valued_path_expression |
        aggregate_expression |
        identification_variable |
        OBJECT(identification_variable) |
        constructor_expression
    constructor_expression ::=
        NEW constructor_name(constructor_item {,
        constructor_item}*)
    constructor_item ::= single_valued_path_expression |
        aggregate_expression
    aggregate_expression ::=
        {AVG |MAX |MIN |SUM} ([DISTINCT]
            state_field_path_expression) |
        COUNT ([DISTINCT] identification_variable |
            state_field_path_expression |
            single_valued_association_path_expression)
    where_clause ::= WHERE conditional_expression
    groupby_clause ::= GROUP BY groupby_item {, groupby_item}*
    groupby_item ::= single_valued_path_expression
    having_clause ::= HAVING conditional_expression
    orderby_clause ::= ORDER BY orderby_item {, orderby_item}*
    orderby_item ::= state_field_path_expression [ASC |DESC]
    subquery ::= simple_select_clause subquery_from_clause
        [where_clause] [groupby_clause] [having_clause]
    subquery_from_clause ::=
        FROM subselect_identification_variable_declaration
            {, subselect_identification_variable_declaration}*
    subselect_identification_variable_declaration ::=
        identification_variable_declaration |
        association_path_expression [AS] identification_variable |
        collection_member_declaration
    simple_select_clause ::= SELECT [DISTINCT]
        simple_select_expression
    simple_select_expression::=
        single_valued_path_expression |
        aggregate_expression |
        identification_variable
    conditional_expression ::= conditional_term |
        conditional_expression OR conditional_term
    conditional_term ::= conditional_factor | conditional_term AND
        conditional_factor
    conditional_factor ::= [NOT] conditional_primary
    conditional_primary ::= simple_cond_expression |(
        conditional_expression)
    simple_cond_expression ::=
        comparison_expression |
        between_expression |
        like_expression |
        in_expression |
        null_comparison_expression |
        empty_collection_comparison_expression |
        collection_member_expression |
        exists_expression
    between_expression ::=
        arithmetic_expression [NOT] BETWEEN
            arithmetic_expressionAND arithmetic_expression |
        string_expression [NOT] BETWEEN string_expression AND
            string_expression |
        datetime_expression [NOT] BETWEEN
            datetime_expression AND datetime_expression
    in_expression ::=
        state_field_path_expression [NOT] IN (in_item {, in_item}*
        | subquery)
    in_item ::= literal | input_parameter
    like_expression ::=
        string_expression [NOT] LIKE pattern_value [ESCAPE
            escape_character]
    null_comparison_expression ::=
        {single_valued_path_expression | input_parameter} IS [NOT]
            NULL
    empty_collection_comparison_expression ::=
        collection_valued_path_expression IS [NOT] EMPTY
    collection_member_expression ::= entity_expression
        [NOT] MEMBER [OF] collection_valued_path_expression
    exists_expression::= [NOT] EXISTS (subquery)
    all_or_any_expression ::= {ALL |ANY |SOME} (subquery)
    comparison_expression ::=
        string_expression comparison_operator {string_expression |
        all_or_any_expression} |
        boolean_expression {= |<> } {boolean_expression |
        all_or_any_expression} |
        enum_expression {= |<> } {enum_expression |
        all_or_any_expression} |
        datetime_expression comparison_operator
            {datetime_expression | all_or_any_expression} |
        entity_expression {= |<> } {entity_expression |
        all_or_any_expression} |
        arithmetic_expression comparison_operator
            {arithmetic_expression | all_or_any_expression}
    comparison_operator ::= = |> |>= |< |<= |<>
    arithmetic_expression ::= simple_arithmetic_expression |
        (subquery)
    simple_arithmetic_expression ::=
        arithmetic_term | simple_arithmetic_expression {+ |- }
            arithmetic_term
    arithmetic_term ::= arithmetic_factor | arithmetic_term {* |/ }
        arithmetic_factor
    arithmetic_factor ::= [{+ |- }] arithmetic_primary
    arithmetic_primary ::=
        state_field_path_expression |
        numeric_literal |
        (simple_arithmetic_expression) |
        input_parameter |
        functions_returning_numerics |
        aggregate_expression
    string_expression ::= string_primary | (subquery)
    string_primary ::=
        state_field_path_expression |
        string_literal |
        input_parameter |
        functions_returning_strings |
        aggregate_expression
    datetime_expression ::= datetime_primary | (subquery)
    datetime_primary ::=
        state_field_path_expression |
        input_parameter |
        functions_returning_datetime |
        aggregate_expression
    boolean_expression ::= boolean_primary | (subquery)
    boolean_primary ::=
        state_field_path_expression |
        boolean_literal |
        input_parameter
     enum_expression ::= enum_primary | (subquery)
    enum_primary ::=
        state_field_path_expression |
        enum_literal |
        input_parameter
    entity_expression ::=
        single_valued_association_path_expression |
            simple_entity_expression
    simple_entity_expression ::=
        identification_variable |
        input_parameter
    functions_returning_numerics::=
        LENGTH(string_primary) |
        LOCATE(string_primary, string_primary[,
            simple_arithmetic_expression]) |
        ABS(simple_arithmetic_expression) |
        SQRT(simple_arithmetic_expression) |
        MOD(simple_arithmetic_expression,
            simple_arithmetic_expression) |
        SIZE(collection_valued_path_expression)
    functions_returning_datetime ::=
        CURRENT_DATE |
        CURRENT_TIME |
        CURRENT_TIMESTAMP
    functions_returning_strings ::=
        CONCAT(string_primary, string_primary) |
        SUBSTRING(string_primary,
            simple_arithmetic_expression,
            simple_arithmetic_expression)|
        TRIM([[trim_specification] [trim_character] FROM]
            string_primary) |
        LOWER(string_primary) |
        UPPER(string_primary)
    trim_specification ::= LEADING | TRAILING | BOTH

### FROM Clause

The `FROM` clause defines the domain of the query by declaring identification variables.

The following topics are addressed here:

- Identifiers

- Identification Variables

- Range Variable Declarations

- Collection Member Declarations

- Joins

#### Identifiers

An identifier is a sequence of one or more characters. The first character must be a valid first character (letter, `$`, ``) in an identifier of the Java programming language, hereafter in this chapter called simply "Java." Each subsequent character in the sequence must be a valid nonfirst character (letter, digit, `$`, ``) in a Java identifier. (For details, see the Java SE API documentation of the `isJavaIdentifierStart` and `isJavaIdentifierPart` methods of the `Character` class.) The question mark (`?`) is a reserved character in the query language and cannot be used in an identifier.

A query language identifier is case-sensitive, with two exceptions:

- Keywords

- Identification variables

An identifier cannot be the same as a query language keyword. Here is a list of query language keywords:

`ABS`
`ALL`
`AND`
`ANY`
`AS`
`ASC`
`AVG`
`BETWEEN`
`BIT_LENGTH`
`BOTH`
`BY`
`CASE`
`CHAR_LENGTH`
`CHARACTER_LENGTH`
`CLASS`
`COALESCE`
`CONCAT`
`COUNT`
`CURRENT_DATE`
`CURRENT_TIMESTAMP`
`DELETE`
`DESC`
`DISTINCT`
`ELSE`
`EMPTY`
`END`
`ENTRY`
`ESCAPE`
`EXISTS`
`FALSE`
`FETCH`
`FROM`
`GROUP`
`HAVING`
`IN`
`INDEX`
`INNER`
`IS`
`JOIN`
`KEY`
`LEADING`
`LEFT`
`LENGTH`
`LIKE`
`LOCATE`
`LOWER`
`MAX`
`MEMBER`
`MIN`
`MOD`
`NEW`
`NOT`
`NULL`
`NULLIF`
`OBJECT`
`OF`
`OR`
`ORDER`
`OUTER`
`POSITION`
`SELECT`
`SET`
`SIZE`
`SOME`
`SQRT`
`SUBSTRING`
`SUM`
`THEN`
`TRAILING`
`TRIM`
`TRUE`
`TYPE`
`UNKNOWN`
`UPDATE`
`UPPER`
`VALUE`
`WHEN`
`WHERE`

It is not recommended that you use an SQL keyword as an identifier, because the list of keywords may expand to include other reserved SQL words in the future.

#### Identification Variables

An identification variable is an identifier declared in the `FROM` clause. Although they can reference identification variables, the `SELECT` and `WHERE` clauses cannot declare them. All identification variables must be declared in the `FROM` clause.

Because it is an identifier, an identification variable has the same naming conventions and restrictions as an identifier, with the exception that an identification variable is case-insensitive. For example, an identification variable cannot be the same as a query language keyword. (See Identifiers for more naming rules.) Also, within a given persistence unit, an identification variable name must not match the name of any entity or abstract schema.

The `FROM` clause can contain multiple declarations, separated by commas. A declaration can reference another identification variable that has been previously declared (to the left). In the following `FROM` clause, the variable `t` references the previously declared variable `p`:

    FROM Player p, IN (p.teams) AS t

Even if it is not used in the `WHERE` clause, an identification variable’s declaration can affect the results of the query. For example, compare the next two queries. The following query returns all players, whether or not they belong to a team:

    SELECT p
    FROM Player p

In contrast, because it declares the `t` identification variable, the next query fetches all players who belong to a team:

    SELECT p
    FROM Player p, IN (p.teams) AS t

The following query returns the same results as the preceding query, but the `WHERE` clause makes it easier to read:

    SELECT p
    FROM Player p
    WHERE p.teams IS NOT EMPTY

An identification variable always designates a reference to a single value whose type is that of the expression used in the declaration. There are two kinds of declarations: range variable and collection member.

#### Range Variable Declarations

To declare an identification variable as an abstract schema type, you specify a range variable declaration. In other words, an identification variable can range over the abstract schema type of an entity. In the following example, an identification variable named `p` represents the abstract schema named `Player`:

    FROM Player p

A range variable declaration can include the optional `AS` operator:

    FROM Player AS p

To obtain objects, a query usually uses path expressions to navigate through the relationships. But for those objects that cannot be obtained by navigation, you can use a range variable declaration to designate a starting point, or query root.

If the query compares multiple values of the same abstract schema type, the `FROM` clause must declare multiple identification variables for the abstract schema:

    FROM Player p1, Player p2

For an example of such a query, see Comparison Operators.

#### Collection Member Declarations

In a one-to-many relationship, the multiple side consists of a collection of entities. An identification variable can represent a member of this collection. To access a collection member, the path expression in the variable’s declaration navigates through the relationships in the abstract schema. (For more information on path expressions, see Path Expressions.) Because a path expression can be based on another path expression, the navigation can traverse several relationships. See Traversing Multiple Relationships.

A collection member declaration must include the `IN` operator but can omit the optional `AS` operator.

In the following example, the entity represented by the abstract schema named `Player` has a relationship field called `teams`. The identification variable called `t` represents a single member of the `teams` collection:

    FROM Player p, IN (p.teams) t

#### Joins

The `JOIN` operator is used to traverse over relationships between entities and is functionally similar to the `IN` operator.

In the following example, the query joins over the relationship between customers and orders:

    SELECT c
    FROM Customer c JOIN c.orders o
    WHERE c.status = 1 AND o.totalPrice > 10000

The `INNER` keyword is optional:

    SELECT c
    FROM Customer c INNER JOIN c.orders o
    WHERE c.status = 1 AND o.totalPrice > 10000

These examples are equivalent to the following query, which uses the `IN` operator:

    SELECT c
    FROM Customer c, IN (c.orders) o
    WHERE c.status = 1 AND o.totalPrice > 10000

You can also join a single-valued relationship:

    SELECT t
    FROM Team t JOIN t.league l
    WHERE l.sport = :sport

A `LEFT JOIN` or `LEFT OUTER JOIN` retrieves a set of entities where matching values in the join condition may be absent. The `OUTER` keyword is optional:

    SELECT c.name, o.totalPrice
    FROM CustomerOrder o LEFT JOIN o.customer c

A `FETCH JOIN` is a join operation that returns associated entities as a side effect of running the query. In the following example, the query returns a set of departments and, as a side effect, the associated employees of the departments, even though the employees were not explicitly retrieved by the `SELECT` clause:

    SELECT d
    FROM Department d LEFT JOIN FETCH d.employees
    WHERE d.deptno = 1

### Path Expressions

Path expressions are important constructs in the syntax of the query language for several reasons. First, path expressions define navigation paths through the relationships in the abstract schema. These path definitions affect both the scope and the results of a query. Second, path expressions can appear in any of the main clauses of a query (`SELECT`, `DELETE`, `HAVING`, `UPDATE`, `WHERE`, `FROM`, `GROUP BY`, `ORDER BY`). Finally, although much of the query language is a subset of SQL, path expressions are extensions not found in SQL.

The following topics are addressed here:

- Examples of Path Expressions

- Expression Types

- Navigation

#### Examples of Path Expressions

Here, the `WHERE` clause contains a `single_valued_path_expression`; the `p` is an identification variable, and `salary` is a persistent field of `Player`:

    SELECT DISTINCT p
    FROM Player p
    WHERE p.salary BETWEEN :lowerSalary AND :higherSalary

Here, the `WHERE` clause also contains a `single_valued_path_expression`; `t` is an identification variable, `league` is a single-valued relationship field, and `sport` is a persistent field of `league`:

    SELECT DISTINCT p
    FROM Player p, IN (p.teams) t
    WHERE t.league.sport = :sport

Here, the `WHERE` clause contains a `collection_valued_path_expression`; `p` is an identification variable, and `teams` designates a collection-valued relationship field:

    SELECT DISTINCT p
    FROM Player p
    WHERE p.teams IS EMPTY

#### Expression Types

The type of a path expression is the type of the object represented by the ending element, which can be one of the following:

- Persistent field

- Single-valued relationship field

- Collection-valued relationship field

For example, the type of the expression `p.salary` is `double` because the terminating persistent field (`salary`) is a `double`.

In the expression `p.teams`, the terminating element is a collection-valued relationship field (`teams`). This expression’s type is a collection of the abstract schema type named `Team`. Because `Team` is the abstract schema name for the `Team` entity, this type maps to the entity. For more information on the type mapping of abstract schemas, see Return Types.

#### Navigation

A path expression enables the query to navigate to related entities. The terminating elements of an expression determine whether navigation is allowed. If an expression contains a single-valued relationship field, the navigation can continue to an object that is related to the field. However, an expression cannot navigate beyond a persistent field or a collection-valued relationship field. For example, the expression `p.teams.league.sport` is illegal because `teams` is a collection-valued relationship field. To reach the `sport` field, the `FROM` clause could define an identification variable named `t` for the `teams` field:

    FROM Player AS p, IN (p.teams) t
    WHERE t.league.sport = 'soccer'

### WHERE Clause

The `WHERE` clause specifies a conditional expression that limits the values returned by the query. The query returns all corresponding values in the data store for which the conditional expression is `TRUE`. Although usually specified, the `WHERE` clause is optional. If the `WHERE` clause is omitted, the query returns all values. The high-level syntax for the `WHERE` clause is as follows:

    where_clause ::= WHERE conditional_expression

The following topics are addressed here:

- Literals

- Input Parameters

- Conditional Expressions

- Operators and Their Precedence

- BETWEEN Expressions

- IN Expressions

- LIKE Expressions

- NULL Comparison Expressions

- Empty Collection Comparison Expressions

- Collection Member Expressions

- Subqueries

- Functional Expressions

- Case Expressions

- NULL Values

- Equality Semantics

#### Literals

There are four kinds of literals: string, numeric, Boolean, and enum.

- String literals: A string literal is enclosed in single quotes:

      'Duke'

  If a string literal contains a single quote, you indicate the quote by using two single quotes:

      'Duke''s'

  Like a Java `String`, a string literal in the query language uses the Unicode character encoding.

- Numeric literals: There are two types of numeric literals: exact and approximate.

  - An exact numeric literal is a numeric value without a decimal point, such as 65, –233, and +12. Using the Java integer syntax, exact numeric literals support numbers in the range of a Java `long`.

  - An approximate numeric literal is a numeric value in scientific notation, such as 57., –85.7, and +2.1. Using the syntax of the Java floating-point literal, approximate numeric literals support numbers in the range of a Java `double`.

- Boolean literals: A Boolean literal is either `TRUE` or `FALSE`. These keywords are not case-sensitive.

- Enum literals: The Java Persistence query language supports the use of enum literals using the Java enum literal syntax. The enum class name must be specified as a fully qualified class name:

      SELECT e
      FROM Employee e
      WHERE e.status = com.example.EmployeeStatus.FULL_TIME

#### Input Parameters

An input parameter can be either a named parameter or a positional parameter.

- A named input parameter is designated by a colon (`:`) followed by a string; for example, `:name`.

- A positional input parameter is designated by a question mark (`?`) followed by an integer. For example, the first input parameter is `?1`, the second is `?2`, and so forth.

The following rules apply to input parameters.

- They can be used only in a `WHERE` or `HAVING` clause.

- Positional parameters must be numbered, starting with the integer 1.

- Named parameters and positional parameters may not be mixed in a single query.

- Named parameters are case-sensitive.

#### Conditional Expressions

A `WHERE` clause consists of a conditional expression, which is evaluated from left to right within a precedence level. You can change the order of evaluation by using parentheses.

#### Operators and Their Precedence

Table 42-2 lists the query language operators in order of decreasing precedence.

Table 42-2 Query Language Order Precedence

Type | Precedence Order
Navigation | `.` (a period)
Arithmetic | `+ –` (unary) `* /` (multiplication and division) `+ –` (addition and subtraction)
Comparison | `=` `>` `>=` `<` `⇐` `<>` (not equal) `[NOT] BETWEEN` `[NOT] LIKE` `[NOT] IN` `IS [NOT] NULL` `IS [NOT] EMPTY` `[NOT] MEMBER OF`
Logical | `NOT` `AND` `OR`

#### BETWEEN Expressions

A `BETWEEN` expression determines whether an arithmetic expression falls within a range of values.

These two expressions are equivalent:

    p.age BETWEEN 15 AND 19
    p.age >= 15 AND p.age <= 19

The following two expressions also are equivalent:

    p.age NOT BETWEEN 15 AND 19
    p.age < 15 OR p.age > 19

If an arithmetic expression has a `NULL` value, the value of the `BETWEEN` expression is unknown.

#### IN Expressions

An `IN` expression determines whether a string belongs to a set of string literals or whether a number belongs to a set of number values.

The path expression must have a string or numeric value. If the path expression has a `NULL` value, the value of the `IN` expression is unknown.

In the following example, the expression is `TRUE` if the country is `UK` , but `FALSE` if the country is `Peru`:

    o.country IN ('UK', 'US', 'France')

You may also use input parameters:

    o.country IN ('UK', 'US', 'France', :country)

#### LIKE Expressions

A `LIKE` expression determines whether a wildcard pattern matches a string.

The path expression must have a string or numeric value. If this value is `NULL`, the value of the `LIKE` expression is unknown. The pattern value is a string literal that can contain wildcard characters. The underscore (`_`) wildcard character represents any single character. The percent (`%`) wildcard character represents zero or more characters. The `ESCAPE` clause specifies an escape character for the wildcard characters in the pattern value. Table 42-3 shows some sample `LIKE` expressions.

Table 42-3 LIKE Expression Examples

Expression | TRUE | FALSE
`address.phone LIKE '12%3'` | `'123'` `'12993'` | `'1234'`
`asentence.word LIKE 'l_se'` | `'lose'` | `'loose'`
`aword.underscored LIKE '\_%' ESCAPE '\'` | `'_foo'` | `'bar'`
`address.phone NOT LIKE '12%3'` | `'1234'` | `'123'` `'12993'`

#### NULL Comparison Expressions

A `NULL` comparison expression tests whether a single-valued path expression or an input parameter has a `NULL` value. Usually, the `NULL` comparison expression is used to test whether a single-valued relationship has been set:

    SELECT t
    FROM Team t
    WHERE t.league IS NULL

This query selects all teams where the league relationship is not set. Note that the following query is not equivalent:

    SELECT t
    FROM Team t
    WHERE t.league = NULL

The comparison with `NULL` using the equals operator (`=`) always returns an unknown value, even if the relationship is not set. The second query will always return an empty result.

#### Empty Collection Comparison Expressions

The `IS [NOT] EMPTY` comparison expression tests whether a collection-valued path expression has no elements. In other words, it tests whether a collection-valued relationship has been set.

If the collection-valued path expression is `NULL`, the empty collection comparison expression has a `NULL` value.

Here is an example that finds all orders that do not have any line items:

    SELECT o
    FROM CustomerOrder o
    WHERE o.lineItems IS EMPTY

#### Collection Member Expressions

The `[NOT]` `MEMBER [OF]` collection member expression determines whether a value is a member of a collection. The value and the collection members must have the same type.

If either the collection-valued or single-valued path expression is unknown, the collection member expression is unknown. If the collection-valued path expression designates an empty collection, the collection member expression is `FALSE`.

The `OF` keyword is optional.

The following example tests whether a line item is part of an order:

    SELECT o
    FROM CustomerOrder o
    WHERE :lineItem MEMBER OF o.lineItems

#### Subqueries

Subqueries may be used in the `WHERE` or `HAVING` clause of a query. Subqueries must be surrounded by parentheses.

The following example finds all customers who have placed more than ten orders:

    SELECT c
    FROM Customer c
    WHERE (SELECT COUNT(o) FROM c.orders o)> 10

Subqueries may contain `EXISTS`, `ALL`, and `ANY` expressions.

- EXISTS expressions: The `[NOT] EXISTS` expression is used with a subquery and is true only if the result of the subquery consists of one or more values; otherwise, it is false.

  The following example finds all employees whose spouses are also employees:

      SELECT DISTINCT emp
      FROM Employee emp
      WHERE EXISTS (
          SELECT spouseEmp
          FROM Employee spouseEmp
          WHERE spouseEmp = emp.spouse)

- ALL and ANY expressions: The `ALL` expression is used with a subquery and is true if all the values returned by the subquery are true or if the subquery is empty.

  The `ANY` expression is used with a subquery and is true if some of the values returned by the subquery are true. An `ANY` expression is false if the subquery result is empty or if all the values returned are false. The `SOME` keyword is synonymous with `ANY`.

  The `ALL` and `ANY` expressions are used with the `=`, `<`, `⇐`, `>`, `>=`, and `<>` comparison operators.

  The following example finds all employees whose salaries are higher than the salaries of the managers in the employee’s department:

      SELECT emp
      FROM Employee emp
      WHERE emp.salary > ALL (
          SELECT m.salary
          FROM Manager m
          WHERE m.department = emp.department)

#### Functional Expressions

The query language includes several string, arithmetic, and date/time functions that may be used in the `SELECT`, `WHERE`, or `HAVING` clause of a query. The functions are listed in Table 42-4, Table 42-5, and Table 42-6.

In Table 42-4, the `start` and `length` arguments are of type `int` and designate positions in the `String` argument. The first position in a string is designated by 1.

Table 42-4 String Expressions

Function Syntax | Return Type
`CONCAT(String, String)` | `String`
`LENGTH(String)` | `int`
`LOCATE(String, String [, start])` | `int`
`SUBSTRING(String, start, length)` | `String`
`TRIM([[LEADING | TRAILING
BOTH] char) FROM] (String)` | `String`
`LOWER(String)` | `String`
`UPPER(String)` | `String`

The `CONCAT` function concatenates two strings into one string.

The `LENGTH` function returns the length of a string in characters as an integer.

The `LOCATE` function returns the position of a given string within a string. This function returns the first position at which the string was found as an integer. The first argument is the string to be located. The second argument is the string to be searched. The optional third argument is an integer that represents the starting string position. By default, `LOCATE` starts at the beginning of the string. The starting position of a string is `1`. If the string cannot be located, `LOCATE` returns `0`.

The `SUBSTRING` function returns a string that is a substring of the first argument based on the starting position and length.

The `TRIM` function trims the specified character from the beginning and/or end of a string. If no character is specified, `TRIM` removes spaces or blanks from the string. If the optional `LEADING` specification is used, `TRIM` removes only the leading characters from the string. If the optional `TRAILING` specification is used, `TRIM` removes only the trailing characters from the string. The default is `BOTH`, which removes the leading and trailing characters from the string.

The `LOWER` and `UPPER` functions convert a string to lowercase or uppercase, respectively.

In Table 42-5, the `number` argument can be an `int`, a `float`, or a `double`.

Table 42-5 Arithmetic Expressions

Function Syntax | Return Type
`ABS(number)` | `int`, `float`, or `double`
`MOD(int, int)` | `int`
`SQRT(double)` | `double`
`SIZE(Collection)` | `int`

The `ABS` function takes a numeric expression and returns a number of the same type as the argument.

The `MOD` function returns the remainder of the first argument divided by the second.

The `SQRT` function returns the square root of a number.

The `SIZE` function returns an integer of the number of elements in the given collection.

In Table 42-6, the date/time functions return the date, time, or timestamp on the database server.

Table 42-6 Date/Time Expressions

Function Syntax | Return Type
`CURRENT_DATE` | `java.sql.Date`
`CURRENT_TIME` | `java.sql.Time`
`CURRENT_TIMESTAMP` | `java.sql.Timestamp`

#### Case Expressions

Case expressions change based on a condition, similar to the `case` keyword of the Java programming language. The `CASE` keyword indicates the start of a case expression, and the expression is terminated by the `END` keyword. The `WHEN` and `THEN` keywords define individual conditions, and the `ELSE` keyword defines the default condition should none of the other conditions be satisfied.

The following query selects the name of a person and a conditional string, depending on the subtype of the `Person` entity. If the subtype is `Student`, the string `kid` is returned. If the subtype is `Guardian` or `Staff`, the string `adult` is returned. If the entity is some other subtype of `Person`, the string `unknown` is returned:

    SELECT p.name
    CASE TYPE(p)
        WHEN Student THEN 'kid'
        WHEN Guardian THEN 'adult'
        WHEN Staff THEN 'adult'
        ELSE 'unknown'
    END
    FROM Person p

The following query sets a discount for various types of customers. Gold-level customers get a 20% discount, silver-level customers get a 15% discount, bronze-level customers get a 10% discount, and everyone else gets a 5% discount:

    UPDATE Customer c
    SET c.discount =
        CASE c.level
            WHEN 'Gold' THEN 20
            WHEN 'SILVER' THEN 15
            WHEN 'Bronze' THEN 10
            ELSE 5
        END

#### NULL Values

If the target of a reference is not in the persistent store, the target is `NULL`. For conditional expressions containing `NULL`, the query language uses the semantics defined by SQL92. Briefly, these semantics are as follows.

- If a comparison or arithmetic operation has an unknown value, it yields a `NULL` value.

- Two `NULL` values are not equal. Comparing two `NULL` values yields an unknown value.

- The `IS NULL` test converts a `NULL` persistent field or a single-valued relationship field to `TRUE`. The `IS NOT NULL` test converts them to `FALSE`.

- Boolean operators and conditional tests use the three-valued logic defined by Table 42-7 and Table 42-8. (In these tables, T stands for `TRUE`, F for `FALSE`, and U for unknown.)

Table 42-7 AND Operator Logic

AND | T | F | U
T | T | F | U
F | F | F | F
U | U | F | U

Table 42-8 OR Operator Logic

OR | T | F | U
T | T | T | T
F | T | F | U
U | T | U | U

#### Equality Semantics

In the query language, only values of the same type can be compared. However, this rule has one exception: Exact and approximate numeric values can be compared. In such a comparison, the required type conversion adheres to the rules of Java numeric promotion.

The query language treats compared values as if they were Java types and not as if they represented types in the underlying data store. For example, a persistent field that could be either an integer or a `NULL` must be designated as an `Integer` object and not as an `int` primitive. This designation is required because a Java object can be `NULL`, but a primitive cannot.

Two strings are equal only if they contain the same sequence of characters. Trailing blanks are significant; for example, the strings `'abc'` and `'abc '` are not equal.

Two entities of the same abstract schema type are equal only if their primary keys have the same value. Table 42-9 shows the operator logic of a negation, and Table 42-10 shows the truth values of conditional tests.

Table 42-9 NOT Operator Logic

NOT Value | Value
T | F
F | T
U | U

Table 42-10 Conditional Test

Conditional Test | T | F | U
Expression `IS TRUE` | T | F | F
Expression `IS FALSE` | F | T | F
Expression is unknown | F | F | T

### SELECT Clause

The `SELECT` clause defines the types of the objects or values returned by the query.

The following topics are addressed here:

- Return Types

- The DISTINCT Keyword

- Constructor Expressions

#### Return Types

The return type of the `SELECT` clause is defined by the result types of the select expressions contained within it. If multiple expressions are used, the result of the query is an `Object[]`, and the elements in the array correspond to the order of the expressions in the `SELECT` clause and in type to the result types of each expression.

A `SELECT` clause cannot specify a collection-valued expression. For example, the `SELECT` clause `p.teams` is invalid because `teams` is a collection. However, the clause in the following query is valid because `t` is a single element of the `teams` collection:

    SELECT t
    FROM Player p, IN (p.teams) t

The following query is an example of a query with multiple expressions in the `SELECT` clause:

    SELECT c.name, c.country.name
    FROM customer c
    WHERE c.lastname = 'Coss' AND c.firstname = 'Roxane'

This query returns a list of `Object[]` elements; the first array element is a string denoting the customer name, and the second array element is a string denoting the name of the customer’s country.

The result of a query may be the result of an aggregate function, listed in Table 42-11.

Table 42-11 Aggregate Functions in Select Statements

Name | Return Type | Description
`AVG` | `Double` | Returns the mean average of the fields
`COUNT` | `Long` | Returns the total number of results
`MAX` | The type of the field | Returns the highest value in the result set
`MIN` | The type of the field | Returns the lowest value in the result set
`SUM` | `Long` (for integral fields) `Double` (for floating-point fields) `BigInteger` (for `BigInteger` fields) `BigDecimal` (for `BigDecimal` fields) | Returns the sum of all the values in the result set

For select method queries with an aggregate function (`AVG`, `COUNT`, `MAX`, `MIN`, or `SUM`) in the `SELECT` clause, the following rules apply.

- The `AVG`, `MAX`, `MIN`, and `SUM` functions return `null` if there are no values to which the function can be applied.

- The `COUNT` function returns 0 if there are no values to which the function can be applied.

The following example returns the average order quantity:

    SELECT AVG(o.quantity)
    FROM CustomerOrder o

The following example returns the total cost of the items ordered by Roxane Coss:

    SELECT SUM(l.price)
    FROM CustomerOrder o JOIN o.lineItems l JOIN o.customer c
    WHERE c.lastname = 'Coss' AND c.firstname = 'Roxane'

The following example returns the total number of orders:

    SELECT COUNT(o)
    FROM CustomerOrder o

The following example returns the total number of items that have prices in Hal Incandenza’s order:

    SELECT COUNT(l.price)
    FROM CustomerOrder o JOIN o.lineItems l JOIN o.customer c
    WHERE c.lastname = 'Incandenza' AND c.firstname = 'Hal'

#### The DISTINCT Keyword

The `DISTINCT` keyword eliminates duplicate return values. If a query returns a `java.util.Collection`, which allows duplicates, you must specify the `DISTINCT` keyword to eliminate duplicates.

#### Constructor Expressions

Constructor expressions allow you to return Java instances that store a query result element instead of an `Object[]`.

The following query creates a `CustomerDetail` instance per `Customer` matching the `WHERE` clause. A `CustomerDetail` stores the customer name and customer’s country name. So the query returns a `List` of `CustomerDetail` instances:

    SELECT NEW com.example.CustomerDetail(c.name, c.country.name)
    FROM customer c
    WHERE c.lastname = 'Coss' AND c.firstname = 'Roxane'

### ORDER BY Clause

As its name suggests, the `ORDER BY` clause orders the values or objects returned by the query.

If the `ORDER BY` clause contains multiple elements, the left-to-right sequence of the elements determines the high-to-low precedence.

The `ASC` keyword specifies ascending order, the default, and the `DESC` keyword indicates descending order.

When using the `ORDER BY` clause, the `SELECT` clause must return an orderable set of objects or values. You cannot order the values or objects for values or objects not returned by the `SELECT` clause. For example, the following query is valid because the `ORDER BY` clause uses the objects returned by the `SELECT` clause:

    SELECT o
    FROM Customer c JOIN c.orders o JOIN c.address a
    WHERE a.state = 'CA'
    ORDER BY o.quantity, o.totalcost

The following example is not valid, because the `ORDER BY` clause uses a value not returned by the `SELECT` clause:

    SELECT p.product_name
    FROM CustomerOrder o, IN(o.lineItems) l JOIN o.customer c
    WHERE c.lastname = 'Faehmel' AND c.firstname = 'Robert'
    ORDER BY o.quantity

### GROUP BY and HAVING Clauses

The `GROUP BY` clause allows you to group values according to a set of properties.

The following query groups the customers by their country and returns the number of customers per country:

    SELECT c.country, COUNT(c)
    FROM Customer c GROUP BY c.country

The `HAVING` clause is used with the `GROUP BY` clause to further restrict the returned result of a query.

The following query groups orders by the status of their customer and returns the customer status plus the average `totalPrice` for all orders where the corresponding customers have the same status. In addition, it considers only customers with status `1`, `2`, or `3`, so orders of other customers are not taken into account:

    SELECT c.status, AVG(o.totalPrice)
    FROM CustomerOrder o JOIN o.customer c
    GROUP BY c.status HAVING c.status IN (1, 2, 3)

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