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Full Syntax

This section discusses the query language syntax, as defined in the Java Persistence specification. Much of the following material paraphrases or directly quotes the specification.

BNF Symbols

Table 27-1 describes the BNF symbols used in this chapter.

Table 27-1 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 curly braces are grouped together.
[...]	The constructs within the square brackets are optional.
|	An exclusive OR.
BOLDFACE	A keyword (although capitalized in the BNF diagram, keywords are not case-sensitive).
Whitespace	A whitespace character can be a space, a horizontal tab, or a linefeed.

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.

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 J2SE 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:

ALL AND ANY AS ASC AVG BETWEEN BY COUNT CURRENT_DATE CURRENT_TIME CURRENT_TIMESTAMP DELETE DESC DISTINCT EMPTY EXISTS

FALSE FETCH FROM GROUP HAVING IN INNER IS JOIN LEFT LIKE MAX MEMBER MIN MOD NEW

NOT NULL OBJECT OF OUTER OR ORDER SELECT SOME SUM TRIM TRUE UNKNOWN UPDATE UPPER WHERE

It is not recommended that you use a SQL keyword as an identifier, as 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 the SELECT and WHERE clauses can reference identification variables, they cannot declare them. All identification variables must be declared in the FROM clause.

Because an identification variable is an identifier, it has the same naming conventions and restrictions as an identifier with the exception that an identification variables is case-insensitive. For example, an identification variable cannot be the same as a query language keyword. (See the preceding section 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 an identification variable is not used in the WHERE clause, its declaration can affect the results of the query. For an 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 the next query declares the t identification variable, it fetches all players that 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 

In most cases, to obtain objects a query 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 root).

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

FROM Player p1, Player p2 

For a sample 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 the following section.) 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 it 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 

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 Order 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, they define navigation paths through the relationships in the abstract schema. These path definitions affect both the scope and the results of a query. Second, they 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.

Examples

In the following query, 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 

The WHERE clause of the next example also contains a single_valued_path_expression. The 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 

In the next query, the WHERE clause contains a collection_valued_path_expression. The 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 the section 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, then the query returns all values. The high-level syntax for the WHERE clause follows:

where_clause ::= WHERE conditional_expression 

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 fully qualified class name.

SELECT e 
FROM Employee e 
WHERE e.status = com.xyz.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.

An 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 27-2 lists the query language operators in order of decreasing precedence.

Table 27-2 Query Language Operator 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 are also equivalent:

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

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

IN Expressions

An IN expression determines whether or not 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, then the value of the IN expression is unknown.

In the following example, if the country is UK the expression is TRUE. If the country is Peru it is FALSE.

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, then 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 27-3 shows some sample LIKE expressions.

Table 27-3 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 or not 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. Please note, 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 or not a collection-valued relationship has been set.

If the collection-valued path expression is NULL, then 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 Order 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, then the collection member expression is unknown. If the collection-valued path expression designates an empty collection, then 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 Order 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 find all customers who have placed more than 10 orders:

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

Exists Expressions

The [NOT] EXISTS expression is used in conjunction with a subquery, and is true only if the result of the subquery consists of one or more values and that is false otherwise.

The following example finds all employees whose spouse is also an employee:

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 in conjunction 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 in conjunction 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 =, <, <=, >, >=, <> comparison operators.

The following example finds all employees whose salary is higher than the salary 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 and arithmetic functions which may be used in the WHERE or HAVING clause of a query. The functions are listed in the following tables. In Table 27-4, the start and length arguments are of type int. They designate positions in the String argument. The first position in a string is designated by 1. In Table 27-5, the number argument can be either an int, a float, or a double.

Table 27-4 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 positon of a given string within a string. It 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 character(s) from the string. If the optional TRAILING specification is used, TRIM removes only the trailing character(s) from the string. The default is BOTH, which removes the leading and trailing character(s) from the string.

The LOWER and UPPER functions convert a string to lower or upper case, respectively.

Table 27-5 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.

NULL Values

If the target of a reference is not in the persistent store, then 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 27-6 and Table 27-7. (In these tables, T stands for TRUE, F for FALSE, and U for unknown.)

Table 27-6 AND Operator Logic 

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

Table 27-7 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, if a persistent field could be either an integer or a NULL, then it 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 27-8 shows the operator logic of a negation, and Table 27-9 shows the truth values of conditional tests.

Table 27-8 NOT Operator Logic 

NOT Value	Value
T	F
F	T
U	U

Table 27-9 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.

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 the 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' 

It returns a list of Object[] elements where 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.

Aggregate Functions in the SELECT Clause

The result of a query may be the result of an aggregate function.

Table 27-10 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:

For the AVG, MAX, MIN, and SUM functions, the functions return null if there are no values to which the function can be applied.
For the COUNT function, if there are no values to which the function can be applied, COUNT returns returns 0.

The following example returns the average order quantity:

SELECT AVG(o.quantity) 
FROM Order o 

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

SELECT SUM(l.price)
FROM Order 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 Order o 

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

SELECT COUNT(l.price)
FROM Order 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--then 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.xyz.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 Order o, IN(o.lineItems) l JOIN o.customer c
WHERE c.lastname = 'Faehmel' AND c.firstname = 'Robert'
ORDER BY o.quantity 

The GROUP BY Clause

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

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 has the same status. In addition it considers only customer with status 1, 2, or 3, so orders of other customers are not taken into account:

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

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