test

The Java EE 6 Tutorial

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 follows:

where_clause ::= WHERE conditional_expression

Literals

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

Input Parameters

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

The following rules apply to input parameters.

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 22–2 lists the query language operators in order of decreasing precedence.

Table 22–2 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 22–3 shows some sample LIKE expressions.

Table 22–3 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 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, 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 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 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.

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 22–4, Table 22–5, and Table 22–6.

In Table 22–4, 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 22–4 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 22–5, the number argument can be an int, a float, or a double.

Table 22–5 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 22–6, the date/time functions return the date, time, or timestamp on the database server.

Table 22–6 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.

Table 22–7 AND Operator Logic

AND 

T 

F 

U 

T 

T 

F 

U 

F 

F 

F 

F 

U 

U 

F 

U 

Table 22–8 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 22–9 shows the operator logic of a negation, and Table 22–10 shows the truth values of conditional tests.

Table 22–9 NOT Operator Logic

NOT Value 

Value 

T 

F 

F 

T 

U 

U 

Table 22–10 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