示例 6
SELECT DISTINCT OBJECT(p)
FROM Player p, IN (p.teams) AS t
WHERE t.league.sport = ?1
获得的数据:参与指定运动的队员。
Finder方法: findBySport(String sport)
说明:sport持续化字段属于LeagueEJB bean。要访问sport字段,查询必须首先实现从PlayerEJB bean到TeamEJB bean的跨越(p.teams),然后实现从TeamEJB bean到LeagueEJB bean的跨越(t.league)。因为league关联字段不是一个集合,在它后面可以跟随sport持续化字段。
使用其它条件表达式的Finder查询
每一个WHERE子句都必须指定一个条件表达式,这样的条件表达式可以有几种。在之前的示例中,条件表达式是检测是否相等的比较表达式。在下面的例子中,使用了一些其它种类的条件表达式。
示例 7
SELECT OBJECT(p)
FROM Player p
WHERE p.teams IS EMPTY
获得的数据:所有不属于任何运动队的队员。
Finder方法: findNotOnTeam()
说明:PlayerEJB bean的teams关联字段是一个集合。如果一个队员不属于任何运动队,那么teams集合为空,条件表达式的结果将为TRUE。
示例 8
SELECT DISTINCT OBJECT(p)
FROM Player p
WHERE p.salary BETWEEN ?1 AND ?2
获得的数据:薪水位于指定范围的队员。
Finder方法: findBySalaryRange(double low, double high)
说明:BETWEEN表达式是一个三元表达式:一个持续化字段(p.salary)和两个输入参数(?1和?2)。下面的表达式与之前的BETWEEN表达式等价:
p.salary >= ?1 AND p.salary <= ?2
示例 9
SELECT DISTINCT OBJECT(p1)
FROM Player p1, Player p2
WHERE p1.salary > p2.salary
AND p2.name = ?1
获得的数据:所有薪水高于指定姓名的队员薪水的队员。
Finder方法: findByHigherSalary(String name)
说明:FROM子句声明了两个标识变量(p1和p2),这两个标识变量具有同一类型(Player)。之所以在这里需要两个标识变量是因为以WHERE子句中将一个队员(p2)的薪水与另一个队员(p1)进行了比较。
Select查询
这一部分的查询是select方法所使用的。与finder方法不同,一个select方法可以返回持续化字段或其它entity bean。
示例 10
SELECT DISTINCT t.league
FROM Player p, IN (p.teams) AS t
WHERE p = ?1
获得的数据:指定队员所属的联盟。
Select方法:ejbSelectLeagues(LocalPlayer player)
说明:这个查询的返回类型是LeagueEJB entity bean的抽象模式类型。这个抽象模式类型映射到LocalLeagueHome接口。因为表达式t.league不是一个独立的标识变量,OBJECT关键字被省略了。
示例 11
SELECT DISTINCT t.league.sport
FROM Player p, IN (p.teams) AS t
WHERE p = ?1
获得的数据:指定队员所参与的运动。
Select方法: ejbSelectSports(LocalPlayer player)
说明:这个查询返回了一个名为sport的String named sport,这是LeagueEJB entity bean的一个持续化字段。
完整的语法
在这一部分我们讨论Enterprise JavaBean规范中定义的EJB QL语法。下面的素材中的大多数直接取自Enterprise JavaBean规范或是对规范的解释。
EJB QL的BNF语法
这里是EJB QL的全部BNF语法解释:
EJB QL ::= select_clause
from_clause [where_clause]
from_clause ::= FROM
identification_variable_declaration
[, identification_variable_declaration]*
identification_variable_declaration
::=
collection_member_declaration |
range_variable_declaration
collection_member_declaration
::=
IN (collection_valued_path_expression)
[AS] identifier
range_variable_declaration
::=
abstract_schema_name
[AS] identifier
single_valued_path_expression
::=
{single_valued_navigation |
identification_variable}.cmp_field |
single_valued_navigation
single_valued_navigation
::=
identification_variable.
[single_valued_cmr_field.]*
single_valued_cmr_field
collection_valued_path_expression
::=
identification_variable.
[single_valued_cmr_field.]*
collection_valued_cmr_field
select_clause ::= SELECT [DISTINCT]
{single_valued_path_expression |
OBJECT(identification_variable)}
where_clause
::= WHERE conditional_expression
conditional_expression
::= conditional_term |
conditional_expression OR conditional_term
conditional_term
::= conditional_factor |
conditional_term AND conditional_factor
conditional_factor
::= [ NOT ] conditional_test
conditional_test
:: = 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
between_expression
::=
arithmetic_expression [NOT] BETWEEN
arithmetic_expression
AND arithmetic_expression
in_expression
::=
single_valued_path_expression
[NOT] IN (string_literal
[, string_literal]* )
like_expression
::=
single_valued_path_expression
[NOT] LIKE pattern_value
[ESCAPE escape-character]
null_comparison_expression
::=
single_valued_path_expression IS [NOT] NULL
empty_collection_comparison_expression
::=
collection_valued_path_expression
IS [NOT] EMPTY
collection_member_expression
::=
{single_valued_navigation
| identification_variable |
input_parameter}
[NOT] MEMBER [OF]
collection_valued_path_expression
comparison_expression ::=
string_value { =|<>}
string_expression |
boolean_value { =|<>}
boolean_expression} |
datetime_value { = | <> | > | < }
datetime_expression |
entity_bean_value { = | <> }
entity_bean_expression |
arithmetic_value comparison_operator
single_value_designator
arithmetic_value
::= single_valued_path_expression |
functions_returning_numerics
single_value_designator
::= scalar_expression
comparison_operator
::=
= | > | >= | < | <= | <>
scalar_expression
::= arithmetic_expression
arithmetic_expression
::= arithmetic_term |
arithmetic_expression { + | - }
arithmetic_term
arithmetic_term
::= arithmetic_factor |
arithmetic_term { * | / }
arithmetic_factor
arithmetic_factor ::= { + |- }
arithmetic_primary
arithmetic_primary
::= single_valued_path_expression |
literal | (arithmetic_expression) |
input_parameter
| functions_returning_numerics
string_value
::= single_valued_path_expression |
functions_returning_strings
string_expression
::= string_primary | input_expression
string_primary
::= single_valued_path_expression
| literal |
(string_expression)
| functions_returning_strings
datetime_value
::= single_valued_path_expression
datetime_expression
::= datetime_value | input_parameter
boolean_value
::= single_valued_path_expression
boolean_expression
::= single_valued_path_expression |
literal | input_parameter
entity_bean_value
::=
single_valued_navigation
| identification_variable
entity_bean_expression
::= entity_bean_value | input_parameter
functions_returning_strings
::=
CONCAT(string_expression,
string_expression) |
SUBSTRING(string_expression,
arithmetic_expression,
arithmetic_expression)
functions_returning_numerics::=
LENGTH(string_expression) |
LOCATE(string_expression,
string_expression[, arithmetic_expression]) |
ABS(arithmetic_expression) |
SQRT(arithmetic_expression)
FROM子句
FROM子句通过声明标识变量定义了查询的范围。这里是FROM子句的语法:
from_clause
::= FROM
identification_variable_declaration
[,
identification_variable_declaration]* identification_variable_declaration
::=
collection_member_declaration
| range_variable_declaration
collection_member_declaration
::= IN
(collection_valued_path_expression)
[AS]
identifier range_variable_declaration
::= abstract_schema_name
[AS] identifier
标识符
一个标识符是由一个或多个字符组成的序列。在Java编程语言(以下简称"Java")中的标识符的首字符必须是一个有效的首字符(字母、$和_)。在一外Java标识符中的每一个子序列字符都必须是一个有效的非首字符(字母、数字、$和_)。(详细情况请参阅Character类的isJavaIdentifierStart方法和isJavaIdentifierPart方法的J2SE API文档。在EJB QL中问号(?)是一个保留字符,它不可以在一个标识符中使用。与一个Java变量不同,一个EJB QL标识符是对大小写不敏感的。
一个标识符不能与EJB QL关键字发生冲突:
AND
AS
BETWEEN
DISTINCT
EMPTY
FALSE
FROM
IN
IS
LIKE
MEMBER
NOT
NULL
OBJECT
OF
OR
SELECT
TRUE
UNKNOWN
WHERE