【IT168 技术文档】
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)
BNF符号
符号：描述
::=：该符号左边的元素被该符号右边的结构所定义
*：该符号前面的结构可以重复零次或多次
{...}：在这个花括号中的结构一起组合成一组
[...]：方括号中的结构是可选的
|：一个具有排它性的OR
黑体字：一个关键字(尽管在这个BNF定义中使用了大写，但是事实上关键字并不是大小写敏感的)
空白区域：一个空白区域可以是一个空格、横表符或是换页符
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
EJB QL的关键字同时也是SQL的保留字。在将来，EJB QL关键字将扩充到包含其它SQL保留字。Enterprise JavaBeans规范推荐你不要在EJB QL标识符中使用其它SQL保留字。
