SQL Case表达式的完整和正确语法是什么?
答案 0 :(得分:77)
完整语法取决于您正在使用的数据库引擎:
对于SQL Server:
CASE case-expression
WHEN when-expression-1 THEN value-1
[ WHEN when-expression-n THEN value-n ... ]
[ ELSE else-value ]
END
或:
CASE
WHEN boolean-when-expression-1 THEN value-1
[ WHEN boolean-when-expression-n THEN value-n ... ]
[ ELSE else-value ]
END
表达式等:
case-expression - something that produces a value
when-expression-x - something that is compared against the case-expression
value-1 - the result of the CASE statement if:
the when-expression == case-expression
OR the boolean-when-expression == TRUE
boolean-when-exp.. - something that produces a TRUE/FALSE answer
另请注意,WHEN语句的顺序很重要。您可以轻松编写多个重叠的WHEN子句,并使用匹配的第一个。
注意:如果未指定ELSE子句,且未找到匹配的WHEN条件,则CASE表达式的值将为 NULL 。
答案 1 :(得分:21)
考虑到您标记了多个产品,我会说完整正确的语法将是ISO / ANSI SQL-92标准中的语法:
<case expression> ::=
<case abbreviation>
| <case specification>
<case abbreviation> ::=
NULLIF <left paren> <value expression> <comma>
<value expression> <right paren>
| COALESCE <left paren> <value expression>
{ <comma> <value expression> }... <right paren>
<case specification> ::=
<simple case>
| <searched case>
<simple case> ::=
CASE <case operand>
<simple when clause>...
[ <else clause> ]
END
<searched case> ::=
CASE
<searched when clause>...
[ <else clause> ]
END
<simple when clause> ::= WHEN <when operand> THEN <result>
<searched when clause> ::= WHEN <search condition> THEN <result>
<else clause> ::= ELSE <result>
<case operand> ::= <value expression>
<when operand> ::= <value expression>
<result> ::= <result expression> | NULL
<result expression> ::= <value expression>
语法规则
1) NULLIF (V1, V2) is equivalent to the following <case specification>:
CASE WHEN V1=V2 THEN NULL ELSE V1 END
2) COALESCE (V1, V2) is equivalent to the following <case specification>:
CASE WHEN V1 IS NOT NULL THEN V1 ELSE V2 END
3) COALESCE (V1, V2, . . . ,n ), for n >= 3, is equivalent to the
following <case specification>:
CASE WHEN V1 IS NOT NULL THEN V1 ELSE COALESCE (V2, . . . ,n )
END
4) If a <case specification> specifies a <simple case>, then let CO
be the <case operand>:
a) The data type of each <when operand> WO shall be comparable
with the data type of the <case operand>.
b) The <case specification> is equivalent to a <searched case>
in which each <searched when clause> specifies a <search
condition> of the form "CO=WO".
5) At least one <result> in a <case specification> shall specify a
<result expression>.
6) If an <else clause> is not specified, then ELSE NULL is im-
plicit.
7) The data type of a <case specification> is determined by ap-
plying Subclause 9.3, "Set operation result data types", to the
data types of all <result expression>s in the <case specifica-
tion>.
Access Rules
None.
General Rules
1) Case:
a) If a <result> specifies NULL, then its value is the null
value.
b) If a <result> specifies a <value expression>, then its value
is the value of that <value expression>.
2) Case:
a) If the <search condition> of some <searched when clause> in
a <case specification> is true, then the value of the <case
specification> is the value of the <result> of the first
(leftmost) <searched when clause> whose <search condition> is
true, cast as the data type of the <case specification>.
b) If no <search condition> in a <case specification> is true,
then the value of the <case expression> is the value of the
<result> of the explicit or implicit <else clause>, cast as
the data type of the <case specification>.
答案 2 :(得分:19)
以下是CASE
statement examples from the PostgreSQL docs(Postgres遵循SQL标准):
SELECT a,
CASE WHEN a=1 THEN 'one'
WHEN a=2 THEN 'two'
ELSE 'other'
END
FROM test;
或
SELECT a,
CASE a WHEN 1 THEN 'one'
WHEN 2 THEN 'two'
ELSE 'other'
END
FROM test;
显然,当您只是根据可能值列表检查一个字段时,第二种形式更清晰。第一种形式允许更复杂的表达。
答案 3 :(得分:8)
Sybase与SQL Server具有相同的case syntax:
支持条件SQL表达式;可以在任何可以使用值表达式的地方使用。
case
when search_condition then expression
[when search_condition then expression]...
[else expression]
end
case expression
when expression then expression
[when expression then expression]...
[else expression]
end
开始案例表达。
在搜索条件或要比较的表达式之前。
用于设置所选结果的条件。案例表达式的搜索条件类似于where子句中的搜索条件。 “Transact-SQL用户指南”中详细介绍了搜索条件。
在指定case的结果值的表达式之前。
是列名,常量,函数,子查询,或由算术或按位运算符连接的列名,常量和函数的任意组合。有关表达式的更多信息,请参阅。
中的“表达式”select disaster,
case
when disaster = "earthquake"
then "stand in doorway"
when disaster = "nuclear apocalypse"
then "hide in basement"
when monster = "zombie apocalypse"
then "hide with Chuck Norris"
else
then "ask mom"
end
from endoftheworld
答案 4 :(得分:4)
我挖出了相同的Oracle页面,看起来这是相同的语法,只是略有不同。
答案 5 :(得分:3)
Oracle syntax from the 11g Documentation:
CASE { simple_case_expression | searched_case_expression }
[ else_clause ]
END
simple_case_expression 的
expr { WHEN comparison_expr THEN return_expr }...
searched_case_expression 的
{ WHEN condition THEN return_expr }...
else_clause 的
ELSE else_expr
答案 6 :(得分:1)
在Oracle的情况下需要注意的一点是,如果匹配时没有,并且没有其他部分引发异常。
答案 7 :(得分:-1)
在这里您可以找到MySQL case statements in SQL的完整指南。
CASE
WHEN some_condition THEN return_some_value
ELSE return_some_other_value
END
答案 8 :(得分:-2)
SQL SERVER中的case语句语法:
CASE column
WHEN value1 THEN 1
WHEN value3 THEN 2
WHEN value3 THEN 3
WHEN value1 THEN 4
ELSE ''
END
我们也可以使用如下:
CASE
WHEN column=value1 THEN 1
WHEN column=value3 THEN 2
WHEN column=value3 THEN 3
WHEN column=value1 THEN 4
ELSE ''
END