SQL案例语句的语法？
什么是SQL Case语句的完整和正确的语法？
完整的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 要么：
CASE WHEN boolean-when-expression-1 THEN value-1 [ WHEN boolean-when-expression-n THEN value-n ... ] [ ELSE else-value ] END
expression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
链接： CASE（Transact-SQL）
另外请注意，WHEN语句的顺序很重要。 您可以轻松地编写多个重叠的WHEN子句，并使用第一个匹配的子句。
注意 ：如果没有指定ELSE子句，并且没有find匹配的WHEN条件，则CASEexpression式的值将为NULL 。
考虑到你标记了多个产品，我会说完全正确的语法将在ISO / ANSI SQL-92标准中find：
<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>.
以下是PostgreSQL文档中的CASE语句示例 （Postgres在此遵循SQL标准）：
SELECT a, CASE WHEN a=1 THEN 'one' WHEN a=2 THEN 'two' ELSE 'other' END FROM test;
要么
SELECT a, CASE a WHEN 1 THEN 'one' WHEN 2 THEN 'two' ELSE 'other' END FROM test;
显然，当你只是在一个可能的值列表中检查一个字段时，第二种forms更清晰。 第一种forms允许更复杂的expression式。
Sybase与SQL Server具有相同的大小写语法 ：
描述
支持有条件的SQLexpression式; 可以在任何可以使用值expression式的地方使用。
句法
case when search_condition then expression [when search_condition then expression]... [else expression] end
案例和值的语法
case expression when expression then expression [when expression then expression]... [else expression] end
参数
案件
开始案件expression。
什么时候
先于search条件或要比较的expression式。
search_condition
用于设置所选结果的条件。 caseexpression式的search条件与where子句中的search条件类似。 search条件详见“Transact-SQL用户指南”。
然后
在指定case的结果值的expression式之前。
expression
是列名称，常量，函数，子查询或由算术或位运算符连接的列名，常量和函数的任意组合。 有关expression式的更多信息，请参见“expression式”。
例
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
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。
链接： Oracle / PLSQL：案例陈述
Oracle 11g文档中的语法 ：
CASE { simple_case_expression | searched_case_expression } [ else_clause ] END
simple_case_expression
expr { WHEN comparison_expr THEN return_expr }...
searched_case_expression
{ WHEN condition THEN return_expr }...
else_clause
ELSE else_expr
在Oracle的情况下要注意的一点是，如果没有匹配，并且没有其他部分出现exception，则引发。
Case statement syntax in SQL SERVER. CASE column WHEN value1 THEN 1 WHEN value3 THEN 2 WHEN value3 THEN 3 WHEN value1 THEN 4 ELSE '' END And we can use like below also: CASE WHEN column=value1 THEN 1 WHEN column=value3 THEN 2 WHEN column=value3 THEN 3 WHEN column=value1 THEN 4 ELSE '' END
