mysql - 在 SQL 中的 case..when 语句中，当单个条件为真时，我可以执行多个表达式吗？

Question

我想在 SQL 中的 case..when 语句中当单个条件为真时显示多个语句。

例如:

case when (condition is true) then 
print "A" 
print "B"
.
.
.
.
print "Z"
when (condition2 is true) then 
print "Z" 
print "Y"
.
.
.
.
print "A

end

谁能给我提供它的确切语法？提前致谢。

Answer 1

如果您的条件很复杂，您可以将其移至子查询。这样你就不必为每一列重复它:

select  case when Condition = 1 then 'A' else 'B' end
,       case when Condition = 1 then 'C' else 'D' end
,       case when Condition = 1 then 'E' else 'F' end
,       ...
from    (
        select  *
        ,       case
                when ... complex condition ... then 1 
                else 0 
                end as Condition
        from    YourTable
        ) as SubQueryAlias

另一个选项是与 CTE 的联合(并非在所有数据库中都可用。)它允许您在不使用 case 的情况下为两者编写表达式，并且由于 CTE，条件不会重复。

;       with CteAlias as
        (
        select  *
        ,       case
                when ... complex condition ... then 1 
                else 0 
                end as Condition
        from    YourTable
        )
select  'A', 'C', 'E'
from    CteAlias
where   Condition = 1
union all
select  'B', 'D', 'F'
from    CteAlias
where   Condition = 0