sql-server - 如何在 SQL 中使用 CASE 优化查询

Question

我有一个带有 Union 语句的查询，只是为了检查大小写，但我想在 Where 中实现 CASE，但我是 SQL 的初学者，不知道我该怎么做。

我的查询就像

SELECT TOP 1 EndDate
           FROM   (
                      SELECT 1 AS seq,

                             EndDate,

                      FROM   pipeline_rest_envr_info e
                      WHERE  e.tckt_id = imt.tckt_id
                             AND stat = 'INPR'
                      UNION
                      SELECT TOP 1 2 AS seq,

                             EndDate,

                      FROM   pipeline_rest_envr_info e
                      WHERE  e.tckt_id = imt.tckt_id
                             AND stat = 'CMPL'
                      ORDER BY
                             enddate DESC
                      UNION
                      SELECT TOP 1 3 AS seq,

                             EndDate,

                      FROM   pipeline_rest_envr_info e
                      WHERE  e.tckt_id = imt.tckt_id
                             AND stat = 'PLND'
                      ORDER BY
                             strt_dt
                      UNION 
                      SELECT 4 AS seq,

                             NULL,

                      FROM   pipeline_rest_envr_info e
                      WHERE  e.tckt_id = imt.tckt_id
                  ) aa
           ORDER BY
                  aa.seq

Answer 1

SELECT TOP 1 EndDate
FROM（
选择（当 stat = 'INPR' 然后 1
当 stat = 'CMPL' 然后 2
当 stat = 'PLND' 然后 3
else 4 end
）作为 seq,EndDate
from pipeline_rest_envr_info e
left external join table2name imt on e.tckt_id = imt.tckt_id
) aa
ORDER BY
aa.seq

Answer 2

SELECT TOP 1 aa.*
FROM (
select (case when condition1 then result1
when condition2 then result2
...
else
default_result
end ) as result,... from tablename
) aa
ORDER BY
aa.seq

Answer 3

避免在 WHERE 中执行 CASE。而是在 SELECT 中应用它。此外，您还没有提到代表别名 IMT 的表

SELECT TOP 1 EndDate
FROM (
select
(
case when stat = 'INPR' then 1
when stat = 'CMPL' then 2
when stat = 'PLND' then 3
else 4 end
) as seq, EndDate
from pipeline_rest_envr_info e
内连接 不管表名是什么.tckt_id = imt.tckt_id
) aa
ORDER BY aa.seq