sql - 在 WHERE 条件下加倍

Question

@Age是否有可能在 where 语句参数中做类似这里发生的事情？

 @AGE Varchar(8)

 SELECT NAME FROM TABLE WHERE 
 (
 If @AGE='man'
  then (AGE = 'man' or AGE = 'boy')
  else (AGE = 'man')
 )
 AND City IS NULL

Answer 1

是的，像这样：

SELECT NAME FROM TABLE WHERE 
((@AGE='man'
   AND (AGE = 'man' or AGE = 'boy'))
OR
(@AGE <> 'man'
  AND (AGE = 'man')))
AND City IS NULL

请注意，@AGE <> 'man'可能需要根据 @AGE 变量是否可以为空来调整条件。

我在这里回答了一个非常相似的问题。

这是 WHERE 子句中非常常见的技术。如果您想在 WHERE 子句中应用一些“IF”逻辑，您需要做的就是将带有布尔 AND 的额外条件添加到需要应用它的部分。

Answer 2

一种方法是使用EXEC并构建字符串命令If's。

像这样的东西：

declare @t nvarchar(max)
@AGE Varchar(8)
set @t=' SELECT NAME FROM TABLE WHERE '


 If @AGE='man' set @t=@t+'(AGE = ''man'' or AGE = ''boy'')'
  else  set @t=@t+'(AGE = 'man')'

set @t=@t+' AND City IS NULL'

Answer 3

以合取范式（“一系列与”，较少依赖于参数）：

SELECT * 
  FROM TABLE
 WHERE ( @AGE <> 'man' OR (AGE IN ('man', 'boy') ) )
       AND ( @AGE = 'man' OR AGE = 'man' )
       AND CITY IS NULL;

这里的想法是使用隐含重写规则：

( IF x THEN y )  is equivalent to  ( NOT ( x ) OR y )

---

下面是一些测试代码：

WITH T 
     AS
     (
      SELECT * 
        FROM (
              VALUES (1, 'man', NULL), 
                     (2, 'boy', NULL), 
                     (3, 'girl', NULL)
             ) AS T (NAME, AGE, CITY)
     ),
     Params
     AS
     (
      SELECT * 
        FROM (
              VALUES ('man'), 
                     ('boy'), 
                     ('girl')
             ) AS T (p_AGE)
     ), 
     Results
     AS
     (
      SELECT Params.*, T.*, 
             CASE WHEN     ( ( p_AGE <> 'man' OR (AGE IN ('man', 'boy') ) ) AND ( p_AGE = 'man' OR AGE = 'man' ) )  THEN 'TRUE'
                  WHEN NOT ( ( p_AGE <> 'man' OR (AGE IN ('man', 'boy') ) ) AND ( p_AGE = 'man' OR AGE = 'man' ) )  THEN 'FALSE'
                  ELSE 'UNKNOWN' 
             END AS result_onedaywhen, 
             CASE WHEN     ( AGE='man' OR (p_AGE='MAN' and AGE='boy') )  THEN 'TRUE'
                  WHEN NOT ( AGE='man' OR (p_AGE='MAN' and AGE='boy') )  THEN 'FALSE'
                  ELSE 'UNKNOWN' 
             END AS result_Damien_The_Unbeliever
        FROM T, Params
     )
SELECT * 
  FROM Results;

Answer 4

或者你可以使用解码功能