sql-server - 如何删除此 SQL 子句重复？

Question

我有 2 个查询在同一张表上以相同的条件执行 SELECT。你能帮我删除这个重复吗？

if exists(select count(1)    
              from (<table>) t   
              where  (<condition>)   
              having count(1) = 1)   
    set @yes = 1    

if @yes = 1    
    select @x = X   
        from (<table>) t   
        where  (<condition>)

Answer 1

您的代码相当于：

select @yes = 1,@x=max(X) from (<table>) t   
       where  (<condition>)   
       having count(1) = 1

Answer 2

EXISTS可以作为另一个WHERE条件：

SELECT @x = X
FROM   <table> t
WHERE  <condition>
AND    EXISTS (
    SELECT 1
    FROM   <table> t1
    WHERE  <condition>
    HAVING count(*) = 1
    );

基本上它仅在X只有row qualifying时才会选择。 或者（可能更快）：<table>

SELECT @x = X
FROM  <table> t
WHERE <condition>
AND NOT EXISTS (
    SELECT 1
    FROM   <table> t1
    WHERE  <condition>
    AND    t1.id <> t.id     -- no other row satisfies same conditions
    );

id是主键或<table>.
反向逻辑：仅选择符合条件的行，如果没有其他行符合条件。

Answer 3

请检查你是否是这个意思：

    select 
      @x = X   
    from 
      (<table>) t   
    where  
      (<condition>)
    group by X
    having count(*) = 1

或者

select 
    @x = X  
from(
    select 
        count(*) over() cnt, X    
    from 
        (<table>) t  
    where  
        (<condition>)
)a
WHERE a.cnt=1

Answer 4

如果表中的“X”字段不为空，则可以使用此查询：

SET @x = null

SELECT @x = X   
        FROM (<table>) t   
        WHERE (<condition>)
IF (@x IS NOT NULL) BEGIN SET @yes = 1 END