如何刪除此SQL子句重複？

Question

我有兩個查詢，在同一張表上做相同的條件選擇。你能幫我解決這個重複問題嗎？

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

請檢查U是否意味着這樣的：

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

OR

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

Answer 2

EXISTS可以作爲只是另WHERE條件：

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

基本上它選擇來自<table>X僅當存在只有排出線。
或（可能更快）：

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是主密鑰或<table>。
反向邏輯：只選擇合格行，如果沒有其他行合格。

Answer 3

如果你的表沒有空，你可以使用此查詢 'X' 領域：

SET @x = null 

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