Question

我有3个表，我的查询是：

SELECT BRAND, AMOUNT FROM
(
SELECT BRAND, AMOUNT FROM SALES1 
UNION
SELECT BRAND, AMOUNT FROM SALES2
UNION
SELECT BRAND, AMOUNT FROM SALES3
)

SALES 1表有品牌： A 和AMOUNT： 50

SALES 3表有品牌： A AND AMOUNT： 100

我想获得金额50而忽略100.我想问一下使用工会时是否有优先权？

Answer 1

您需要"where not exists"内的"not in"（或"left join"或union}：

(SELECT BRAND, AMOUNT FROM SALES1 )
UNION 
(SELECT BRAND, AMOUNT FROM SALES2) 
UNION 
(SELECT BRAND, AMOUNT FROM SALES3 where brand not in (SELECT BRAND FROM SALES1))

Answer 2

如果您想要第一次出现，则应使用union all而不是union。这对于性能原因很重要，因为union会进行不必要的重复消除。

然后对每个子查询使用not exists子句：

(SELECT BRAND, AMOUNT
 FROM SALES1 
) UNION ALL
(SELECT BRAND, AMOUNT
 FROM SALES2 s2
 WHERE NOT EXISTS (SELECT 1 FROM SALES1 s1 WHERE s1.BRAND = s2.BRAND)
) UNION ALL
(SELECT BRAND, AMOUNT
 FROM SALES3 s3
 WHERE NOT EXISTS (SELECT 1 FROM SALES1 s1 WHERE s1.BRAND = s3.BRAND) AND
       NOT EXISTS (SELECT 1 FROM SALES2 s2 WHERE s2.BRAND = s3.BRAND)
)

Answer 3

SELECT  BRAND, AMOUNT
    FROM  
      ( SELECT  BRAND, AMOUNT, 1 AS priority
            FROM  SALES1
            UNION  ALL 
        SELECT  BRAND, AMOUNT, 2 AS priority
            FROM  SALES2
            UNION  ALL 
        SELECT  BRAND, AMOUNT, 3 AS priority
            FROM  SALES3 
      )
    ORDER BY  priority
    LIMIT  1;

Mysql union优先级

3 个答案: