Mysql联合优先级

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
)

销售 1 表的品牌为:A 且金额:50

销售 3 表的品牌为:A 且金额:100

我要取金额50，忽略100。我想问一下，使用union有没有优先级？

Answer 1

如果您想要第一次出现，则应使用union all 而不是union。出于性能原因，这很重要，因为 union 会消除不必要的重复。

然后对每个子查询使用 not contains 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)
)