为什么Crossjoin效率低于联盟

Question

我有以下快速运行的查询：

SELECT CUSTOMER FROM (
SELECT CUSTOMER FROM
EMAILS INNER JOIN CUSTOMERS 
ON EMAILS.email=CUSTOMERS.email1
UNION
SELECT CUSTOMER FROM
EMAILS INNER JOIN CUSTOMERS 
ON EMAILS.email=CUSTOMERS.email2)

但是交叉加入正在取决于：

SELECT DISTINCT CUSTOMER FROM
EMAILS INNER JOIN CUSTOMERS 
ON EMAILS.email=CUSTOMERS.email1
OR EMAILS.email=CUSTOMERS.email2
-- alternative: EMAILS.email in (CUSTOMERS.email1,CUSTOMERS.email2)

Answer 1

关于OR与Union的讨论已经有很多，你可以在这里查看

• UNION ALL vs OR condition in sql server query
• SQL Performance UNION vs OR
• http://sqlserverplanet.com/optimization/using-union-instead-of-or

由于您要将一列与两个不同的列进行比较，您也可以尝试使用

SELECT DISTINCT CUSTOMER 
FROM EMAILS 
INNER JOIN CUSTOMERS 
ON EMAILS.email in (CUSTOMERS.email1,CUSTOMERS.email2)

此外，您在第一次查询时不需要DISTINCT Union会删除重复项