Question

MySQL是否支持公用表表达式？例如在Oracle中有WITH子句？：

WITH aliasname
AS
  ( SELECT COUNT(*) FROM table_name )
SELECT COUNT(*) FROM dept,aliasname

Answer 1

一种方法是使用子查询：

  SELECT COUNT(*) 
  FROM dept,
  ( 
      SELECT COUNT(*) 
      FROM table_name
  ) AS aliasname

请注意，两个表之间的,将与您发布的查询中的两个表交叉连接。如果它们之间存在任何关系，您可以JOIN代替它们。

Answer 2

SELECT t.name,
       t.num
  FROM TABLE t
  JOIN (SELECT c.id,COUNT(*) 'num1'
          FROM TABLE1 c
         WHERE c.column = 'a'
      GROUP BY c.id) ta1 ON ta1.id = t.id
 JOIN (SELECT d.id,COUNT(*) 'num2'
          FROM TABLE2 d
         WHERE d.column = 'a'
      GROUP BY d.id) ta2 ON ta2.id = t.id

Answer 3

不，MySQL不支持公用表表达式（CTE）。因此，不必使用WITH tablealias as (....)，而是必须执行子查询。

例如，

WITH totalcount AS 
(select userid, count(*) as tot from logins group by userid)
SELECT a.firstname, a.lastname, b.tot
FROM users a
INNER JOIN 
totalcount b 
    on a.userid = b.userid

可以在MySQL中重写为

SELECT a.firstname, a.lastname, b.totalcount
FROM users a
INNER JOIN 
(select userid, count(*) as tot from logins group by userid) b 
    on a.userid = b.userid

MySQL中的WITH子句？

3 个答案: