WITH

WITH子句定义查询子句的命名关系，可以展平嵌套查询或简化子查询语句。

例如下面的查询语句是等价的：

SELECT name, maxprice FROM (SELECT name, MAX(price) AS maxprice FROM fruit GROUP BY name) AS x;

WITH x AS (SELECT name, MAX(price) AS maxprice FROM fruit GROUP BY name) SELECT name, maxprice FROM x;

• 多个子查询

  with 
  t1 as(select name,max(price) as maxprice from fruit group by name),
  t2 as(select name,avg(price) as avgprice from fruit group by name)
  select t1.*,t2.* from t1 join t2 on t1.name = t2.name;

• WITH的链式形式

  WITH
  x AS (SELECT a FROM t),
  y AS (SELECT a AS b FROM x),
  z AS (SELECT b AS c FROM y)
  SELECT c FROM z;