WITH

The WITH clause defines the naming relationship of query clauses, which can flatten nested queries or simplify subquery statements. For example, the following query statements are equivalent:

SELECT name, maxprice FROM (SELECT mame, 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, price FROM x;

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
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;