Full join 和 Cross join

full join 一定需要 on 条件的匹配，一条匹配全部输出

cross join 是笛卡尔积，没有匹配都能输出

例子：

 如何用一个值 替代null？

首先cross join,每一个值都拥有了一行

with
t1 as (select avg(scores) avg_score from testscore2 )
select * from testscore2 cross join t1

 再筛选，

注意遇到null时，这么写

case when scores is NULL

with
t1 as (select avg(scores) avg_score from testscore2 )
select id,subject,(case when scores is NULL then avg_score else scores end)scores 
from testscore2 cross join t1

得到：