Full join 和 Cross join

 

full join 一定需要 on 條件的匹配，一條匹配全部輸出

cross join 是笛卡爾積，沒有匹配都能輸出

例子：

 

 如何用一個值 替代null？

首先cross join,每一個值都擁有了一行

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

 

 再篩選，

注意遇到null時，這么寫

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：