mysql 表关联时执行顺序

left join  左连接

with temp1 as (
select 1 as id ,12 as num union 
select 2 as id ,13 as num union 
select 3 as id ,12 as num union 
select 4 as id ,14 as num union 
select 5 as id ,12 as num union 
select 6 as id ,16 as num  
), temp2 as (
select 1 as id ,'a' as var union 
select 2 as id ,'b' as var union 
select 3 as id ,'c' as var union 
select 4 as id ,'a' as var union 
select 5 as id ,'b' as var union 
select 6 as id ,'f' as var  
)
select  * from temp1 t1 left join temp2 t2 on t1.id =t2.id

left join  左连接 （on 中加条件）

说明：先关联， 再过滤 ；

with temp1 as (
select 1 as id ,12 as num union 
select 2 as id ,13 as num union 
select 3 as id ,12 as num union 
select 4 as id ,14 as num union 
select 5 as id ,12 as num union 
select 6 as id ,16 as num  
), temp2 as (
select 1 as id ,'a' as var union 
select 2 as id ,'b' as var union 
select 3 as id ,'c' as var union 
select 4 as id ,'a' as var union 
select 5 as id ,'b' as var union 
select 6 as id ,'f' as var  
)
-- select  * from temp1 t1 left join temp2 t2 on t1.id =t2.id
select  * from temp1 t1 left join temp2 t2 on t1.id =t2.id and t1.num=12 
-- select  * from temp1 t1 left join temp2 t2 on t1.id =t2.id where t1.num=12

left join  左连接  （末尾 where 条件）

说明：最后过滤

with temp1 as (
select 1 as id ,12 as num union 
select 2 as id ,13 as num union 
select 3 as id ,12 as num union 
select 4 as id ,14 as num union 
select 5 as id ,12 as num union 
select 6 as id ,16 as num  
), temp2 as (
select 1 as id ,'a' as var union 
select 2 as id ,'b' as var union 
select 3 as id ,'c' as var union 
select 4 as id ,'a' as var union 
select 5 as id ,'b' as var union 
select 6 as id ,'f' as var  
)
-- select  * from temp1 t1 left join temp2 t2 on t1.id =t2.id
-- select  * from temp1 t1 left join temp2 t2 on t1.id =t2.id and t1.num=12 
select  * from temp1 t1 left join temp2 t2 on t1.id =t2.id where t1.num=12

left join  左连接  （ on 中加条件 & 末尾 where 条件）

with temp1 as (
select 1 as id ,12 as num union 
select 2 as id ,13 as num union 
select 3 as id ,12 as num union 
select 4 as id ,14 as num union 
select 5 as id ,12 as num union 
select 6 as id ,16 as num  
), temp2 as (
select 1 as id ,'a' as var union 
select 2 as id ,'b' as var union 
select 3 as id ,'c' as var union 
select 4 as id ,'a' as var union 
select 5 as id ,'b' as var union 
select 6 as id ,'f' as var  
)
select  * from temp1 t1 left join temp2 t2 on t1.id =t2.id and t1.num=12  where  t1.num=12