SQL关联查询

普通关联查询

SELECT A1,B2 FROM A ,B WHERE A1=B1

子查询关联

SELECT A1,(SELECT B2 FROM B WHERE A1=B1) AS A2 ,A3, (SELECT B2 FROM B WHERE A3=B1)AS A4 FROM A

左连接left join

select A1,B.B2,A3,TB.B2
from A
left join B on A.A1=B.B1
left join B TB on A.A3=TB.B1