DQL data Query Language--connection query

--Internal connection two kinds of equivalent connection
Select R.*,b.bummc from T_hq_ryxx R, t_hq_bm b where R.BUMBM = B.BUMBM

Select R.*,b.bummc from T_hq_ryxx r Inner joins T_HQ_BM B on r.bumbm = B.BUMBM

--No equivalent connection
Select R.*,b.bummc from T_hq_ryxx R, t_hq_bm b where R.BUMBM > B.BUMBM

Select R.*,b.bummc from T_hq_ryxx r Inner joins T_HQ_BM B on R.BUMBM < B.BUMBM

--Self-connected
SELECT * from T_hq_ryxx A, t_hq_ryxx b where A.BUMBM = B.BUMBM

--Outer connection left connection to ensure the integrity of the left table information
Select R.*,b.bummc from T_hq_ryxx R left joins T_HQ_BM B on r.bumbm = B.BUMBM
--Right connection ensures right table information integrity
Select R.*,b.bummc from T_hq_ryxx R right joins T_HQ_BM B on r.bumbm = B.BUMBM

--Full connection guarantees integrity of table information on both sides
Select R.*,b.bummc from T_hq_ryxx R full join T_HQ_BM b on R.BUMBM = B.BUMBM

DQL data Query Language--connection query