Syntax of SQL union query (inline, left, right, and full)

SQL The joint query （ inline 、 league of the left-wing writers 、 Right couplet 、 All in one ） The grammar of

summary ：

The efficiency of joint query is higher , Give an example to illustrate the joint query ： inline inner join 、 league of the left-wing writers left outer join 、 Right couplet right outer join 、 All in one full outer join Benefits and usage of .

The efficiency of joint query is higher , The following example illustrates a federated query ( inline 、 league of the left-wing writers 、 Right couplet 、 All in one ) The benefits of ：

First of all ： inline (inner join)

　　 If you want to put user information 、 integral 、 The grades are listed , So it's usually written like this ：select * from T1 ,T3 where T1.userid = T3.userid( In fact, this result is equivalent to select * from T1 inner join T3 on T1.userid=T3.userid ).

　　 Put both tables in userid Put your lines together ( That's inline ), But the latter will be much more efficient than the former , The latter is recommended ( inline ) Writing .

　　SQL sentence ：select * from T1 inner join T2 on T1.userid=T2.userid

second ： league of the left-wing writers (left outer join)

　　 Show left table T1 All rows in , And put the right watch T2 In the left table T1 in ; Right table T2 Does not meet the conditions , You don't have to add it to the result table , also NULL Express .

　　SQL sentence ：select * from T1 left outer join T2 on T1.userid=T2.userid

Third ： Right couplet (right outer join)

　　 Show right table T2 All rows in , And put the left Watch T1 In the right table T2 in ; The left table T1 Does not meet the conditions , You don't have to add it to the result table , also NULL Express .

　　SQL sentence ：select * from T1 right outer join T2 on T1.userid=T2.userid

Fourth ： All in one (full outer join)

　　 Show left table T1、 Right table T2 All lines in both sides , That is to say, the results of the leftist League + The right join result table is combined , Then filter out the duplicates .

　　SQL sentence ：select * from T1 full outer join T2 on T1.userid=T2.userid

summary

About Union query , The efficiency is really high ,4 If you can flexibly use the combination methods , Basically, the complex sentence structure will also be simplified . this 4 One way is ：1)Inner join 2)left outer join 3)right outer join 4)full outer join

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