Connection query of SQL Server database

I learned it today SQL Server Connection query for , Let's record .

What is connection query ？

Query multiple tables at the same time , Query with internal connection , External connection query , Cross query . External connection query is divided into left external connection query 、 Right outer connection query and all outer connection query , However, a join query can also query a table called a self join query .

INNER JOIN： Indicates internal connection ,INNER You can omit and write only JOIN

ON： Give the connection conditions

（JOIN ... ON ...） Equate to （,... where...）

Prepare first 2 A table to query Table01,Table02：

                  Table01

                                                         Table02

that , First come Let's talk about the inner join query of multiple tables

select Table01.SID ,SName,CID,CContact,CPhone

from Table01 JOIN Table02

ON Table01.SID=Table02.SID

In the multi table join query, if there are the same fields in the joined tables , The table prefix must be added before the reference （ Inquire about Table01 Medium SID Just write Table01.SID）, If the query field is unique in multiple tables , The table name prefix can be omitted .

Query results ：

next Let's talk about the self join query for querying a table , In fact, it can be regarded as an alternative inner connection .

A self join query must specify two aliases for the table , In fact, it is to copy one's own table , Make it two tables .

select S1.SID,S1.SName,S1.SDID,s1.SPostID

from Table01 AS S1 JOIN Table01 AS S2

ON S1.SDID!=S2.SDID and S1.SPostID=S2.SPostID

Table Define an alias S1 and S2

Query results ：

Let's talk about external link query , There are left join queries （LEFT OUTER JOIN）, Right connection query （RIGHT OUTER JOIN） And full connection query （FULL OUTER JOIN）. OUTER It can be omitted

Prepare first 2 A watch ,Table01,Table02：

                           Table01                                                         Table02

        

1、 Left outer join query is also called left join query （LEFT OUTER JOIN）

Concept ： Returns all rows in the left table , If the row in the left table does not match the row in the right table , The corresponding behavior in the right table is null .

select * from  Table01 left join Table02

on Table01.classId = Table02.classId

Query results ：

2、 Right outer join query is also called right join query （RIGHT OUTER JOIN）

  Concept ： The opposite of the left link , Returns all the rows in the right table , If the row in the right table does not match the row in the left table , The corresponding behavior in the left table is null .

select * from  Table01 right join Table02

on Table01.classId = Table02.classId

Query results ：

3、 Full external connection query is also called full connection query （FULL OUTER JOIN）

Concept ： Returns all rows in the left and right tables . When a row has no matching row in another table , The other table is null （ First, check the right table according to the left table , If we can't find out , The behavior of the right table is null , After checking, the rest of the right table will be displayed , All rows in the left table are null ）

select * from  Table01 full join Table02

on Table01.classId = Table02.classId

Query results ：

The last one is cross query （CROSS JOIN） Also called The cartesian product , The query table uses two tables with external links Table01,Table02

Concept ： No WHERE Conditional clause , It will return the... Of the two connected tables The cartesian product , That is, each row of the left table is combined with each row of the right table , The number of rows returned is equal to the product of the number of rows in two tables （ for example ：Table01t and Table02, return 4*4=16 Bar record ）, If belt where, Returns or displays the number of matching rows .

1、 No WHERE：

select * from  Table01 cross join Table02 Or you could write it as select * from  Table01,Table02

Query results ：

2、 belt WHERE Clause , First the 2 Table product combination , Then query according to the conditions

notes ： Cross band conditional query must use WHERE, Out of commission ON

select * from  Table01 CROSS join Table02

WHERE Table01.classId = Table02.classId

Query results ：

Finally, I have finished writing , That's all for connection query , After reading it, do you think it's very simple ？