A good memory is better than a bad pen
There are three kinds of SQL connection: Inner connection, outer connection, cross connection.
Inner connection includes: equivalent connection, non-equivalent connection, natural connection
Outer connection includes: Left join (left outer connection), right connection (right outer connection)
Concrete theory, see my blog post http://blog.csdn.net/jdfkldjlkjdl/article/details/41485127
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The following is an example of the introduction, I believe this more intuitive.
Two tables of existing A and B
Table A records the following:
AID Anum
1 a20050111
2 a20050112
3 a20050113
4 a20050114
5 a20050115
Table B records the following:
BID bname
1 2006032401
2 2006032402
3 2006032403
4 2006032404
8 2006032408
1.left join (left join)
SELECT * from a left JOIN B on A.aid =b.bid
The results are as follows:
AID Anum BID bname
1 a20050111 1 2006032401
2 a20050112 2 2006032402
3 a20050113 3 2006032403
4 a20050114 4 2006032404
5 a20050115 NULL NULL
(The number of rows affected is 5 rows)
Result Description:
The left join is based on the records of Table A, a can be regarded as the right table, and B can be regarded as left table.
In other words, the records of the left table (A) will all be represented, and the right table (B) will only display records that match the search criteria (in the example: A.aid = b.bid). The low-record of table B is null.
2.right join (right join)
SELECT * from a right joing b on a.aid = B.bid
The results are as follows:
AID Anum BID bname
1 a20050111 1 2006032401
2 a20050112 2 2006032402
3 a20050113 3 2006032403
4 a20050114 4 2006032404
NULL NULL 8 2006032408
(The number of rows affected is 5 rows)
Result Description:
Looking closely, you will find that the result of the left join is exactly the opposite, this time it is based on the right table (B), where a table is not enough to fill with null.
3.inner Join (equal join or inner join)
SELECT * from a INNER JOIN B on A.aid =b.bid
Equivalent to the following SQL sentence:
SELECT * from a b WHERE a.aid = B.bid
The results are as follows:
AID Anum BID bname
1 a20050111 1 2006032401
2 a20050112 2 2006032402
3 a20050113 3 2006032403
4 a20050114 4 2006032404
Result Description:
Obviously, only a.aid = B.bid records are shown here. This shows that inner join is not based on who, it only shows records that match the criteria.
MySQL left JOIN right join INNER JOIN
