A summary of the different points of the MySQL multiple table association left JOIN, right join, INNER join

definition:

Left join (left-hand join) returns a record that includes all records in the left table and the join fields in the right table
Right join (right-click join) Returns a record that includes all the records in the right table and the join fields in the left table
INNER join (equivalent connection) returns only rows of two tables that have a join field equal

For example:
—————————————— –
Table A record is as follows:
AID anum
1 a20050111
2 a20050112
3 a20050113
4 a20050114
5 A2 0050115

Table B is recorded as follows:
BID bname
1 2006032401
2 2006032402
3 2006032403
4 200603 2404
8 2006032408

—————————————— –

1.left join

SQL statements are as follows:

The code is as follows	Copy Code
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)

Results show:

The left join is based on the records of a table, and a can be viewed as the left-hand table, B can be viewed as the right table, and left table.
In other words, the records in 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).
b tables where the records are insufficient are null.

——————————————–

2.right Join

The SQL statement is as follows:

The code is as follows	Copy Code
SELECT * FROM A Right 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
NULL NULL 8 2006032408

(The number of rows affected is 5 rows)
Results show:
With a closer look, you will find that the result of the left join is just the opposite, this time based on the right table (B), where a table is deficient with null padding.

——————————————–

3.inner Join

The SQL statement is as follows:

code is as follows	copy code
SELECT * From A Innerjoin 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

Result Description:

Obviously, here Only a record of A.aid = B.bid is displayed. This shows that the inner join is not based on who, it only shows the records that match the criteria.

—————————————— –
Note:
The LEFT JOIN operation is used to combine records from the source table in any from clause. Use the left JOIN operation to create a left-hand outer join. The left outer join will contain all records from the first (left) two tables, even if there are no records in the second (right) table that match the values.

Syntax: from table1 left JOIN table2 on table1.field1 compopr table2.field2
Description: table1, the table2 parameter is used to specify the name of the table to combine records with.
Field1, the Field2 parameter specifies the name of the joined field. These fields must have the same data type and contain the same type of data, but they do not need to have the same name. The
COMPOPR parameter specifies the relational comparison operator: "=", "<", ">", "<=", ">=", or "<>".
If you want to join a field that contains Memo data type or OLE Object data type data in a inner JOIN operation, an error occurs.