Left join returns records that include all the records in the left table and the equivalent of the junction fields in the right table
Right join returns records that include all records in the right table and the junction fields in the left table
INNER JOIN (equivalent join) returns only rows that have the same join field in two tables
Examples are as follows:
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Table A records the following:
AID Anum
1 a20050111
2 a20050112
3 a20050113
4 a20050114
5 a20050115
bid bname
1 2006032401
2 2006032402
3 2006032403
Span style= "Font-family:microsoft Yahei; font-size:16px; " >4 2006032404
8 2006032408
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sql statement is as follows:  
Select * from A
Left join B  
on a.aid = b.bid
aid anum BID bname
1 a20050111 1 2006032401
2 a20050112 2 200603240 2
3 a20050113 3 2006032403
4 a20050114 4 2006032404
5 a2005011 5 NULL NULL
left join is based on the records of Table A, a can be seen as the left table, B can be considered as the right table, and the left join is based on 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).
b table records are null.
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2.right join
sql statement is as follows:  
Select * from A
Right join B 
on a.aid = b.bid
aid anum BID bname
1 a20050111 1 2006032401
2 a20050112 2 200603240 2
3 a20050113 3 2006032403
4 a20050114 4 2006032404
null NULL 8 2006032408
look closely and you'll see that the result of the left join is exactly the opposite, This time is based on the right table (B), where a is not sufficient to fill with null.
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3.inner join
sql statement is as follows:  
Select * from A
Innerjoin B  
on a.aid = b.bid
aid anum BID bname
1 a20050111 1 2006032401
2 a20050112 2 200603240 2
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.
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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 an outer join of the side. The left outer join will contain all of the records from the first (left) two tables, even if there are no records of the corresponding values in the second (right) table.
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 field being joined. And 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 are joining a field in a inner JOIN operation that contains data of Memo data type or OLE Object data type, an error occurs.
The difference between the left join of SQL, right join, INNER join