Analysis of join usage in MySQL

The example database is as follows:
student table:

mysql> select * from student; +-----------+-----------+------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | +-----------+-----------+------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | | 201215122 | 刘晨 | 女 | 19 | CS | | 201215123 | 王敏 | 女 | 18 | MA | | 201215125 | 张立 | 男 | 19 | IS | | 201215128 | 陈冬 | 男 | 18 | IS | | 201215126 | 张成民 | 男 | 18 | CS | +-----------+-----------+------+------+-------+6 rows in set (0.00 sec)

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sc Table:

mysql> select * from sc; +-----------+------+-------+ | Sno | Cno | Grade | +-----------+------+-------+ | 201215121 | 1 | 92 | | 201215121 | 2 | 85 | | 201215121 | 3 | 88 | | 201215122 | 2 | 90 | | 201215122 | 3 | 80 | | 201215128 | 1 | 78 | +-----------+------+-------+6 rows in set (0.00 sec)

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LEFT JOIN (left join)

Analysis of join usage in MySQL
The sql statement is as follows:

select * from student left join sc on student.Sno=sc.Sno;

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The running results are as follows:

+-----------+-----------+------+------+-------+-----------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade | +-----------+-----------+------+------+-------+-----------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 | | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | NULL | +-----------+-----------+------+------+-------+-----------+------+-------+

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in In this example, the left join is based on the records in the student table. The student table can be regarded as the left table, and the sc table can be regarded as the right table. The records in the left table will be fully displayed, plus the matching right table. If If there is no match on the left side, the remaining parts will be displayed as null.

USING clause

The using clause and the on clause are similar, but the results are slightly different.
For example:

mysql> select student.Sno,Sname,Grade from student left join sc on student.Sno=sc.Sno; +-----------+-----------+-------+ | Sno | Sname | Grade | +-----------+-----------+-------+ | 201215121 | 李勇 | 92 | | 201215121 | 李勇 | 85 | | 201215121 | 李勇 | 88 | | 201215122 | 刘晨 | 90 | | 201215122 | 刘晨 | 80 | | 201215128 | 陈冬 | 78 | | 201215123 | 王敏 | NULL | | 201215125 | 张立 | NULL | | 201215126 | 张成民 | NULL | +-----------+-----------+-------+ 9 rows in set (0.00 sec)

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The above is equivalent to

select Sno,Sname,Grade from student left join sc using(Sno); +-----------+-----------+-------+ | Sno | Sname | Grade | +-----------+-----------+-------+ | 201215121 | 李勇 | 92 | | 201215121 | 李勇 | 85 | | 201215121 | 李勇 | 88 | | 201215122 | 刘晨 | 90 | | 201215122 | 刘晨 | 80 | | 201215128 | 陈冬 | 78 | | 201215123 | 王敏 | NULL | | 201215125 | 张立 | NULL | | 201215126 | 张成民 | NULL | +-----------+-----------+-------+

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in different places, for example:

select * from student left join sc on student.Sno=sc.Sno; +-----------+-----------+------+------+-------+-----------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade | +-----------+-----------+------+------+-------+-----------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 | | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | NULL | +-----------+-----------+------+------+-------+-----------+------+-------+

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select * from student left join sc using (sno); +-----------+-----------+------+------+-------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Cno | Grade | +-----------+-----------+------+------+-------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 1 | 78 | | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | +-----------+-----------+------+------+-------+------+-------+

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The repeated Sno column will be output twice if the on word is used.

RIGHT JOIN (right join)

Same as LEFT JOIN, but based on the right table, for example:

select * from student right join sc using (sno); +-----------+------+-------+--------+------+------+-------+ | Sno | Cno | Grade | Sname | Ssex | Sage | Sdept | +-----------+------+-------+--------+------+------+-------+ | 201215121 | 1 | 92 | 李勇 | 男 | 22 | CS | | 201215121 | 2 | 85 | 李勇 | 男 | 22 | CS | | 201215121 | 3 | 88 | 李勇 | 男 | 22 | CS | | 201215122 | 2 | 90 | 刘晨 | 女 | 19 | CS | | 201215122 | 3 | 80 | 刘晨 | 女 | 19 | CS | | 201215128 | 1 | 78 | 陈冬 | 男 | 18 | IS | +-----------+------+-------+--------+------+------+-------+

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INNER JOIN (equal join or inner join)

Analysis of join usage in MySQL

It will not show who it is based on, only the records that meet the conditions will be displayed

select * from student inner join sc on student.Sno=sc.Sno; +-----------+--------+------+------+-------+-----------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade | +-----------+--------+------+------+-------+-----------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 | +-----------+--------+------+------+-------+-----------+------+-------+

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The above statement is equivalent to:

select * from student,sc where student.Sno=sc.Sno; +-----------+--------+------+------+-------+-----------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade | +-----------+--------+------+------+-------+-----------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 | +-----------+--------+------+------+-------+-----------+------+-------+

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Extension

If you only want to retrieve some records from table A, but do not include table B

Analysis of join usage in MySQL

You can add a where statement after the left join

select * from student left join sc using(Sno) where sc.Sno is null; +-----------+-----------+------+------+-------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Cno | Grade | +-----------+-----------+------+------+-------+------+-------+ | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | +-----------+-----------+------+------+-------+------+-------+

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Find the difference set

Analysis of join usage in MySQL

#You can combine union words. Since in this example, all the ones on the right are corresponding, so the displayed result is consistent with the previous one.

select * from student left join sc using(Sno) where student.Sno is null union select * from student left join sc using(Sno) where sc.Sno is null; +-----------+-----------+------+------+-------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Cno | Grade | +-----------+-----------+------+------+-------+------+-------+ | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | +-----------+-----------+------+------+-------+------+-------+

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FULL JOIN

Analysis of join usage in MySQL

select * from student left join sc on student.Sno=sc.Sno union select * from student right join sc on student.Sno=sc.Sno; +-----------+-----------+------+------+-------+-----------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade | +-----------+-----------+------+------+-------+-----------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 | | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | NULL | +-----------+-----------+------+------+-------+-----------+------+-------+

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Note: A left join B is equivalent to B right join A

mysql> select * from student left join sc using(Sno); +-----------+-----------+------+------+-------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Cno | Grade | +-----------+-----------+------+------+-------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 1 | 78 | | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | +-----------+-----------+------+------+-------+------+-------+ 9 rows in set (0.00 sec)mysql> select * from sc right join student using(Sno); +-----------+-----------+------+------+-------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Cno | Grade | +-----------+-----------+------+------+-------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 1 | 78 | | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | +-----------+-----------+------+------+-------+------+-------+

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Join usage analysis in MySQL

The instance database is as follows:
student table:

mysql> select * from student; +-----------+-----------+------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | +-----------+-----------+------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | | 201215122 | 刘晨 | 女 | 19 | CS | | 201215123 | 王敏 | 女 | 18 | MA | | 201215125 | 张立 | 男 | 19 | IS | | 201215128 | 陈冬 | 男 | 18 | IS | | 201215126 | 张成民 | 男 | 18 | CS | +-----------+-----------+------+------+-------+ 6 rows in set (0.00 sec)

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sc table:

mysql> select * from sc; +-----------+------+-------+ | Sno | Cno | Grade | +-----------+------+-------+ | 201215121 | 1 | 92 | | 201215121 | 2 | 85 | | 201215121 | 3 | 88 | | 201215122 | 2 | 90 | | 201215122 | 3 | 80 | | 201215128 | 1 | 78 | +-----------+------+-------+ 6 rows in set (0.00 sec)

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LEFT JOIN (left join)

Analysis of join usage in MySQL ##sql The statement is as follows:

select * from student left join sc on student.Sno=sc.Sno;

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The running result is as follows:

+-----------+-----------+------+------+-------+-----------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade | +-----------+-----------+------+------+-------+-----------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 | | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | NULL | +-----------+-----------+------+------+-------+-----------+------+-------+

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In this example, the left join is based on the records in the student table. The student table can be regarded as the left table, and the sc table can be regarded as In the right table, the records in the left table will be fully displayed, plus the matched right table. If the left table is not matched, the remaining parts will be displayed as null.

USING clause

The using clause and the on clause are similar, but the results are slightly different.

For example:

mysql> select student.Sno,Sname,Grade from student left join sc on student.Sno=sc.Sno; +-----------+-----------+-------+| Sno | Sname | Grade | +-----------+-----------+-------+| 201215121 | 李勇 | 92 | | 201215121 | 李勇 | 85 | | 201215121 | 李勇 | 88 | | 201215122 | 刘晨 | 90 | | 201215122 | 刘晨 | 80 | | 201215128 | 陈冬 | 78 | | 201215123 | 王敏 | NULL | | 201215125 | 张立 | NULL || 201215126 | 张成民 | NULL | +-----------+-----------+-------+9 rows in set (0.00 sec)

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The above is equivalent to

select Sno,Sname,Grade from student left join sc using(Sno); +-----------+-----------+-------+| Sno | Sname | Grade | +-----------+-----------+-------+| 201215121 | 李勇 | 92 | | 201215121 | 李勇 | 85 | | 201215121 | 李勇 | 88 | | 201215122 | 刘晨 | 90 | | 201215122 | 刘晨 | 80 | | 201215128 | 陈冬 | 78 | | 201215123 | 王敏 | NULL | | 201215125 | 张立 | NULL || 201215126 | 张成民 | NULL | +-----------+-----------+-------+

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in different places, for example:

select * from student left join sc on student.Sno=sc.Sno; +-----------+-----------+------+------+-------+-----------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade | +-----------+-----------+------+------+-------+-----------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 | | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | NULL | +-----------+-----------+------+------+-------+-----------+------+-------+

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select * from student left join sc using (sno); +-----------+-----------+------+------+-------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Cno | Grade | +-----------+-----------+------+------+-------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 1 | 78 | | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | +-----------+-----------+------+------+-------+------+-------+

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The repeated Sno column will be output twice if the on word is used.

RIGHT JOIN (right join)

Same as LEFT JOIN, but based on the right table, for example:

select * from student right join sc using (sno); +-----------+------+-------+--------+------+------+-------+ | Sno | Cno | Grade | Sname | Ssex | Sage | Sdept | +-----------+------+-------+--------+------+------+-------+ | 201215121 | 1 | 92 | 李勇 | 男 | 22 | CS | | 201215121 | 2 | 85 | 李勇 | 男 | 22 | CS | | 201215121 | 3 | 88 | 李勇 | 男 | 22 | CS | | 201215122 | 2 | 90 | 刘晨 | 女 | 19 | CS | | 201215122 | 3 | 80 | 刘晨 | 女 | 19 | CS | | 201215128 | 1 | 78 | 陈冬 | 男 | 18 | IS | +-----------+------+-------+--------+------+------+-------+

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INNER JOIN (equal join or inner join)

Analysis of join usage in MySQL

It will not show who it is based on, only the records that meet the conditions will be displayed

select * from student inner join sc on student.Sno=sc.Sno; +-----------+--------+------+------+-------+-----------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade | +-----------+--------+------+------+-------+-----------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 | +-----------+--------+------+------+-------+-----------+------+-------+

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The above statement is equivalent to:

select * from student,sc where student.Sno=sc.Sno; +-----------+--------+------+------+-------+-----------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade | +-----------+--------+------+------+-------+-----------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 | +-----------+--------+------+------+-------+-----------+------+-------+

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Extension

If you only want to retrieve some records from table A, but do not include table B

Analysis of join usage in MySQL

You can add a where statement after the left join

select * from student left join sc using(Sno) where sc.Sno is null; +-----------+-----------+------+------+-------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Cno | Grade | +-----------+-----------+------+------+-------+------+-------+ | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | +-----------+-----------+------+------+-------+------+-------+

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Find the difference set

Analysis of join usage in MySQL

#You can combine union words. Since in this example, all the ones on the right are corresponding, so the displayed result is consistent with the previous one.

select * from student left join sc using(Sno) where student.Sno is null union select * from student left join sc using(Sno) where sc.Sno is null; +-----------+-----------+------+------+-------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Cno | Grade | +-----------+-----------+------+------+-------+------+-------+ | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | +-----------+-----------+------+------+-------+------+-------+

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FULL JOIN

Analysis of join usage in MySQL

select * from student left join sc on student.Sno=sc.Sno union select * from student right join sc on student.Sno=sc.Sno; +-----------+-----------+------+------+-------+-----------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Sno | Cno | Grade | +-----------+-----------+------+------+-------+-----------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 201215121 | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 201215128 | 1 | 78 | | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | NULL | +-----------+-----------+------+------+-------+-----------+------+-------+

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Note: A left join B is equivalent to B right join A

mysql> select * from student left join sc using(Sno); +-----------+-----------+------+------+-------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Cno | Grade | +-----------+-----------+------+------+-------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 1 | 78 | | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | +-----------+-----------+------+------+-------+------+-------+ 9 rows in set (0.00 sec)mysql> select * from sc right join student using(Sno); +-----------+-----------+------+------+-------+------+-------+ | Sno | Sname | Ssex | Sage | Sdept | Cno | Grade | +-----------+-----------+------+------+-------+------+-------+ | 201215121 | 李勇 | 男 | 22 | CS | 1 | 92 | | 201215121 | 李勇 | 男 | 22 | CS | 2 | 85 | | 201215121 | 李勇 | 男 | 22 | CS | 3 | 88 | | 201215122 | 刘晨 | 女 | 19 | CS | 2 | 90 | | 201215122 | 刘晨 | 女 | 19 | CS | 3 | 80 | | 201215128 | 陈冬 | 男 | 18 | IS | 1 | 78 | | 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | | 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | | 201215126 | 张成民 | 男 | 18 | CS | NULL | NULL | +-----------+-----------+------+------+-------+------+-------+

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