mysql中的自连接用于什么

这个自联接是什么？为什么我们需要这个自联接？。到目前为止，我从未使用过自联接。

原因有很多，web上提供了大量示例

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原因有很多，网上有很多例子

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查看这些链接是否有助于您

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祝你好运

看看这些链接是否有助于您

mysql> SELECT * FROM pr WHERE id>80;
+----+------+--------+
| id | ids  | status |
+----+------+--------+
| 81 |    4 |      4 |
| 82 |    2 |      3 |
| 83 |    2 |      4 |
+----+------+--------+
3 rows in set (0.00 sec)

mysql> SELECT * FROM pr WHERE id<18;
+----+------+--------+
| id | ids  | status |
+----+------+--------+
|  1 |    1 |      1 |
|  5 | NULL |      2 |
+----+------+--------+
2 rows in set (0.01 sec)

祝你好运

mysql>从pr中选择*id>80；
mysql> SELECT * FROM pr WHERE id>80;
+----+------+--------+
| id | ids  | status |
+----+------+--------+
| 81 |    4 |      4 |
| 82 |    2 |      3 |
| 83 |    2 |      4 |
+----+------+--------+
3 rows in set (0.00 sec)

mysql> SELECT * FROM pr WHERE id<18;
+----+------+--------+
| id | ids  | status |
+----+------+--------+
|  1 |    1 |      1 |
|  5 | NULL |      2 |
+----+------+--------+
2 rows in set (0.01 sec)

+----+------+--------+
|id | id |状态|
+----+------+--------+
| 81 |    4 |      4 |
| 82 |    2 |      3 |
| 83 |    2 |      4 |
+----+------+--------+
一组3行（0.00秒）
mysql>SELECT*FROM pr其中id SELECT*FROM pr为t1，pr为t2其中t1.id80；
+----+------+--------+----+------+--------+
|id | ids |状态| id | ids |状态|
+----+------+--------+----+------+--------+
|  1 |    1 |      1 | 81 |    4 |      4 |
|5 |空| 2 | 81 | 4 | 4|
|  1 |    1 |      1 | 82 |    2 |      3 |
|5 |空| 2 | 82 | 2 | 3|
|  1 |    1 |      1 | 83 |    2 |      4 |
|5 |空| 2 | 83 | 2 | 4|
+----+------+--------+----+------+--------+
一组6行（0.00秒）
mysql>在t1.id80上选择*从pr作为t1连接pr作为t2；
+----+------+--------+----+------+--------+
|id | ids |状态| id | ids |状态|
+----+------+--------+----+------+--------+
|  1 |    1 |      1 | 81 |    4 |      4 |
|5 |空| 2 | 81 | 4 | 4|
|  1 |    1 |      1 | 82 |    2 |      3 |
|5 |空| 2 | 82 | 2 | 3|
|  1 |    1 |      1 | 83 |    2 |      4 |
|5 |空| 2 | 83 | 2 | 4|
+----+------+--------+----+------+--------+
一组6行（0.00秒）

mysql>从pr中选择*id>80；
+----+------+--------+
|id | id |状态|
+----+------+--------+
| 81 |    4 |      4 |
| 82 |    2 |      3 |
| 83 |    2 |      4 |
+----+------+--------+
一组3行（0.00秒）
mysql>SELECT*FROM pr其中id SELECT*FROM pr为t1，pr为t2其中t1.id80；
+----+------+--------+----+------+--------+
|id | ids |状态| id | ids |状态|
+----+------+--------+----+------+--------+
|  1 |    1 |      1 | 81 |    4 |      4 |
|5 |空| 2 | 81 | 4 | 4|
|  1 |    1 |      1 | 82 |    2 |      3 |
|5 |空| 2 | 82 | 2 | 3|
|  1 |    1 |      1 | 83 |    2 |      4 |
|5 |空| 2 | 83 | 2 | 4|
+----+------+--------+----+------+--------+
一组6行（0.00秒）
mysql>在t1.id80上选择*从pr作为t1连接pr作为t2；
+----+------+--------+----+------+--------+
|id | ids |状态| id | ids |状态|
+----+------+--------+----+------+--------+
|  1 |    1 |      1 | 81 |    4 |      4 |
|5 |空| 2 | 81 | 4 | 4|
|  1 |    1 |      1 | 82 |    2 |      3 |
|5 |空| 2 | 82 | 2 | 3|
|  1 |    1 |      1 | 83 |    2 |      4 |
|5 |空| 2 | 83 | 2 | 4|
+----+------+--------+----+------+--------+
一组6行（0.00秒）

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