4-5：MySQL 单表删除记录DELETE

删除记录（单标删除）的语法结构是：

DELETE FROM tbl_name [WHERE where_condition]

比如删除id为6的用户记录：

mysql> DELETE FROM users WHERE id = 6;
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM users;
+----+----------+----------------------------------+-----+------+
| id | username | password                         | age | sex  |
+----+----------+----------------------------------+-----+------+
|  1 | Tom      | 123                              |  29 |    0 |
|  2 | John     | 123                              |  38 |    0 |
|  3 | Tom      | 123                              |  27 |    0 |
|  4 | John     | 123                              |  35 |    0 |
|  5 | John     | 123                              |  10 |    0 |
|  7 | Rose     | 202cb962ac59075b964b07152d234b70 |   8 |    0 |
|  8 | BEn      | 456                              |  17 |    0 |
+----+----------+----------------------------------+-----+------+
7 rows in set (0.00 sec)

可以看到id为6的用户已经被删除，如果此时再插入一条记录，id会为9，而不是6：

mysql> INSERT users VALUEs(NULL,'111','222',33,NULL);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM users;
+----+----------+----------------------------------+-----+------+
| id | username | password                         | age | sex  |
+----+----------+----------------------------------+-----+------+
|  1 | Tom      | 123                              |  29 |    0 |
|  2 | John     | 123                              |  38 |    0 |
|  3 | Tom      | 123                              |  27 |    0 |
|  4 | John     | 123                              |  35 |    0 |
|  5 | John     | 123                              |  10 |    0 |
|  7 | Rose     | 202cb962ac59075b964b07152d234b70 |   8 |    0 |
|  8 | BEn      | 456                              |  17 |    0 |
|  9 | 111      | 222                              |  33 | NULL |
+----+----------+----------------------------------+-----+------+
8 rows in set (0.00 sec)