mysql学习02

清空数据库的内容

drop table tb1; #直接删除表

delete from tb1; #清空表的内容
truncate table tb1;清空标的内容，速度快 自增数字会从零开始重新计数

mysql> desc student;
+----------+-------------+------+-----+---------+----------------+
| Field    | Type        | Null | Key | Default | Extra          |
+----------+-------------+------+-----+---------+----------------+
| sid      | int(11)     | NO   | PRI | NULL    | auto_increment |
| sname    | varchar(25) | YES  |     | NULL    |                |
| gender   | varchar(2)  | YES  |     | NULL    |                |
| class_id | int(11)     | YES  |     | NULL    |                |
+----------+-------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)

mysql> desc class;
+---------+-------------+------+-----+---------+----------------+
| Field   | Type        | Null | Key | Default | Extra          |
+---------+-------------+------+-----+---------+----------------+
| cid     | int(11)     | NO   | PRI | NULL    | auto_increment |
| caption | varchar(25) | YES  |     | NULL    |                |
+---------+-------------+------+-----+---------+----------------+
2 rows in set (0.01 sec)

mysql> alter table student add constraint fk_stu_cls foreign key student(class_id) references class(cid);
Query OK, 0 rows affected (0.38 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql>

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https://www.cnblogs.com/wupeiqi/articles/5713315.html

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