mysql common commands

修改表名称

 mysql> ALTER TABLE test_foreing_key RENAME test_foreign_key;

修改字段排列位置

ALTER TABLE tableName【表名】 MODIFY column1【字段名 1】 varchar(80)【数据类型】 FIRST|AFTER bookName【字段名2】;

• FIRST; 将【字段名1】修改为表的第一个字段
  AFTER bookName【字段名2】; 将【字段名1】更新到【字段名2】的后面

ALTER TABLE book MODIFY owner varchar(80) AFTER bookName;

删除表中的外键约束

ALTER TABLE 【表名】DOPR FOREIGN KEY 【外键约束】

// 准备测试表
mysql> CREATE TABLE test1 (id INT(11) PRIMARY KEY, name VARCHAR(10));
mysql> CREATE TABLE test_foreing_key(
    -> id INT(11) PRIMARY KEY,
    -> foreKeyTestId INT(11),
    -> CONSTRAINT fk_test_id FOREIGN KEY (foreKeyTestId) REFERENCES test1(id));REFERENCES test1(id));
// 删除外键约束
mysql> ALTER TABLE test_foreign_key DROP FOREIGN KEY fk_test_id;

Drop a table without constraints

	When dropping a table if no table does not exist,MySQL 会报错误信息 “ERROR 1051(42S02):Unknown table”, 参数“IF EXISTS ”
It is used to determine whether the deleted table exists before deleting it again,加上参数后,在删除表时,If it does not exist, it can be executed smoothly,But there will be a warning【warning】

DROP TABLE 【IF EXISTS】table1, table2, table3, ..., tableN;

删除被其他表关联的主表

	数据表之间存在外键关联的情况下,如果直接删除父表,会失败,The reason is that dropping the parent table directly breaks the referential integrity of the table.
如果必须要删除,可以先删除与它关联的子表,再删除父表,只是这样同时删除了两个表中的数据.但有的情况下
Subtables may be preserved,In this case, delete the parent table separately,It is only necessary to cancel the foreign key constraint of the associated table,然后就可以删除父表.

// 查看所有表
mysql> SHOW TABLES;
+------------------+
| Tables_in_reader |
+------------------+
| test1            |
| test2            |
| test_foreign_key |
+------------------+
// Directly delete the parent table prompts an error
mysql> DROP TABLE test1;
ERROR 3730 (HY000): Cannot drop table 'test1' referenced by a foreign key constraint 'fk_test_id' on table 'test_foreing_key'.
// 删除外键约束
mysql> ALTER TABLE test_foreign_key DROP FOREIGN KEY fk_test_id;
// 继续删除
mysql> DROP TABLE test1;
Query OK, 0 rows affected (0.16 sec)
// 查看所有表
mysql> SHOW TABLES;
+------------------+
| Tables_in_reader |
+------------------+
| test2            |
| test_foreign_key |
+------------------+