MySQL 约束与索引

2019-03-27  本文已影响0人  Manchangdx

一、主键约束

1.1 CONSTRAINT PRIMARY KEY 创建双主键数据表

如果将一个字段设为主键，可以在字段定义时在后面添加 PRIMARY KEY
所谓双主键，其实是两个字段共同成为一个主键，每张表只能有一个主键

mysql> CREATE TABLE test1 (
    -> field1 INT,
    -> field2 INT,
    -> CONSTRAINT PRIMARY KEY (field1, field2)
    -> );
Query OK, 0 rows affected (0.04 sec)

mysql> DESC test1;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| field1 | int(11) | NO   | PRI | NULL    |       |
| field2 | int(11) | NO   | PRI | NULL    |       |
+--------+---------+------+-----+---------+-------+
2 rows in set (0.01 sec)
1.2 ALTER TABLE 数据表名 DROP PRIMARY KEY 删除主键
mysql> ALTER TABLE test1 DROP PRIMARY KEY;
Query OK, 1 row affected (0.07 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> DESC test1;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| field1 | int(11) | NO   |     | NULL    |       |
| field2 | int(11) | NO   |     | NULL    |       |
+--------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)
1.3 创建数据表后，ALTER TABLE 数据表名 ADD PRIMARY KEY 补增主键
mysql> ALTER TABLE test1 ADD PRIMARY KEY (field1, field2);
Query OK, 0 rows affected (0.24 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> DESC test1;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| field1 | int(11) | NO   | PRI | NULL    |       |
| field2 | int(11) | NO   | PRI | NULL    |       |
+--------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

二、唯一约束

2.1 CONSTRAINT UNIQUE 给字段增加唯一约束

注意，CONSTRAINT UNIQUE 后面括号内如果写两个列名
表示这两个列合在一起的值是唯一的，类似双主键的唯一性
同主键一样，对单列设置唯一约束，在定义列那一行后面加 UNIQUE 即可

mysql> CREATE TABLE test2 (
    -> field1 INT PRIMARY KEY,
    -> field2 INT,
    -> field3 INT,
    -> CONSTRAINT UNIQUE (field2),
    -> CONSTRAINT UNIQUE (field3)
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql> DESC test2;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| field1 | int(11) | NO   | PRI | NULL    |       |
| field2 | int(11) | YES  | UNI | NULL    |       |
| field3 | int(11) | YES  | UNI | NULL    |       |
+--------+---------+------+-----+---------+-------+
3 rows in set (0.01 sec)
2.2 删除唯一约束

首先，执行 SHOW CREATE TABLE 数据表名 查看约束名：

mysql> show create table test2\G
*************************** 1. row ***************************
       Table: test2
Create Table: CREATE TABLE `test2` (
  `field1` int(11) NOT NULL,
  `field2` int(11) DEFAULT NULL,
  `field3` int(11) DEFAULT NULL,
  PRIMARY KEY (`field1`),   # 主键约束
  UNIQUE KEY `field2` (`field2`),   # 唯一约束
  UNIQUE KEY `field3` (`field3`)    # 唯一约束
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

执行 ALTER TABLE 数据表名 DROP KEY 唯一约束名 删除唯一约束：

mysql> ALTER TABLE test2 DROP KEY field2;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> DESC test2;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| field1 | int(11) | NO   | PRI | NULL    |       |
| field2 | int(11) | YES  |     | NULL    |       |
| field3 | int(11) | YES  | UNI | NULL    |       |
+--------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)
2.3 创建数据表后，ALTER TABLE 数据表名 ADD UNIQUE 字段名 添加唯一约束
mysql> ALTER TABLE test2 ADD UNIQUE (field2);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> DESC test2;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| field1 | int(11) | NO   | PRI | NULL    |       |
| field2 | int(11) | YES  | UNI | NULL    |       |
| field3 | int(11) | YES  | UNI | NULL    |       |
+--------+---------+------+-----+---------+-------+
3 rows in set (0.01 sec)

三、外键约束

3.1 FOREIGN KEY (外键字段) REFERENCES 关联表(主键) 设置外键约束
mysql> CREATE TABLE test3 (
    -> id INT PRIMARY KEY,
    -> create_at DATE,
    -> test2_id INT,
    -> FOREIGN KEY (test2_id) REFERENCES test2(field1)
    -> );
Query OK, 0 rows affected (0.04 sec)

mysql> DESC test3;
+-----------+---------+------+-----+---------+-------+
| Field     | Type    | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+-------+
| id        | int(11) | NO   | PRI | NULL    |       |
| create_at | date    | YES  |     | NULL    |       |
| test2_id  | int(11) | YES  | MUL | NULL    |       |
+-----------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)
3.2 DROP FOREIGN KEY 删除外键
mysql> SHOW CREATE TABLE test3\G
*************************** 1. row ***************************
       Table: test3
Create Table: CREATE TABLE `test3` (
  `id` int(11) NOT NULL,
  `create_at` date DEFAULT NULL,
  `test2_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `test2_id` (`test2_id`),
  CONSTRAINT `test3_ibfk_1` FOREIGN KEY (`test2_id`) 
  REFERENCES `test2` (`field1`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> ALTER TABLE test3 DROP FOREIGN KEY test3_ibfk_1;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0
3.3 创建数据表后，ADD CONSTRAINT FOREIGN KEY 补增外键
mysql> ALTER TABLE test3
    -> ADD CONSTRAINT FOREIGN KEY (test2_id) REFERENCES test2(field1)
    -> ON DELETE CASCADE;  # 如果 test2 中数据被删，外键关联的 test3 数据也被删
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

四、索引

索引分两种 BTREE 和 HASH ，前者为默认索引，又称「B+树」索引
执行 SHOW INDEX FROM 数据表名 可以查看索引详情

4.1 创建数据表时顺便设置索引
mysql> CREATE TABLE test4 (
    -> id INT PRIMARY KEY,
    -> name VARCHAR(64) UNIQUE,
    -> addr VARCHAR(64),
    -> INDEX (id),   # 设置索引
    -> INDEX USING HASH (name)   # 设置 HASH 类型的索引
    -> );
Query OK, 0 rows affected, 1 warning (0.04 sec)

mysql> SHOW CREATE TABLE test4\G
*************************** 1. row ***************************
       Table: test4
Create Table: CREATE TABLE `test4` (
  `id` int(11) NOT NULL,
  `name` varchar(64) DEFAULT NULL,
  `addr` varchar(64) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `name` (`name`),
  KEY `id` (`id`),   # 索引
  KEY `name_2` (`name`)   # 索引
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
4.2 删除索引方法有二

DROP INDEX 索引名 ON 数据表名;
ALTER TABLE 数据表名 DROP INDEX 索引名;

mysql> DROP INDEX id ON test4;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> ALTER TABLE test4 DROP INDEX name_2;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW CREATE TABLE test4\G
*************************** 1. row ***************************
       Table: test4
Create Table: CREATE TABLE `test4` (
  `id` int(11) NOT NULL,
  `name` varchar(64) DEFAULT NULL,
  `addr` varchar(64) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
4.3 对已有数据表创建索引

CREATE INDEX 索引名 USING 索引类型 ON 数据表名(字段名); 索引名可以省略

mysql> CREATE INDEX 主键_index ON test4(id);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> CREATE INDEX name_索引 USING HASH ON test4(name);
Query OK, 0 rows affected, 1 warning (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 1

mysql> SHOW CREATE TABLE test4\G
*************************** 1. row ***************************
       Table: test4
Create Table: CREATE TABLE `test4` (
  `id` int(11) NOT NULL,
  `name` varchar(64) DEFAULT NULL,
  `addr` varchar(64) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `name` (`name`),
  KEY `主键_index` (`id`),
  KEY `name_索引` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
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