MySql数据库的学习（基础篇三）

概述

上篇，我们简单的学习了一下，数据表的约束操作，今天，我们接着上篇的内容来学习。

学习

修改数据表

我们有时候进行数据表操作时，会去增删约束、增删字段等，不可能每次都去删表重建，这里就有了语句对应这些操作。

添加单列

语法 ALTER TABLE tbl_name ADD [COLUMNS] col_name colimn_definition [FIRST | AFTER col_name]
如：我们用户表的字段为：

mysql> show columns from userinfor1;
+----------+------------------+------+-----+---------+----------------+
| Field    | Type             | Null | Key | Default | Extra          |
+----------+------------------+------+-----+---------+----------------+
| _id      | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(20)      | NO   |     | NULL    |                |
| pid      | int(10) unsigned | YES  | MUL | NULL    |                |
+----------+------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

我们如果给它加上一个工资salary属性要怎么处理呢？
这里就可以这么做：

mysql> alter table userinfor1 add salary float(8,2) unsigned not null after user
name;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show columns from userinfor1;
+----------+---------------------+------+-----+---------+----------------+
| Field    | Type                | Null | Key | Default | Extra          |
+----------+---------------------+------+-----+---------+----------------+
| _id      | int(10) unsigned    | NO   | PRI | NULL    | auto_increment |
| username | varchar(20)         | NO   |     | NULL    |                |
| salary   | float(8,2) unsigned | NO   |     | NULL    |                |
| pid      | int(10) unsigned    | YES  | MUL | NULL    |                |
+----------+---------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

添加多列

语法 ALTER TABLE tbl_name ADD [COLUMNS] (col_name column_definition,…)

我们再给用户表加上两个字段sex和phone

mysql> alter table userinfor1 add (sex varchar(5) not null,phone varchar(11) not
 null);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show columns from userinfor1;
+----------+---------------------+------+-----+---------+----------------+
| Field    | Type                | Null | Key | Default | Extra          |
+----------+---------------------+------+-----+---------+----------------+
| _id      | int(10) unsigned    | NO   | PRI | NULL    | auto_increment |
| username | varchar(20)         | NO   |     | NULL    |                |
| salary   | float(8,2) unsigned | NO   |     | NULL    |                |
| pid      | int(10) unsigned    | YES  | MUL | NULL    |                |
| sex      | varchar(5)          | NO   |     | NULL    |                |
| phone    | varchar(11)         | NO   |     | NULL    |                |
+----------+---------------------+------+-----+---------+----------------+
6 rows in set (0.00 sec)

删除列

语法 ALTER TABLE tbl_name DROP [COLUMN] col_name (多列用’,'隔开)

如这里我们将添加的列全删掉

mysql> alter table userinfor1 drop salary,drop sex;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table userinfor1 drop phone;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

添加/删除主键约束

语法 ALTER TABLE tbl_name ADD [CONSTRAINT [symbol] PRIMARY KEY [index_type]] (index_col_name,…)

mysql> show tables;
+--------------------+
| Tables_in_myschool |
+--------------------+
| provinces          |
| students           |
| tab1               |
| tab2               |
| tab3               |
| tab4               |
| tab5               |
| userinfor1         |
+--------------------+
8 rows in set (0.00 sec)

如这里为表tab1添加主键

mysql> show columns from tab1;
+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20)         | YES  |     | NULL    |       |
| age      | tinyint(3) unsigned | YES  |     | NULL    |       |
| salary   | float(8,2) unsigned | YES  |     | NULL    |       |
+----------+---------------------+------+-----+---------+-------+
3 rows in set (0.03 sec)

mysql> alter table tab1 add _id int unsigned;
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table tab1 add primary key (_id);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show columns from tab1;
+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20)         | YES  |     | NULL    |       |
| age      | tinyint(3) unsigned | YES  |     | NULL    |       |
| salary   | float(8,2) unsigned | YES  |     | NULL    |       |
| _id      | int(10) unsigned    | NO   | PRI | 0       |       |
+----------+---------------------+------+-----+---------+-------+
4 rows in set (0.01 sec)

删除主键

语法 ALTER TABLE tbl_name DROP PRIMARY KEY

mysql> show columns from tab1;
+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20)         | YES  |     | NULL    |       |
| age      | tinyint(3) unsigned | YES  |     | NULL    |       |
| salary   | float(8,2) unsigned | YES  |     | NULL    |       |
| _id      | int(10) unsigned    | NO   | PRI | 0       |       |
+----------+---------------------+------+-----+---------+-------+
4 rows in set (0.01 sec)

mysql> alter table tab1 drop primary key;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show columns from tab1;
+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20)         | YES  |     | NULL    |       |
| age      | tinyint(3) unsigned | YES  |     | NULL    |       |
| salary   | float(8,2) unsigned | YES  |     | NULL    |       |
| _id      | int(10) unsigned    | NO   |     | 0       |       |
+----------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

添加/删除唯一约束

语法 ALTER TABLE tbl_name ADD [CONSTRAINT [symbol]] UNIQUE [INDEX | KEY] [index_type] (index_col_name,…)

同样，我们对tab1进行操作

mysql> alter table tab1 add unique (username);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show columns from tab1;
+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20)         | YES  | UNI | NULL    |       |
| age      | tinyint(3) unsigned | YES  |     | NULL    |       |
| salary   | float(8,2) unsigned | YES  |     | NULL    |       |
| _id      | int(10) unsigned    | NO   | PRI | 0       |       |
+----------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

删除唯一约束

语法 ALTER TABLE tbl_name DROP {INDEX | KEY} index_name

mysql> alter table tab1 drop index username;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show columns from tab1;
+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20)         | YES  |     | NULL    |       |
| age      | tinyint(3) unsigned | YES  |     | NULL    |       |
| salary   | float(8,2) unsigned | YES  |     | NULL    |       |
| _id      | int(10) unsigned    | NO   | PRI | 0       |       |
+----------+---------------------+------+-----+---------+-------+
4 rows in set (0.02 sec)

添加/删除外键约束

语法 ALTER TABLE tbl_name ADD FOREIGN KEY (col_name) REFERENCES (father_col_name)

mysql> alter table tab2 add id int unsigned;
Query OK, 0 rows affected (0.13 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show columns from tab2;
+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20)         | NO   |     | NULL    |       |
| age      | tinyint(3) unsigned | NO   |     | NULL    |       |
| salary   | float(8,2) unsigned | NO   |     | NULL    |       |
| id       | int(10) unsigned    | YES  |     | NULL    |       |
+----------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table tab2 add foreign key(id) references tab1(_id) on delete casca
de;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show columns from tab2;
+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20)         | NO   |     | NULL    |       |
| age      | tinyint(3) unsigned | NO   |     | NULL    |       |
| salary   | float(8,2) unsigned | NO   |     | NULL    |       |
| id       | int(10) unsigned    | YES  | MUL | NULL    |       |
+----------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

删除外键约束

语法 ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol
这里的fk_symbol我们可以从表结构得到

mysql> show create table tab2\G;
*************************** 1. row ***************************
       Table: tab2
Create Table: CREATE TABLE `tab2` (
  `username` varchar(20) NOT NULL,
  `age` tinyint(3) unsigned NOT NULL,
  `salary` float(8,2) unsigned NOT NULL,
  `id` int(10) unsigned DEFAULT NULL,
  KEY `id` (`id`),
  CONSTRAINT `tab2_ibfk_1` FOREIGN KEY (`id`) REFERENCES `tab1` (`_id`) ON DELET
E CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

ERROR:
No query specified

其中的 ‘tab2_ibfk_1’ 就是。

mysql> alter table tab2 drop foreign key tab2_ibfk_1;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show create table tab2\G;
*************************** 1. row ***************************
       Table: tab2
Create Table: CREATE TABLE `tab2` (
  `username` varchar(20) NOT NULL,
  `age` tinyint(3) unsigned NOT NULL,
  `salary` float(8,2) unsigned NOT NULL,
  `id` int(10) unsigned DEFAULT NULL,
  KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

ERROR:
No query specified

添加/删除默认约束

语法 ALTER TABLE tbl_name ALTER col_name SET DEFAULT number;

mysql> show columns from tab1;
+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20)         | YES  |     | NULL    |       |
| age      | tinyint(3) unsigned | YES  |     | NULL    |       |
| salary   | float(8,2) unsigned | YES  |     | NULL    |       |
| _id      | int(10) unsigned    | NO   | PRI | 0       |       |
+----------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table tab1 alter age set default 20;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show columns from tab1;
+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20)         | YES  |     | NULL    |       |
| age      | tinyint(3) unsigned | YES  |     | 20      |       |
| salary   | float(8,2) unsigned | YES  |     | NULL    |       |
| _id      | int(10) unsigned    | NO   | PRI | 0       |       |
+----------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

删除默认约束
语法 ALTER TABLE tbl_name ALTER col_name DROP DEFAULT;

mysql> alter table tab1 alter age drop default;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show columns from tab1;
+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20)         | YES  |     | NULL    |       |
| age      | tinyint(3) unsigned | YES  |     | NULL    |       |
| salary   | float(8,2) unsigned | YES  |     | NULL    |       |
| _id      | int(10) unsigned    | NO   | PRI | 0       |       |
+----------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

基础三到此，转到基础四

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