MySQL数据表的基本操作札记

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数据库是一个可以存放数据库对象的容器，数据库对象包括：表、视图、存储过程、函数、触发器、事件。其中，表是数据库最基本的元素，是其他数据库对象的前提条件。

表中的一列称为一个字段，一行称为一条记录。

1. 数据表的创建、查看数据表、查看数据表结构

MySQL> CREATE DATABASE test1;
Query OK, 1 row affected (0.02 sec)

mysql> USE test1;
Database changed

mysql> CREATE TABLE table1(-> id SMALLINT UNSIGNED,
    -> username VARCHAR(20),
    -> age TINYINT
    -> );
Query OK, 0 rows affected (0.40 sec)

mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
| test1              |
+--------------------+
5 rows in set (0.00 sec)

mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| test1      |
+------------+
1 row in set (0.00 sec)

mysql> SHOW TABLES;
+-----------------+
| Tables_in_test1 |
+-----------------+
| table1          |
+-----------------+
1 row in set (0.00 sec)

mysql> SHOW COLUMNS FROM table1;
+----------+----------------------+------+-----+---------+-------+
| Field    | Type                 | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+-------+
| id       | smallint(5) unsigned | YES  |     | NULL    |       |
| username | varchar(20)          | YES  |     | NULL    |       |
| age      | tinyint(4)           | YES  |     | NULL    |       |
+----------+----------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> DESCRIBE table1;
+----------+----------------------+------+-----+---------+-------+
| Field    | Type                 | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+-------+
| id       | smallint(5) unsigned | YES  |     | NULL    |       |
| username | varchar(20)          | YES  |     | NULL    |       |
| age      | tinyint(4)           | YES  |     | NULL    |       |
+----------+----------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

2. 数据表的删除 DROP TABLE table_name;

mysql> USE test1;
Database changed

mysql> SHOW TABLES;
+-----------------+
| Tables_in_test1 |
+-----------------+
| table1          |
| tb2             |
+-----------------+
2 rows in set (0.00 sec)

mysql> DROP TABLE tb2;
Query OK, 0 rows affected (0.20 sec)

mysql> DESCRIBE tb2;
ERROR 1146 (42S02): Table 'test1.tb2' doesn't exist

3. 插入记录、查看记录 INSERT table_name [(col_name, …)] VALUES(…);

mysql> SHOW COLUMNS FROM table1;
+----------+----------------------+------+-----+---------+-------+
| Field    | Type                 | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+-------+
| id       | smallint(5) unsigned | YES  |     | NULL    |       |
| username | varchar(20)          | YES  |     | NULL    |       |
| age      | tinyint(4)           | YES  |     | NULL    |       |
+----------+----------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> INSERT table1 VALUES(5,'Tom',22);
Query OK, 1 row affected (0.05 sec)

mysql> INSERT table1 (id,username) VALUES(5,'Tom');
Query OK, 1 row affected (0.10 sec)

mysql> SELECT * FROM table1;
+------+----------+------+
| id   | username | age  |
+------+----------+------+
|    5 | Tom      |   22 |
|    5 | Tom      | NULL |
+------+----------+------+
2 rows in set (0.00 sec)

4. 字段的空值与非空 NULL,NOT NULL

mysql> CREATE TABLE table2(-> username VARCHAR(20) NOT NULL,
    -> #NULL 可加可不加，不加默认可以为空#
    -> age TINYINT
    -> );
Query OK, 0 rows affected (0.25 sec)

mysql> SHOW COLUMNS FROM table2;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| username | varchar(20) | NO   |     | NULL    |       |
| age      | tinyint(4)  | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> INSERT table2 VALUES(NULL,25);
ERROR 1048 (23000): Column 'username' cannot be null

5. 主键约束与自动编号 PRIMARY KEY,AUTO_INCREMENT

※ 一张数据表只能存在一个主键

※ 主键能保证 记录的唯一性

※ 主键自动为 NOT NULL

※ AUTO_INCREMENT 只能配合 PRIMARY KEY 使用，不能单独使用。PRIMARY KEY 可以单独使用。

mysql> CREATE TABLE table3(-> id SMALLINT PRIMARY KEY AUTO_INCREMENT,
    -> username VARCHAR(20)
    -> );
Query OK, 0 rows affected (0.32 sec)

mysql> DESCRIBE table3;
+----------+-------------+------+-----+---------+----------------+
| Field    | Type        | Null | Key | Default | Extra          |
+----------+-------------+------+-----+---------+----------------+
| id       | smallint(6) | NO   | PRI | NULL    | auto_increment |
| username | varchar(20) | YES  |     | NULL    |                |
+----------+-------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

mysql> INSERT table3 (username) VALUES('XingyaZhao');
Query OK, 1 row affected (0.05 sec)

mysql> INSERT table3 (username) VALUES('XuebiBaby');
Query OK, 1 row affected (0.06 sec)

mysql> INSERT table3 VALUES(4,'David');
Query OK, 1 row affected (0.07 sec)

mysql> INSERT table3 (username) VALUES('Somebody');
Query OK, 1 row affected (0.09 sec)

mysql> SELECT * FROM table3;
+----+------------+
| id | username   |
+----+------------+
|  1 | XingyaZhao |
|  2 | XuebiBaby  |
|  4 | David      |
|  5 | Somebody   |
+----+------------+
4 rows in set (0.00 sec)

mysql> CREATE TABLE table4(-> id SMALLINT UNSIGNED PRIMARY KEY,
    -> username VARCHAR(20)
    -> );
Query OK, 0 rows affected (0.30 sec)

mysql> SHOW COLUMNS FROM table4;
+----------+----------------------+------+-----+---------+-------+
| Field    | Type                 | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+-------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    |       |
| username | varchar(20)          | YES  |     | NULL    |       |
+----------+----------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> INSERT table4 VALUES(6,'ChuanDao');
Query OK, 1 row affected (0.07 sec)

mysql> INSERT table4 VALUES(3,'Pigiu');
Query OK, 1 row affected (0.04 sec)

mysql> INSERT table4 VALUES(3,'York');
ERROR 1062 (23000): Duplicate entry '3' for key 'PRIMARY'
mysql> SELECT * FROM table4;
+----+----------+
| id | username |
+----+----------+
|  3 | Pigiu    |
|  6 | ChuanDao |
+----+----------+
2 rows in set (0.00 sec)

6. 唯一约束 UNIQUE KEY

※ 唯一约束可以保证 某个字段中每个记录的唯一性

※ 唯一约束的字段可以为空值

※ 每张数据表可以存在多个唯一约束

mysql> CREATE TABLE table5(-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    -> username VARCHAR(20) NOT NULL UNIQUE KEY,
    -> age TINYINT UNSIGNED NOT NULL
    -> );
Query OK, 0 rows affected (0.60 sec)

mysql> SHOW COLUMNS FROM table5;
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(20)          | NO   | UNI | NULL    |                |
| age      | tinyint(3) unsigned  | NO   |     | NULL    |                |
+----------+----------------------+------+-----+---------+----------------+
3 rows in set (0.02 sec)

mysql> INSERT table5 (username,age) VALUES('XingyaZhao',22);
Query OK, 1 row affected (0.05 sec)

mysql> INSERT table5 (username,age) VALUES('XuebiBaby',21);
Query OK, 1 row affected (0.07 sec)

mysql> INSERT table5 (username,age) VALUES('XingyaZhao',18);
ERROR 1062 (23000): Duplicate entry 'XingyaZhao' for key 'username'

mysql> SELECT * FROM table5;
+----+------------+-----+
| id | username   | age |
+----+------------+-----+
|  1 | XingyaZhao |  22 |
|  2 | XuebiBaby  |  21 |
+----+------------+-----+
3 rows in set (0.00 sec)

7. 默认约束 DEFAULT

mysql> CREATE TABLE table6(-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    -> username VARCHAR(20) NOT NULL UNIQUE KEY,
    -> age TINYINT UNSIGNED DEFAULT 20
    -> );
Query OK, 0 rows affected (0.29 sec)

mysql> DESCRIBE table6;
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(20)          | NO   | UNI | NULL    |                |
| age      | tinyint(3) unsigned  | YES  |     | 20      |                |
+----------+----------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> INSERT table6 (username) VALUES('Xingya');
Query OK, 1 row affected (0.08 sec)

mysql> INSERT table6 (username) VALUES('Jude');
Query OK, 1 row affected (0.05 sec)

mysql> INSERT table6 (username,age) VALUES('Tom',21);
Query OK, 1 row affected (0.08 sec)

mysql> SELECT * FROM table6;
+----+----------+------+
| id | username | age  |
+----+----------+------+
|  1 | Xingya   |   20 |
|  2 | Jude     |   20 |
|  3 | Tom      |   21 |
+----+----------+------+
3 rows in set (0.00 sec)