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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)本文永久更新链接地址：http://www.linuxidc.com/Linux/2016-11/137207.htm
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