mysql的数据类型主要分为数值类型、日期/时间类型、字符串类型三种
数值类型可以指定为无符号(unsigned),表示不取负数。 1字节(bytes)= 8bit。 对于整型类型的范围:
- 有符号范围:-2^(类型字节数*8-1)到2^(类型字节数*8-1)-1,如int是4字节,就是-2^31到2^31-1
- 无符号范围:0到2^(类型字节数*8)-1,如int就是2^32-1 尽量不使用unsigned,对于int类型可能存放不下的数据,int unsigned同样可能存放不下,与其如此,还不如设计时,将int类型提升为bigint类型。
语法
SHOW DATABASES;
示例
MariaDB [(none)]> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
+--------------------+
3 rows in set (0.017 sec)
语法
CREATE DATABASE [IF NOT EXISTS] [数据库名];
//数据库名只能是数字、字母、下划线,IF NOT EXISTS为可选选项,如果数据库存在则不再创建
示例
MariaDB [(none)]> CREATE DATABASE test;
Query OK, 1 row affected (0.001 sec)
MariaDB [(none)]> SHOW DATABASES;
+--------------------+
| Database |
+--------------------+
| information_schema |
| lee |
| mysql |
| performance_schema |
| test |
+--------------------+
5 rows in set (0.000 sec)
语法
USE [数据库名];
示例
MariaDB [(none)]> USE test;
Database changed
语法
DROP [IF EXISTS] DATABASE [数据库名];
示例
MariaDB [test]> DROP DATABASE test;
Query OK, 0 rows affected (0.032 sec)
MariaDB [(none)]> SHOW DATABASES;
+--------------------+
| Database |
+--------------------+
| information_schema |
| lee |
| mysql |
| performance_schema |
+--------------------+
4 rows in set (0.000 sec)
语法
CREATE TABLE [IF NOT EXISTS] [表名] (
[数据名] [数据类型],
[数据名] [数据类型],
[数据名] [数据类型]
);
示例
MariaDB [lee]> CREATE TABLE stu(
-> id int,
-> age int,
-> name varchar(8),
-> birth date,
-> math decimal,
-> english decimal
-> );
Query OK, 0 rows affected (0.086 sec)
语法
SHOW TABLES;
示例
MariaDB [lee]> SHOW TABLES;
+---------------+
| Tables_in_lee |
+---------------+
| stu |
+---------------+
1 row in set (0.001 sec)
语法
DESC [数据库名];
示例
MariaDB [lee]> DESC stu;
+---------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+---------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
| name | varchar(8) | YES | | NULL | |
| birth | date | YES | | NULL | |
| math | decimal(10,0) | YES | | NULL | |
| english | decimal(10,0) | YES | | NULL | |
+---------+---------------+------+-----+---------+-------+
6 rows in set (0.021 sec)
语法
DROP [IF EXISTS] TABLE [数据库名];
示例
MariaDB [lee]> DROP TABLE stu;
Query OK, 0 rows affected (0.006 sec)
MariaDB [lee]> SHOW TABLES;
Empty set (0.001 sec)