day02_mysql：数据表创建

基于不同的操作目的，为提高效率，创建数据库时可使用不同的数据库存储引擎，最常见时事务型InnoDB

mysql 常见数据类型

字符串： char 固定长度， varchar(n) 可变长度
数值： int ， float(m, d) ， double(m, d) m为整数位数，d为小数位数
日期/时间： date 日期， time 时间， datetime 日期和时间

创建数据表时，五大完整性约束：

primary key 主键约束，唯一且不能为空

not null　　非空约束

unique　　唯一性约束

default <值>　　默认约束

foreign key　　外键约束　　foreign key(<字段>) references <表名>(<字段>)
check　　检查约束

create table <表名> (

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　　<字段1> <类型1> [约束1],

　　...

　　<字段n> <类型n> [约束n],

);

### 创建数据表

举例：

- 员工信息表：employees

1. 设计表结构、字段；
字段：工号姓名性别年龄部门薪水
id name gender age department salary

2. 创建员工信息表

mysql> create database pgdb;
Query OK, 1 row affected (0.31 sec)

mysql> use pgdb;
Database changed
mysql> show tables;
Empty set (0.12 sec)

mysql> create table employees(
-> id int,
-> name varchar(20),
-> gender char(1),
-> age int,
-> department varchar(30),
-> salary float(8, 2)
-> );
Query OK, 0 rows affected (1.33 sec)

mysql> show tables;
+----------------+
| Tables_in_pgdb |
+----------------+
| employees |
+----------------+
1 row in set (0.01 sec)

mysql> desc employees;
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| gender | char(1) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
| department | varchar(30) | YES | | NULL | |
| salary | float(8,2) | YES | | NULL | |
+------------+-------------+------+-----+---------+-------+
6 rows in set (0.01 sec)

- 部门信息表 departments
1. 设计表结构
字段：部门id 部门名称所在城市
id name location
2. 创建部门信息表
`

mysql> create table departments
-> (
-> id int,
-> name varchar(100),
-> location varchar(50)
-> );
Query OK, 0 rows affected (0.46 sec)

mysql> show tables;
+----------------+
| Tables_in_pgdb |
+----------------+
| departments |
| employees |
+----------------+
2 rows in set (0.01 sec)

mysql> desc departments;
+----------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(100) | YES | | NULL | |
| location | varchar(50) | YES | | NULL | |
+----------+--------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

- 查看创建表的语句 show create table employees; show create table departments;

mysql> show create table employees;
+-----------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-----------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| employees | CREATE TABLE `employees` (
`id` int(11) DEFAULT NULL,
`name` varchar(20) DEFAULT NULL,
`gender` char(1) DEFAULT NULL,
`age` int(11) DEFAULT NULL,
`department` varchar(30) DEFAULT NULL,
`salary` float(8,2) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+-----------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.19 sec)

day02_mysql：数据表创建

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