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MySQL 常用 SQL 命令 （1. DDL语句）
SQL : Structure Query Language 结构化查询语言。
SQL 语句分类：
- DDL (Data Definition Language): 数据定义语言。如 create, drop, alter等。
- DML (Data Manipulation Language): 数据操纵语言。如 insert, delete, update, select等。
- DCL (Data Control Language): 数据控制语言。如 grant, revoke等。
连接数据库：
mysql -u <user_name> -p
eg.
192:~ youngbear$ mysql -u bearyang -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 9
Server version: 8.0.11 MySQL Community Server - GPL
Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql>
本文章使用的 MySQL 版本是 8.0.11。
1. DDL 语句
1.1 创建数据库:
CREATE DATABASE <dbname>;
显示数据库：
show databases;
使用数据库：
USE <dbname>;
显示数据库中的表：
show tables;
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使用表：
USE <table_name>;
1.2 删除数据库
DROP DATABASE <dbname>;
eg.
mysql> drop database db201810;
Query OK, 0 rows affected (0.10 sec)
注意： 在 MySQL 里面，drop 语句操作的结果都是显示 “0 rows affected” 。
1.3 创建表
创建表的基本语句：
CREATE TABLE <table_name> (
column_name_1 column_type_1 constraints,
column_name_2 column_type_2 constraints,
column_name_n column_type_n constraints
);
eg.
mysql> create table emp(
-> ename varchar(10),
-> hiredate date,
-> sal decimal(10,2),
-> deptno int(2)
-> );
Query OK, 0 rows affected (0.12 sec)
查看表的定义：
DESC <table_name>;
eg.
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename | varchar(10) | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
4 rows in set (0.01 sec)
查看创建表的 SQL 语句：
show create table <table_name> \G;
eg.
mysql> show create table emp \G;
*************************** 1. row ***************************
Table: emp
Create Table: CREATE TABLE `emp` (
`ename` varchar(10) DEFAULT NULL,
`hiredate` date DEFAULT NULL,
`sal` decimal(10,2) DEFAULT NULL,
`deptno` int(2) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)
ERROR:
No query specifieds
其中，engine 表示存储引擎， charset 表示字符集， collate 表示 校对集。
“\G” 选项的含义是使得记录能够按照字段竖向排列，以便更好地显示内容较长的记录。
1.4 删除表
DROP TABLE <table_name>;
eg.
mysql> drop table emp;
Query OK, 0 rows affected (0.04 sec)
1.5 修改表
1.5.1 修改表类型
ALTER TABLE <table_name> MODIFY [COLUMN] <column_definition> [FIRST|AFTER <col_name>];
eg. 修改表emp的ename字段定义，将 varchar(10)改为varchar(20):
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename | varchar(10) | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
4 rows in set (0.01 sec)
mysql> alter table emp modify ename varchar(20);
Query OK, 0 rows affected (0.08 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename | varchar(20) | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
1.5.2 增加表字段
ALTER TABLE <table_name> ADD [COLUMN] <column_definition> [FIRST|AFTER <col_name>];
eg. 在表 emp 中新增加字段 age，类型为 int(3):
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename | varchar(20) | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
mysql> alter table emp add column age int(3);
Query OK, 0 rows affected (0.12 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename | varchar(20) | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
| age | int(3) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.01 sec)
其中，column可以省略。
1.5.3 删除表字段
ALTER TABLE <table_name> DROP [COLUMN] <col_name>;
eg. 删除 emp 的 age 字段：
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename | varchar(20) | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
| age | int(3) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.01 sec)
mysql> alter table emp drop age;
Query OK, 0 rows affected (0.08 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename | varchar(20) | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
4 rows in set (0.01 sec)
1.5.4 字段改名
ALTER TABLE <table_name> CHANGE [COLUMN] <old_col_name> <column_definition> [FIRST|AFTER <col_name>];
eg. 将 emp 的 age 改名为 age1, 同时修改字段类型为 int(4);
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename | varchar(20) | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
| age | int(3) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
mysql> alter table emp change age age1 int(4);
Query OK, 0 rows affected (0.10 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename | varchar(20) | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
| age1 | int(4) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
注意：
change 和 modify 都可以修改表的定义，不同的是 change 后面需要写两次列名，不方便。但是 change 的优点是修改列名称， modify 则不能。
1.5.5 修改字段排列顺序
前面的字段增加和修改语法（ADD/CHANGE/MODIFY）中，都有一个可选项[FIRST|AFTER <col_name>]，表示修改字段在表中的位置。ADD 增加新字段默认在表的最后位置，而 CHANGE/MODIFY 默认都不会改变字段的位置。
eg1. 将新增字段 birth 加在 ename 之后：
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename | varchar(20) | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
| age1 | int(4) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.01 sec)
mysql> alter table emp add birth date after ename;
Query OK, 0 rows affected (0.06 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename | varchar(20) | YES | | NULL | |
| birth | date | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
| age1 | int(4) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
eg. 2. 修改字段 age1 为 age,类型改为 int(3)， 并将它放在最前面：
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename | varchar(20) | YES | | NULL | |
| birth | date | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
| age1 | int(4) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
mysql> alter table emp change age1 age int(3) first;
Query OK, 0 rows affected (0.07 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| age | int(3) | YES | | NULL | |
| ename | varchar(20) | YES | | NULL | |
| birth | date | YES | | NULL | |
| hiredate | date | YES | | NULL | |
| sal | decimal(10,2) | YES | | NULL | |
| deptno | int(2) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
1.5.6 更改表名
ALTER TABLE <table_name> RENAME [TO] <new_table_name>;
eg. 将 emp 改为 emp1;
mysql> alter table emp rename emp1;
Query OK, 0 rows affected (0.05 sec)
mysql> show tables;
+--------------------+
| Tables_in_db201810 |
+--------------------+
| emp1 |
+--------------------+
1 row in set (0.00 sec)
todo:
DML 和DCL。