查看已有的数据库：
mysql> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | test | +--------------------+ 2 rows in set (0.05 sec)
新建数据库语句:
mysql> create database wzu; Query OK, 1 row affected (0.05 sec) mysql> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | test | | wzu | +--------------------+ 3 rows in set (0.05 sec)
要对某个数据库进行操作时首先要选中这个数据库,使用use <dbname>：
mysql> use wzu
Database changed
创建表格:
命令:create table ＜表名＞ ( ＜属性名 属性类型 ［修饰］＞ );
create table student(sno char(9) primary key,sname char(20) unique,ssex char(2),sage smallint, sdept char(20)); create table course(cno char(4) primary key,cname char(40) not null,cpno char(4),ccredit smallint,foreign key(cpno) references course(cno)); create table sc(sno char(9),cno char(4),grade smallint,primary key(sno,cno),foreign key(sno) references student(sno),foreign key(cno) references course(cno));
删除表格:
命令：drop table ＜表名＞;
mysql> drop table emp;
Query OK, 0 rows affected (0.41 sec)
查看数据库中有哪些表格:
mysql> show tables;
+---------------+
| Tables_in_wzu |
+---------------+
| course |
| sc |
| student |
+---------------+
更改外键约束:
1 set foreign_key_checks=0; //忽略外键检查 2 set foreign_key_checks=1; //重启外键检查
更改表中某属性的数据类型:
命令:alter table ＜表名＞ modify ＜需要更改的属性＞ ＜想要更改的类型＞；
如:student 表中的sno字段改成varchar(11)
mysql> alter table student modify sno varchar(11); Query OK, 0 rows affected (0.15 sec) Records: 0 Duplicates: 0 Warnings: 0
可以用desc ＜表名＞ 来查看表格结构：
mysql> desc student; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | sno | varchar(11) | NO | PRI | | | | sname | varchar(20) | YES | UNI | NULL | | | ssex | varchar(2) | YES | | NULL | | | sage | smallint(6) | YES | | NULL | | | sdept | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+
表格插入数据:
命令：insert into <表名> values( 填,入,相,应,的,值 );
1 insert into student values('1024','测试','男','18','CS');
删除一行数据:
命令:delete from <表名> where [条件];
1 mysql> delete from emp where empno='232';
修改一行数据：
命令：update <表名> set [属性名]=[值] [where] [条件]
1 mysql> update course set cpno=null where cpno='NULL';
表格中的简单数据查询:
select * from [表名]
如:select * from student;
* 表示查询表格中所有的列,也可以用列名加','隔开来选择自己需要的数据
mysql> select * from student; +-------------+--------+------+------+-------+ | sno | sname | ssex | sage | sdept | +-------------+--------+------+------+-------+ | 1024 | 测试 | 男 | 18 | CS | | 1621116 | 周帅 | 男 | 21 | CS | +-------------+--------+------+------+-------+ mysql> select sno,sname from student; +-------------+--------+ | sno | sname | +-------------+--------+ | 1024 | 测试 | | 1621116 | 周帅 | +-------------+--------+
给已创建的表格增加一栏属性:
alter table <表名> add <属性名> <属性域>;
如:
mysql> alter table dept add type varchar(11); Query OK, 4 rows affected (0.62 sec) Records: 4 Duplicates: 0 Warnings: 0 mysql> desc dept; +--------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+--------------+------+-----+---------+-------+ | deptno | decimal(2,0) | NO | PRI | NULL | | | dname | varchar(14) | NO | | NULL | | | loc | varchar(13) | YES | | NULL | | | type | varchar(11) | YES | | NULL | | +--------+--------------+------+-----+---------+-------+
在已创建的表格中删除一栏属性:
alter table <表名> drop <属性名>;
如:
mysql> alter table dept drop type; Query OK, 4 rows affected (0.57 sec) Records: 4 Duplicates: 0 Warnings: 0 mysql> desc dept; +--------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+--------------+------+-----+---------+-------+ | deptno | decimal(2,0) | NO | PRI | NULL | | | dname | varchar(14) | NO | | NULL | | | loc | varchar(13) | YES | | NULL | | +--------+--------------+------+-----+---------+-------+
当发现一个操作有warning时 可以通过show warnings；来查看警告信息：
mysql> show warnings; +---------+------+-----------------------------------------------------------------------------------+ | Level | Code | Message | +---------+------+-----------------------------------------------------------------------------------+ | Warning | 1366 | Incorrect string value: 'xE6x9Dx8ExE5x8Bx87' for column 'sname' at row 1 | | Warning | 1366 | Incorrect string value: 'xE7x94xB7' for column 'ssex' at row 1 | | Warning | 1366 | Incorrect string value: 'xE6x95xB0xE6x8DxAE...' for column 'cname' at row 1 | +---------+------+-----------------------------------------------------------------------------------+
此处的问题主要是因为插入的编码格式不兼容（插入的数据编码为utf8 表格的编码为latinl）
解决方法见mysql/mariadb学习过程中出现的问题与解决
mysql查看当前选择的数据库:
mysql> select database(); +------------+ | database() | +------------+ | groupdb | +------------+
//或者：
mysql> show tables;
+-------------------+
| Tables_in_groupdb |//此处为当前数据库
+-------------------+
| course |
| dept |
| emp |
| jwc |
| salgrade |
| sc |
| student |
| temp |
+-------------------+
//或者：
mysql> status;
--------------
mysql Ver 14.14 Distrib 5.7.14, for Win64 (x86_64)
Connection id: 2721
Current database: groupdb //此处为当前数据库
Current user: group@122.228.131.86
SSL: Not in use
Using delimiter: ;
Server version: 5.5.56-MariaDB MariaDB Server
Protocol version: 10
Connection: AAA.AAA.AAA.AAA via TCP/IP
Server characterset: utf8
Db characterset: utf8
Client characterset: utf8
Conn. characterset: utf8
TCP port: 3306
Uptime: 2 days 5 hours 25 min 51 sec
Threads: 2 Questions: 549 Slow queries: 0 Opens: 26 Flush tables: 2 Open tables: 44 Queries per second avg: 0.002
2018-04-30
判断一个值是否为NULL：
使用 is 关键字而不能用 = 来判断
mysql> select * from course where cpno is null; +-----+--------------+------+---------+ | cno | cname | cpno | ccredit | +-----+--------------+------+---------+ | 2 | 高等数学 | NULL | 2 | | 6 | 数据处理 | NULL | 2 | +-----+--------------+------+---------+ 2 rows in set (0.05 sec) mysql> select * from course where cpno is not null; +-----+--------------+------+---------+ | cno | cname | cpno | ccredit | +-----+--------------+------+---------+ | 1 | 数据库 | 5 | 4 | | 3 | 信息系统 | 1 | 4 | | 4 | 操作系统 | 6 | 3 | | 5 | 数据结构 | 7 | 4 | | 7 | C语言 | 6 | 4 | +-----+--------------+------+---------+