mysql/mariadb学习记录——创建删除数据库、表的基本命令

查看已有的数据库：

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| test               |
+--------------------+
 rows in set (0.05 sec)

新建数据库语句:

mysql> create database wzu;
Query OK,  row affected (0.05 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| test               |
| wzu                |
+--------------------+
 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,  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:

 set foreign_key_checks=;  //忽略外键检查
 set foreign_key_checks=; //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();
Query OK,  rows affected (0.15 sec)
Records:   Duplicates:   Warnings: 

可以用desc ＜表名＞ 来查看表格结构：

mysql> desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| sno   | varchar() | NO   | PRI |         |       |
| sname | varchar() | YES  | UNI | NULL    |       |
| ssex  | varchar()  | YES  |     | NULL    |       |
| sage  | smallint() | YES  |     | NULL    |       |
| sdept | varchar() | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

表格插入数据:

命令：insert into <表名> values( 填,入,相,应,的,值 );

 insert into student values('','测试','男','','CS');

删除一行数据:

命令:delete from <表名> where [条件];

 mysql> delete from emp where empno='';

修改一行数据：

命令：update <表名> set [属性名]=[值] [where] [条件]

 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 |
+-------------+--------+------+------+-------+
|         | 测试    | 男   |    | CS    |
|      | 周帅    | 男   |    | CS    |
+-------------+--------+------+------+-------+

mysql> select sno,sname from student;
+-------------+--------+
| sno         | sname  |
+-------------+--------+
|         | 测试    |
|      | 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();
Query OK,  rows affected (0.62 sec)
Records:   Duplicates:   Warnings: 

mysql> desc dept;
+--------+--------------+------+-----+---------+-------+
| Field  | Type         | Null | Key | Default | Extra |
+--------+--------------+------+-----+---------+-------+
| deptno | decimal(,) | NO   | PRI | NULL    |       |
| dname  | varchar()  | NO   |     | NULL    |       |
| loc    | varchar()  | YES  |     | NULL    |       |
| type   | varchar()  | 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,  rows affected (0.57 sec)
Records:   Duplicates:   Warnings: 

mysql> desc dept;
+--------+--------------+------+-----+---------+-------+
| Field  | Type         | Null | Key | Default | Extra |
+--------+--------------+------+-----+---------+-------+
| deptno | decimal(,) | NO   | PRI | NULL    |       |
| dname  | varchar()  | NO   |     | NULL    |       |
| loc    | varchar()  | 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: '\xE6\x9D\x8E\xE5\x8B\x87' for column 'sname' at row 1    |
| Warning | 1366 | Incorrect string value: '\xE7\x94\xB7' for column 'ssex' at row 1                 |
| Warning | 1366 | Incorrect string value: '\xE6\x95\xB0\xE6\x8D\xAE...' 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 |
+-----+--------------+------+---------+
|    | 高等数学      | NULL |        |
|    | 数据处理      | NULL |        |
+-----+--------------+------+---------+
 rows in set (0.05 sec)

mysql> select * from course where cpno is not null;
+-----+--------------+------+---------+
| cno | cname        | cpno | ccredit |
+-----+--------------+------+---------+
|    | 数据库        |     |        |
|    | 信息系统      |     |        |
|    | 操作系统      |     |        |
|    | 数据结构      |     |        |
|    | C语言         |     |        |
+-----+--------------+------+---------+