MySQL基本命令

mysql安装完成后，默认有3个数据库
information_schema：是mysql运行过程中位于内存中的信息，保存mysql运行时数据，只有mysql启动时才有数据，平时是空的。

test：测试时才用到的数据库

mysql：mysql 的数据库

MySQL数据库目录：

[root@host2 ~]# ls /var/lib/mysql/
ibdata1  ib_logfile0  ib_logfile1  mysql  mysql.sock  test

可以看到没有information_schema数据库，因为它是存在内存中的。

MySQL常用命令：

DDL：定义数据库对象：
CREATE：

ALTER：

DROP：

DML：数据操纵语言
INSERT

UPDATE

DELETE

DCL数据控制语言
GRANT：

REVOKE：

创建数据库：
CREATE DATABASE db_name;

CREATE DATABASES IF NOT EXISTS testdb;

mysql> CREATE DATABASE test_db;
Query OK, 1 row affected (0.00 sec)
mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| test               |
| test_db            |
+--------------------+
4 rows in set (0.01 sec)

删除数据库：

DROP DATABASE db_name;

mysql> DROP DATABASE test_db;
Query OK, 0 rows affected (0.00 sec)
mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| test               |
+--------------------+
3 rows in set (0.00 sec)

创建表：
CREATE TABLE tb_name(col1,col2,...) col1,col2为字段名

注意：表是依赖于数据库的，所以在创建表之前，一定要指定默认数据库，使用命令
USER DATABASE; 来指定数据库
创建一个表，名为student，包含三个字段，Name、Age、Gender

mysql> CREATE TABLE students(Name CHAR(20) NOT NULL, Age TINYINT UNSIGNED,Gender CHAR(1) NOT NULL);
Query OK, 0 rows affected (0.01 sec)
mysql> SHOW TABLES;
+-------------------+
| Tables_in_test_db |
+-------------------+
| students          |
+-------------------+
1 row in set (0.00 sec)

查看某个数据库中的表：
SHOW TABLES FROM db_name;

查看表结构：
DESC table_name;

mysql> DESC students;
+--------+---------------------+------+-----+---------+-------+
| Field  | Type                | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+-------+
| Name   | char(20)            | NO   |     | NULL    |       |
| Age    | tinyint(3) unsigned | YES  |     | NULL    |       |
| Gender | char(1)             | NO   |     | NULL    |       |
+--------+---------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

删除表：
DROP TABLE tb_name;

修改表：
ALTER TABLE tb_name;

MODIFY：修改某个字段，修改字段属性，字段名不改

CHANGE：改变某个字段，改变字段名称

ADD：添加字段

DROP：删除字段

可以通过help命令查询帮助信息：

help CREATE TABLE;

增加一个字段
例如：修改前面创建的student表，增加一个字段课程course,

mysql> ALTER TABLE students ADD course VARCHAR(100);
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> DESC students;
+--------+---------------------+------+-----+---------+-------+
| Field  | Type                | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+-------+
| Name   | char(20)            | NO   |     | NULL    |       |
| Age    | tinyint(3) unsigned | YES  |     | NULL    |       |
| Gender | char(1)             | NO   |     | NULL    |       |
| course | varchar(100)        | YES  |     | NULL    |       |
+--------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

修改字段名称，例如将上面添加的course字段名改为Course

mysql> ALTER TABLE students CHANGE course Course VARCHAR(100);
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> DESC students;
+--------+---------------------+------+-----+---------+-------+
| Field  | Type                | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+-------+
| Name   | char(20)            | NO   |     | NULL    |       |
| Age    | tinyint(3) unsigned | YES  |     | NULL    |       |
| Gender | char(1)             | NO   |     | NULL    |       |
| Course | varchar(100)        | YES  |     | NULL    |       |
+--------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

INSERT  INTO  tb_name (col1,col2,...) VALUES|VALUE ('STRING',NUM...);

上面命令意思是：往某个表中插入数据，字段名为col1，col2,... 插入的值为字符串则，用引号引起来，如果为数值，则直接用数字。如果每一个字段都给值，则不用写字段名称

例如：在students表中插入两条数据，张三和李四

mysql> INSERT INTO students (Name,Gender) VALUE ('ZhangSan','M'),('LiSi','F');
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0

查看数据

mysql> SELECT * FROM students;
+----------+------+--------+--------+
| Name     | Age  | Gender | Course |
+----------+------+--------+--------+
| ZhangSan | NULL | M      | NULL   |
| LiSi     | NULL | F      | NULL   |
+----------+------+--------+--------+
2 rows in set (0.01 sec)

插入一个用户，所有字段都有值：
注意，所有字段都有值，就不需要指定字段名

mysql> INSERT INTO students VALUES ('ZengChengpeng',28,'M','IT');
Query OK, 1 row affected (0.00 sec)
mysql> SELECT * FROM students WHERE Name='ZengChengpeng';
+---------------+------+--------+--------+
| Name          | Age  | Gender | Course |
+---------------+------+--------+--------+
| ZengChengpeng |   28 | M      | IT     |
+---------------+------+--------+--------+
1 row in set (0.00 sec)

修改数据：

UPDATE tb_name SET column=value WHERE 条件

例如：将ZengChengpeng的Course课程名称改为Develop

mysql> UPDATE students SET Course='Develop' WHERE Name='ZengChengpeng';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0
mysql> SELECT * FROM students WHERE Name='ZengChengpeng';
+---------------+------+--------+---------+
| Name          | Age  | Gender | Course  |
+---------------+------+--------+---------+
| ZengChengpeng |   28 | M      | Develop |
+---------------+------+--------+---------+
1 row in set (0.00 sec)

SELETE语句：

selete语句分为两种情况，

选择：指定以某字段作为搜索码，做逻辑比较，筛选符合条件的行。WHERE指定选择