MySQL SQL语句
SQL语句
SQL语句有多种类型:
-
数据查询语言(DQL)
数据查询语言(Data Query Language, DQL)是SQL语言中,负责进行数据查询而不会对数据本身进行修改的语句,这是最基本的SQL语句。SELECT是DQL(也是所有SQL)用得最多的动词,其他DQL常用的保留字有FROM,WHERE,GROUP BY,HAVING和ORDER BY。这些DQL保留字常与其他类型的SQL语句一起使用。 -
数据定义语言(DDL)
数据定义语言 (Data Definition Language, DDL) 是SQL语言集中,负责数据结构定义与数据库对象定义的语言,由CREATE、ALTER与DROP三个语法所组成。 -
数据操纵语言(DML)
数据操纵语言(Data Manipulation Language, DML)是SQL语言中,负责对数据库对象运行数据访问工作的指令集,以INSERT、UPDATE、DELETE三种指令为核心,分别代表插入、更新与删除。 -
数据控制语言(DCL)
数据控制语言 (Data Control Language) 在SQL语言中,是一种可对数据访问权进行控制的指令,它可以控制特定用户账户对数据表、查看表、预存程序、用户自定义函数等数据库对象的控制权。由GRANT和REVOKE两个指令组成。DCL以控制用户的访问权限为主,GRANT为授权语句,对应的REVOKE是撤销授权语句。 -
指针控制语言(CCL)
它的语句,像DECLARE CURSOR,FETCH INTO和UPDATE WHERE CURRENT用于对一个或多个表单独行的操作。 -
事务处理语言(TPL)
它的语句能确保被DML语句影响的表的所有行及时得以更新。TPL语句包括BEGIN TRANSACTION,COMMIT和ROLLBACK。
1、DDL操作
1.1 数据库DDL操作
#创建数据库
语法:CREATE DATABASE [IF NOT EXISTS] 'DB_NAME';
#创建数据库hzz
mysql> CREATE DATABASE IF NOT EXISTS hzz;
Query OK, 1 row affected (0.00 sec)
#查看当前实例有哪些数据库
mysql> SHOW DATABASES;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| hzz |
+--------------------+
5 rows in set (0.00 sec)
#删除数据库
语法:DROP DATABASE [IF EXISTS] 'DB_NAME';
#删除数据库hzz
mysql> DROP DATABASE IF EXISTS hzz;
Query OK, 0 rows affected (0.00 sec)
mysql> SHOW DATABASES;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
+--------------------+
4 rows in set (0.00 sec)
#创建数据库时,设置数据库的字符集:
#CHARACTER SET:指定数据库采用的字符集,utf8不能写成utf-8,建议使用utf8mb4字符集
#COLLATE:指定数据库字符集的排序规则,utf8mb4的默认排序规则为utf8mb4_general_ci(通过show character set查看)
mysql> CREATE DATABASE dbtest CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;
#数据库客户端字符编码需要和服务端字符集保持一致
#SET NAMES:指定客户端字符集
mysql> SET NAMES utf8mb4;
#字符集配置都可写入MySQL配置文件中启动MySQL服务时自动加载
1.2 表DDL操作
字段修饰符(约束) | 含义 |
---|---|
PRIMARY KEY | 主键约束,在建立数据表的时候,一般情况下,为了方便更快地查找表中的记录,都会要求在表中设置一个“主键”。”主键“是表里面的一个特殊字段,这个字段能够唯一标识该表中的每条信息;一个表只能定义一个主键;主键值必须唯一标识表中的每一行,并且不能出现null 的情况且主键数据不能重复,严格遵守唯一性原则 |
UNIQUE | 唯一,该字段数据不可重复,保证唯一性但是可以为空 |
AUTO_INCREMENT | 自增,只能用于INT类型的字段;插入数据时该字段内所有数据中最大的值加1,也可手动指定值,值不可重复 |
Default ‘默认内容’ | 指定默认值,若插入数据时未填写数据,则自动填入默认值 |
NOT NULL | 该字段不能为空 |
NULL | 空值,实际任然占用空间,插入数据时若未给字段值,则默认为空 |
FOREIGN KEY(字段) REFERENCES 关联的表名(关联的表字段名) | 外键约束,外键约束是指一个表中的一个或多个列(称为外键)与另一个表中的一个或多个列(称为主键)之间的关系。使用外键约束可以确保数据的完整性,避免在表之间出现不一致的情况。 |
… | … |
数据类型 | 含义 |
---|---|
INT | 整数类型,范围-2147483648-2147483647 |
TINYINT | 小整数类型,范围-128-127 |
BIGINT | 大整数类型,范围-9223372036854775808~9223372036854775807 |
DOUBLE | 浮点数类型,两位小数 |
VARCHAR | 字符串类型,最大长度255 |
TEXT | 长字符串类型,最大长度2^16 |
DATE | 日期类型,日期格式YYYY-MM-DD ’ 或者 ‘YYYYMMDD ’ |
… | … |
#创建表
语法:CREATE TABLE table_name (col1 datatype 修饰符(约束)...,col2 datatype 修饰符(约束)...) ENGINE='存储引擎类型';
#在数据库hzz里创建表hzztable
mysql> CREATE DATABASE hzz;
Query OK, 1 row affected (0.00 sec)
#进入hzz数据库,再创建表
mysql> use hzz;
Database changed
#创建hzztable表指定id字段为主键,同时设置主键自增
CREATE TABLE hzztable(id INT(9) AUTO_INCREMENT PRIMARY KEY, NAME VARCHAR(10) NOT NULL, age TINYINT(3) );
# INT(9) 代表数据长度为9;如:123456789 长度为9
# VARCHAR(10) 代表数据长度为10,可存储10个汉字
#创建联合主键,多个字段为主键
CREATE TABLE hzztabletest(id INT(9) AUTO_INCREMENT, student_number INT(9), name VARCHAR(10) NOT NULL, age TINYINT, PRIMARY KEY(id, student_number));
#查看当前数据库有哪些表,注意进入数据库再查看表
mysql> SHOW TABLES;
+----------------------+
| Tables_in_hzz |
+----------------------+
| hzztable |
+----------------------+
1 row in set (0.00 sec)
#删除表
语法:DROP TABLE [ IF EXISTS ] 'table_name';
#删除表hzztable
mysql> DROP TABLE hzztable;
Query OK, 0 rows affected (0.01 sec)
mysql> SHOW TABLES;
Empty set (0.00 sec)
1.3 用户DDL操作
MySQL用户帐号由两部分组成,如’USERNAME’@‘HOST’,表示此USERNAME只能从此HOST上远程登录;HOST用于限制此用户可通过哪些主机远程连接mysql程序。
HOST的值可为:
- IP地址,如:172.16.12.129
- 通配符
- %:匹配任意长度的任意字符,常用于设置允许从任何主机登录
- _:匹配任意单个字符
#数据库用户创建
语法:CREATE USER 'username'@'host' [IDENTIFIED BY 'password'];
#创建数据库用户hzz
mysql> CREATE USER 'hzz'@'127.0.0.1' IDENTIFIED BY 'hzz123!';
Query OK, 0 rows affected (0.00 sec)
#刷新授权表
mysql> FLUSH PRIVILEGES;
Query OK, 0 rows affected (0.006 sec)
#使用新创建的用户和密码登录
[root@localhost ~]# mysql -uhzz -phzz123! -h127.0.0.1
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 6
Server version: 5.7.23 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>
#删除数据库用户
语法:DROP USER 'username'@'host';
#删除数据库用户hzz
mysql> DROP USER 'hzz'@'127.0.0.1';
Query OK, 0 rows affected (0.00 sec)
#刷新授权表
mysql> FLUSH PRIVILEGES;
Query OK, 0 rows affected (0.006 sec)
1.4 查看命令SHOW
#查看支持的所有字符集
mysql> SHOW CHARACTER SET;
+----------+---------------------------------+---------------------+--------+
| Charset | Description | Default collation | Maxlen |
+----------+---------------------------------+---------------------+--------+
| big5 | Big5 Traditional Chinese | big5_chinese_ci | 2 |
| dec8 | DEC West European | dec8_swedish_ci | 1 |
| cp850 | DOS West European | cp850_general_ci | 1 |
| hp8 | HP West European | hp8_english_ci | 1 |
| koi8r | KOI8-R Relcom Russian | koi8r_general_ci | 1 |
| latin1 | cp1252 West European | latin1_swedish_ci | 1 |
......
......
#查看客户端的字符编码
mysql> SHOW VARIABLES LIKE '%char%';
.....
.....
#查看某表中某字段使用的字符编码
mysql> SELECT CHARSET(email) FROM tbtest;
.....
.....
#查看当前数据库支持的所有存储引擎
mysql> SHOW ENGINES;
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| Engine | Support | Comment
| Transactions | XA | Savepoints |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| InnoDB | DEFAULT | Supports transactions, row-level locking, and
foreign keys | YES | YES | YES |
| MRG_MYISAM | YES | Collection of identical MyISAM tables
| NO | NO | NO |
| MEMORY | YES | Hash based, stored in memory, useful for temporary tables | NO | NO | NO |
| BLACKHOLE | YES | /dev/null storage engine (anything you write to it disappears) | NO | NO | NO |
| MyISAM | YES | MyISAM storage engine | NO | NO | NO |
| CSV | YES | CSV storage engine | NO | NO | NO |
| ARCHIVE | YES | Archive storage engine | NO | NO | NO |
| PERFORMANCE_SCHEMA | YES | Performance Schema | NO | NO | NO |
| FEDERATED | NO | Federated MySQL storage engine | NULL | NULL | NULL |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
9 rows in set (0.00 sec)
#查看数据库信息
mysql> SHOW DATABASES;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| hzz |
+--------------------+
5 rows in set (0.00 sec)
#不进入某数据库而列出其包含的所有表
mysql> SHOW TABLES FROM hzz;
+----------------------+
| Tables_in_hzz |
+----------------------+
| hzztable |
+----------------------+
1 row in set (0.00 sec)
#查看表结构
语法:DESC [db_name.]table_name;
mysql> DESC hzz.hzztable;
+-------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id | int(9) | NO | | NULL | |
| name | varchar(10) | NO | | NULL | |
| age | tinyint(3) | YES | | NULL | |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.03 sec)
#查看某表的创建命令,可以看到创建表时设置的参数
语法:SHOW CREATE TABLE table_name;
mysql> SHOW CREATE TABLE hzz.hzztable;
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------+
| hzztable | CREATE TABLE `hzz` (
`id` int NOT NULL AUTO_INCREMENT,
`name` varchar(10) NOT NULL,
`age` tinyint(3) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
#查看某表的状态
语法:SHOW TABLE STATUS LIKE 'table_name'\G
#进入数据库hzz
mysql> use hzz;
Database changed
#查看hzztable表的状态
mysql> SHOW TABLE STATUS LIKE 'hzztable'\G
*************************** 1. row ***************************
Name: hzztable
Engine: InnoDB
Version: 10
Row_format: Dynamic
Rows: 0
Avg_row_length: 0
Data_length: 16384
Max_data_length: 0
Index_length: 0
Data_free: 0
Auto_increment: NULL
Create_time: 2018-08-13 00:53:21
Update_time: NULL
Check_time: NULL
Collation: latin1_swedish_ci
Checksum: NULL
Create_options:
Comment:
1 row in set (0.00 sec)
1.5 ALTER修改操作
#修改数据库属性
语法:ALTER DATABASE 'DB_NAME' CHARACTER SET charset_name | COLLATE collation_name
#修改数据库字符编码
mysql> ALTER DATABASE hzz CHARACTER SET utf8;
mysql> ALTER DATABASE hzz CHARACTER SET utf8 COLLATE utf8_general_ci;
#修改表
语法:ALTER TABLE <table_name> [option]
#给hzztable表添加新的一列,先查看表结构
mysql> DESC hzztable;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(9) | NO | | NULL | |
| name | varchar(10) | NO | | NULL | |
| age | tinyint(3) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.001 sec)
#给hzztable表添加一列'phone'
mysql> ALTER TABLE hzztable ADD phone bigint(11);
Query OK, 0 rows affected (0.001 sec)
Records: 0 Duplicates: 0 Warnings: 0
#查看表结构
mysql> DESC hzztable;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(9) | NO | | NULL | |
| name | varchar(10) | NO | | NULL | |
| age | tinyint(3) | YES | | NULL | |
| phone | bigint(11) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.000 sec)
#修改hzztable表中'phone'列为'sex'
mysql> ALTER TABLE hzztable CHANGE phone sex varchar(4);
Query OK, 0 rows affected (0.005 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> DESC hzztable;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(9) | NO | | NULL | |
| name | varchar(10) | NO | | NULL | |
| age | tinyint(3) | YES | | NULL | |
| sex | varchar(4) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.001 sec)
#修改表名为zzhtable
mysql> ALTER TABLE hzztable RENAME TO zzhtable;
Query OK, 0 rows affected (0.002 sec)
mysql> SHOW TABLES;
+---------------+
| Tables_in_hzz |
+---------------+
| zzhtable |
+---------------+
1 row in set (0.000 sec)
#修改表字符编码
mysql> ALTER TABLE tbtest CHARACTER SET utf8 COLLATE utf8_general_ci;
1.6 获取帮助
#获取命令使用帮助
语法:HELP commond;
#获取创建表的帮助
mysql> HELP CREATE TABLE;
Name: 'CREATE TABLE'
Description:
Syntax:
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
(create_definition,...)
[table_options]
[partition_options]
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
[(create_definition,...)]
[table_options]
[partition_options]
[IGNORE | REPLACE]
[AS] query_expression
......
......
2、DML操作
DML操作包括增(INSERT)、删(DELETE)、改(UPDATE)、查(SELECT),均属针对表的操作。
2.1 INSERT语句
#DML操作之增操作insert
语法:INSERT [INTO] table_name [(column_name,...)] {VALUES | VALUE} (value1,...),(...),...
mysql> use hzz;
Database changed
#一次插入一条记录
mysql> INSERT INTO hzztable (id,name,age) VALUE (1,'tom',20);
Query OK, 1 row affected (0.01 sec)
#一次插入多条记录
mysql> INSERT INTO hzztable (id,name,age) VALUES (2,'jerry',23),(3,'hzz',25),(4,'sean',28),(5,'zhangshan',26),(6,'zhangshan',20),(7,'lisi',NULL);
Query OK, 4 rows affected (0.01 sec)
Records: 4 Duplicates: 0 Warnings: 0
2.2 SELECT语句(DQL)
字段column匹配方式:
表示符 | 含义 |
---|---|
* | 查询结果显示所有字段内容 |
字段名称 | 查询结果只显示该字段内容 |
as | 字段别名,如col1 AS alias1 当表名很长时用别名代替 |
条件判断语句WHERE:
操作类型 | 常用操作符 |
---|---|
操作符 | >,<,>=,<=,=,!= BETWEEN column# AND column# LIKE:模糊匹配 RLIKE:基于正则表达式进行模式匹配 IS NOT NULL:非空 IS NULL:空 |
条件逻辑操作 | AND OR NOT |
#DML操作之查操作select
语法:SELECT column1,column2,... FROM table_name [WHERE clause] [ORDER BY 'column_name' [DESC]] [LIMIT [m,]n];
mysql> use hzz;
Database changed
mysql> select * from hzztable;
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 1 | tom | 20 |
| 2 | jerry | 23 |
| 3 | hzz | 25 |
| 4 | sean | 28 |
| 5 | zhangshan | 26 |
| 6 | zhangshan | 20 |
| 7 | lisi | NULL |
+----+-----------+------+
7 rows in set (0.00 sec)
mysql> SELECT name FROM hzztable;
+-----------+
| name |
+-----------+
| tom |
| jerry |
| hzz |
| sean |
| zhangshan |
| zhangshan |
| lisi |
+-----------+
7 rows in set (0.01 sec)
mysql> SELECT * FROM hzztable WHERE age >= 25;
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 3 | hzz | 25 |
| 4 | sean | 28 |
| 5 | zhangshan | 26 |
+----+-----------+------+
3 rows in set (0.00 sec)
mysql> SELECT * FROM hzztable WHERE age >= 25 AND name = 'zhangshan';
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 5 | zhangshan | 26 |
+----+-----------+------+
1 row in set (0.00 sec)
mysql> SELECT * FROM hzztable WHERE age BETWEEN 23 and 28;
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 2 | jerry | 23 |
| 3 | hzz | 25 |
| 4 | sean | 28 |
| 5 | zhangshan | 26 |
+----+-----------+------+
4 rows in set (0.00 sec)
mysql> select * from hzztable where age is not null;
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 1 | tom | 20 |
| 2 | jerry | 23 |
| 3 | hzz | 25 |
| 4 | sean | 28 |
| 5 | zhangshan | 26 |
| 6 | zhangshan | 20 |
+----+-----------+------+
6 rows in set (0.00 sec)
mysql> select * from hzztable where age is null;
+----+------+------+
| id | name | age |
+----+------+------+
| 7 | lisi | NULL |
+----+------+------+
1 row in set (0.00 sec)
mysql> select * from hzztable where name like '%z%';
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 3 | hzz | 25 |
| 5 | zhangshan | 26 |
| 6 | zhangshan | 20 |
+----+-----------+------+
3 rows in set (0.00 sec)
GROUP BY 分组聚合函数
group by 分组功能原理:
1.按照group by指定的列进⾏排序;
2.然后根据group by指定的列去重复,也就是将相同的值分成一组;
3.最后通过聚合函数将其他列的结果进⾏聚合。
- 聚合函数
count() 统计数量
sum() 求和
avg() 平均数
max() 最⼤值
min() 最⼩值
#首先以name字段进行分组,然后对该字段进行排序、去重,最后通过count()聚合函数将name字段对应的age字段的数量统计出来
mysql> select name,count(age) from hzztable group by name;
+-----------+-------------+
| name | count(age) |
+-----------+-------------+
| hzz | 1 |
| jerry | 1 |
| sean | 1 |
| tom | 1 |
| zhangshan | 2 |
+----+-----------+--------+
5 rows in set (0.00 sec)
HAVING 聚合判断
HAVING:聚合判断,主要应⽤在GROUP BY之后再次进行判断。
select name,count(age) from hzztable group by name having count(age)>=2;
+-----------+-------------+
| name | count(age) |
+-----------+-------------+
| zhangshan | 2 |
+----+-----------+--------+
ORDER BY 聚合排序
ORDER BY:聚合排序,默认为升序(ASC),也可降序(DESC)。
ORDER BY语句 | 含义 |
---|---|
ORDER BY ‘column_name’ | 根据column_name进行升序排序 |
ORDER BY ‘column_name’ DESC | 根据column_name进行降序排序 |
mysql> SELECT * FROM hzztable ORDER BY age;
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 7 | lisi | NULL |
| 1 | tom | 20 |
| 6 | zhangshan | 20 |
| 2 | jerry | 23 |
| 3 | hzz | 25 |
| 5 | zhangshan | 26 |
| 4 | sean | 28 |
+----+-----------+------+
7 rows in set (0.00 sec)
mysql> SELECT * FROM hzztable ORDER BY age DESC;
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 4 | sean | 28 |
| 5 | zhangshan | 26 |
| 3 | hzz | 25 |
| 2 | jerry | 23 |
| 1 | tom | 20 |
| 6 | zhangshan | 20 |
| 7 | lisi | NULL |
+----+-----------+------+
7 rows in set (0.00 sec)
LIMIT 分⻚查询
LIMIT:分⻚查询,主要对数据进行分页,查询想要的内容。
LIMIT语句 | 含义 |
---|---|
ORDER BY ’column_name’ LIMIT 2 | 根据column_name进行升序排序并只取前2个结果 |
ORDER BY ‘column_name’ LIMIT 1,2 | 根据column_name进行升序排序并且略过第1个结果取后面的2个结果 |
mysql> SELECT * FROM hzztable ORDER BY age limit 2;
+----+------+------+
| id | name | age |
+----+------+------+
| 7 | lisi | NULL |
| 1 | tom | 20 |
+----+------+------+
2 rows in set (0.00 sec)
mysql> SELECT * FROM hzztable ORDER BY age limit 1,2;
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 1 | tom | 20 |
| 6 | zhangshan | 20 |
+----+-----------+------+
2 rows in set (0.00 sec)
2.3 update语句
#DML操作之改操作update
语法:UPDATE table_name SET column1 = new_value1[,column2 = new_value2,...] [WHERE clause] [ORDER BY 'column_name' [DESC]] [LIMIT [m,]n];
mysql> select * from hzztable;
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 1 | tom | 20 |
| 2 | jerry | 23 |
| 3 | hzz | 25 |
| 4 | sean | 28 |
| 5 | zhangshan | 26 |
| 6 | zhangshan | 20 |
| 7 | lisi | NULL |
+----+-----------+------+
7 rows in set (0.00 sec)
mysql> update hzztable set age = 30 where name = 'lisi';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from hzztable where name = 'lisi';
+----+------+------+
| id | name | age |
+----+------+------+
| 7 | lisi | 30 |
+----+------+------+
1 row in set (0.00 sec)
2.4 delete语句
#DML操作之删操作delete
语法:DELETE FROM table_name [WHERE clause] [ORDER BY 'column_name' [DESC]] [LIMIT [m,]n];
mysql> select * from hzztable;
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 1 | tom | 20 |
| 2 | jerry | 23 |
| 3 | hzz | 25 |
| 4 | sean | 28 |
| 5 | zhangshan | 26 |
| 6 | zhangshan | 20 |
| 7 | lisi | 30 |
+----+-----------+------+
7 rows in set (0.00 sec)
#删除某条记录
mysql> delete from hzztable where id = 7;
Query OK, 1 row affected (0.00 sec)
mysql> select * from hzztable;
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 1 | tom | 20 |
| 2 | jerry | 23 |
| 3 | hzz | 25 |
| 4 | sean | 28 |
| 5 | zhangshan | 26 |
| 6 | zhangshan | 20 |
+----+-----------+------+
6 rows in set (0.00 sec)
#删除整张表的内容
mysql> delete from hzztable;
Query OK, 6 rows affected (0.00 sec)
mysql> select * from hzztable;
Empty set (0.00 sec)
#表结构不会被删除
mysql> desc hzztable;
+-------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id | int(9) | NO | | NULL | |
| name | varchar(10) | NO | | NULL | |
| age | tinyint(3) | YES | | NULL | |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
2.5 truncate语句
truncate与delete的区别:
语句类型 | 特点 |
---|---|
delete | DELETE删除表内容时仅删除内容,但会保留表结构 DELETE语句每次删除一行,并在事务日志中为所删除的每行记录一项 可以通过回滚事务日志恢复数据 非常占用空间 |
truncate | 删除表中所有数据,且无法恢复 表结构、约束和索引等保持不变,新添加的行计数值重置为初始值 执行速度比DELETE快,且使用的系统和事务日志资源少 通过释放存储表数据所用的数据页来删除数据,并且只在事务日志中记录页的释放 对于有外键约束引用的表,不能使用TRUNCATE TABLE删除数据 不能用于加入了索引视图的表 |
语法:TRUNCATE table_name;
mysql> select * from hzztable;
+----+-----------+------+
| id | name | age |
+----+-----------+------+
| 1 | tom | 20 |
| 2 | jerry | 23 |
| 3 | hzz | 25 |
| 4 | sean | 28 |
| 5 | zhangshan | 26 |
| 6 | zhangshan | 20 |
| 7 | lisi | NULL |
+----+-----------+------+
7 rows in set (0.00 sec)
mysql> truncate hzztable;
Query OK, 0 rows affected (0.01 sec)
mysql> select * from hzztable;
Empty set (0.00 sec)
mysql> desc hzztable;
+-------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id | int(9) | NO | | NULL | |
| name | varchar(10) | NO | | NULL | |
| age | tinyint(3) | YES | | NULL | |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
3、DCL操作
3.1 创建授权grant
权限类型(priv_type)
权限类型 | 含义 |
---|---|
ALL | 所有权限 |
SELECT | 读取内容的权限 |
INSERT | 插入内容的权限 |
UPDATE | 更新内容的权限 |
DELETE | 删除内容的权限 |
… | … |
指定要操作的对象db_name.table_name
表示方式 | 含义 |
---|---|
*.* | 所有库的所有表 |
db_name | 指定库的所有表 |
db_name.table_name | 指定库的指定表 |
WITH GRANT OPTION:被授权的用户可将自己的权限副本转赠给其他用户,说白点就是将自己的权限完全复制给另一个用户。不建议使用。
语法:GRANT priv_type,... ON [object_type] db_name.table_name TO ‘username'@'host' [IDENTIFIED BY 'password'] [WITH GRANT OPTION];
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| hzz |
+--------------------+
5 rows in set (0.00 sec)
#授权hzz用户在数据库本机上登录访问所有数据库
mysql> GRANT ALL ON *.* TO 'hzz'@'localhost' IDENTIFIED BY 'hzz123!';
Query OK, 0 rows affected, 1 warning (0.00 sec)
#也可表示为:
mysql> GRANT ALL ON *.* TO 'hzz'@'127.0.0.1' IDENTIFIED BY 'hzz123!';
Query OK, 0 rows affected, 1 warning (0.00 sec)
#授权hzz用户在172.16.12.129上远程登录访问hzz数据库
mysql> GRANT ALL ON hzz.* TO 'hzz'@'172.16.12.129' IDENTIFIED BY 'hzz123!';
Query OK, 0 rows affected, 1 warning (0.00 sec)
#授权hzz用户在所有位置上远程登录访问hzz数据库
mysql> GRANT ALL ON hzz.* TO 'hzz'@'%' IDENTIFIED BY 'hzz123!';
Query OK, 0 rows affected, 1 warning (0.00 sec)
#刷新授权表
mysql> FLUSH PRIVILEGES;
3.2 查看授权
#查看当前登录用户的授权信息
mysql> SHOW GRANTS;
+---------------------------------------------------------------------+
| Grants for root@localhost |
+---------------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' WITH GRANT OPTION |
| GRANT PROXY ON ''@'' TO 'root'@'localhost' WITH GRANT OPTION |
+---------------------------------------------------------------------+
2 rows in set (0.00 sec)
#查看指定用户hzz的授权信息
mysql> SHOW GRANTS FOR 'hzz'@'localhost';
+-------------------------------------------------------+
| Grants for hzz@localhost |
+-------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'hzz'@'localhost' |
+-------------------------------------------------------+
1 row in set (0.00 sec)
mysql> SHOW GRANTS FOR 'hzz'@'127.0.0.1';
+-------------------------------------------------------+
| Grants for hzz@127.0.0.1 |
+-------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'hzz'@'127.0.0.1' |
+-------------------------------------------------------+
1 row in set (0.00 sec)
3.3 取消授权REVOKE
语法:REVOKE priv_type,... ON db_name.table_name FROM 'username'@'host';
mysql> REVOKE ALL ON *.* FROM 'hzz'@'172.16.12.129';
Query OK, 0 rows affected (0.00 sec)
mysql> FLUSH PRIVILEGES;
Query OK, 0 rows affected (0.00 sec)
注意:mysql服务进程启动时会读取mysql库中的所有授权表至内存中:
- GRANT或REVOKE等执行权限操作会保存于表中,mysql的服务进程会自动重读授权表,并更新至内存中
- 对于不能够或不能及时重读授权表的命令,可手动让mysql的服务进程重读授权表
mysql> FLUSH PRIVILEGES;
4、找回root密码
#1.停⽌MySQL服务
[root@localhost ~]# systemctl stop mysqld
#2.编辑MySQL服务配置文件,跳过授权登陆验证
[root@localhost ~]# vim /etc/my.cnf
[mysqld]
skip-grant-tables #添加此行,表示跳过授权表
#3.开启MySQL服务,免密登陆MySQL
[root@localhost ~]# systemctl start mysqld
[root@localhost ~]# mysql #直接使用mysql命令即可登录
#4.登录成功后修改MySQL密码
mysql> UPDATE mysql.user SET authentication_string=PASSWORD("123456") WHERE user='root' and host='localhost';
#5.关闭MySQL服务,删除配置文件中添加的跳过授权表配置
[root@localhost ~]# systemctl stop mysqld
[root@localhost ~]# vim /etc/my.cnf
[mysqld]
skip-grant-tables #删除该行
#6.再启动MySQL服务,使⽤修改后的密码登陆
[root@localhost ~]# systemctl start mysqld
[root@localhost ~]# mysql -uroot -p123456
#7.注意
#跳过授权表之后,所有的本地和远程⽤户都不需要⽤户名密码认证即可连接,⾮常危险。