MySQL SQL介绍
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mysql SQL介绍
目录
一、MySQL SQL介绍
1.1.1 SQL 应用基础
- SQL（结构化查询语句）介绍
SQL标准：SQL 92 SQL99
5.7版本后启用SQL_Mode 严格模式
- SQL作用
SQL 用来管理和操作MySQL内部的对象
SQL对象：
库：库名，库属性
表：表名，表属性，列名，记录，数据类型，列属性和约束
- SQL语句的类型
DDL：数据定义语言 data definition language
DCL：数据控制语言 data control language
DML：数据操作语言 data manipulation language
DQL：数据查询语言 data query language
- 数据类型
1、 作用：
控制数据的规范性，让数据有具体含义，在列上进行控制
2、字符串
char(32)
定长长度为32的字符串。存储数据时，一次性提供32字符长度的存储空间，存不满，用空格填充。
varchar(32)：
可变长度的字符串类型。存数据时，首先进行字符串长度判断，按需分配存储空间
会单独占用一个字节来记录此次的字符长度
超过255之后，需要两个字节长度记录字符长度。
定长 的字符串类型,在存储字符串时，最大字符长度11个，立即分配11个字符长度的存储空间，如果存不满，空格填充。
变长的字符串类型看，最大字符长度11个。在存储字符串时，自动判断字符长度，按需分配存储空间。
3、 enum 枚举类型
enum(‘bj‘,‘sh‘,‘sz‘,‘cq‘,‘hb‘,......)
数据行较多时，会影响到索引的应用
枚举类型，比较适合于将来此列的值是固定范围内的特点，可以使用enum,可以很大程度的优化我们的索引结构。
注意：数字类禁止使用enum类型
4、 数字
tinyint ： -128~127
int ：-231~231-1
说明：手机号是无法存储到int的。一般是使用char类型来存储收集号
5、时间
timestamp
datetime
- 表属性
存储引擎：engine =（myisam innodb）
字符集：charset = （utf8mb4）
utf8 中文 三个字节长度
utf8mb4 中文 四个字节长度 才是真正的utf8
支持emoji（表情）字符
排序规则（校对规则） collation
针对英文字符串大小写问题
- 列的类型与约束
6.1 主键： primary key (PK)
说明：
唯一
非空
数字列，整数列，无关列，自增的.
聚集索引列？
是一种约束，也是一种索引类型，在一张表中只能有一个主键。
6.2 非空： Not NULL
说明：
我们建议，对于普通列来讲，尽量设置not null
默认值 default ： 数字列的默认值使用0 ,字符串类型，设置为一个nil null
6.3 唯一：unique
不能重复
6.4 自增 auto_increment
针对数字列，自动生成顺序值
6.5 无符号 unsigned
针对数字列
6.6 注释 comment
二、MySQL DDL 语句操作
2.1.1 MySQL DDL 库表语句操作
- DDL：数据定义语言 库操作
#mysql帮助命令
mysql> help create database;
Name: ‘CREATE DATABASE‘
Description:
Syntax:
CREATE {DATABASE | SCHEMA} [IF NOT EXISTS] db_name
[create_specification] ...
create_specification:
[DEFAULT] CHARACTER SET [=] charset_name
| [DEFAULT] COLLATE [=] collation_name
CREATE DATABASE creates a database with the given name. To use this
statement, you need the CREATE privilege for the database. CREATE
SCHEMA is a synonym for CREATE DATABASE.
URL: http://dev.mysql.com/doc/refman/5.7/en/create-database.html
#创建数据库
mysql> create database chenhj character set utf8mb4 collate utf8mb4_general_ci; #后面是字符集合校对规则
Query OK, 1 row affected (0.00 sec)
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| chenhj |
| kaode |
| mysql |
| performance_schema |
| sys |
| wordpress |
+--------------------+
7 rows in set (0.08 sec)
#查看建库语句
mysql> show create database chenhj;
+----------+--------------------------------------------------------------------+
| Database | Create Database |
+----------+--------------------------------------------------------------------+
| chenhj | CREATE DATABASE `chenhj` /*!40100 DEFAULT CHARACTER SET utf8mb4 */ |
+----------+--------------------------------------------------------------------+
1 row in set (0.00 sec)
#修改库
mysql> alter database chenhj character set utf8; #修改字符集
Query OK, 1 row affected (0.01 sec)
mysql> show create database chenhj;
+----------+-----------------------------------------------------------------+
| Database | Create Database |
+----------+-----------------------------------------------------------------+
| chenhj | CREATE DATABASE `chenhj` /*!40100 DEFAULT CHARACTER SET utf8 */ |
+----------+-----------------------------------------------------------------+
1 row in set (0.00 sec)
#删除数据库
mysql> drop database kaode;
Query OK, 22 rows affected (0.17 sec)
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| chenhj |
| mysql |
| performance_schema |
| sys |
| wordpress |
+--------------------+
6 rows in set (0.00 sec)
- 表操作
#建表建库规范：
1、库名和表名是小写字母不要大写字母
为啥？不同的开发和系统平台可能会出现问题，比如window环境不区分大小写，
但是Linux平台的区分大小写的。
2、不能以数字开头
3、不支持- 支持_
4、内部函数名不能使用
5、名字和业务功能有关（his,jf,yz,oss,erp,crm...）
mysql> create user root@‘10.0.0.%‘ identified by ‘123456‘ ;
Query OK, 0 rows affected (0.00 sec)
mysql> grant all on *.* to root@‘10.0.0.%‘;
Query OK, 0 rows affected (0.00 sec)
#建表
mysql> CREATE TABLE `NewTable` (
`id` int NOT NULL COMMENT ‘学号‘ ,
`name` varchar(255) NOT NULL COMMENT ‘学生姓名‘ ,
`age` tinyint UNSIGNED ZEROFILL NOT NULL COMMENT ‘学生年龄‘ ,
`gender` enum(‘f‘,‘n‘,‘m‘) NOT NULL DEFAULT ‘n‘ COMMENT ‘性别‘ ,
PRIMARY KEY (`id`)
)
;
#查看建表语句
mysql> show create table chenhj.stu;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| stu | CREATE TABLE `stu` (
`id` int(11) NOT NULL COMMENT ‘学号‘,
`name` varchar(255) NOT NULL COMMENT ‘学生姓名‘,
`age` tinyint(3) unsigned zerofill NOT NULL COMMENT ‘学生年龄‘,
`gender` enum(‘f‘,‘n‘,‘m‘) NOT NULL DEFAULT ‘n‘ COMMENT ‘性别‘,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
CREATE TABLE oldchen(
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT COMMENT ‘学号‘,
name varchar(255) not null comment ‘姓名‘,
age TINYINT UNSIGNED NOT NULL DEFAULT ‘0‘ COMMENT ‘年龄‘,
gender ENUM(‘M‘,‘F‘,‘N‘) NOT NULL default ‘n‘ COMMENT ‘性别‘
)charset=utf8mb4 engine=innodb;
#改表
#添加一个手机号码列 （add）
mysql> alter table oldchen add telnum char(11) unique not null comment ‘手机号码‘;
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc oldchen;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(255) | NO | | NULL | |
| age | tinyint(3) unsigned | NO | | 0 | |
| gender | enum(‘M‘,‘F‘,‘N‘) | NO | | N | |
| telnum | char(11) | NO | | NULL | |
+--------+---------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)
mysql> alter table oldchen add state tinyint unsigned not null default 1 comment ‘状态列‘;
Query OK, 0 rows affected (0.34 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc oldchen;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(255) | NO | | NULL | |
| age | tinyint(3) unsigned | NO | | 0 | |
| gender | enum(‘M‘,‘F‘,‘N‘) | NO | | N | |
| telnum | char(11) | NO | UNI | NULL | |
| state | tinyint(3) unsigned | NO | | 1 | |
+--------+---------------------+------+-----+---------+----------------+
6 rows in set (0.00 sec)
#name列后面添加qq列 （add 和 after）
mysql> alter table oldchen add qq varchar(255) unique not null comment ‘qq号码‘ after name;
Query OK, 0 rows affected (1.68 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc oldchen;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(255) | NO | | NULL | |
| qq | varchar(255) | NO | UNI | NULL | |
| age | tinyint(3) unsigned | NO | | 0 | |
| gender | enum(‘M‘,‘F‘,‘N‘) | NO | | N | |
| telnum | char(11) | NO | UNI | NULL | |
| state | tinyint(3) unsigned | NO | | 1 | |
+--------+---------------------+------+-----+---------+----------------+
7 rows in set (0.00 sec)
#在name 之前添加wechat列
mysql> alter table oldchen add wechat varchar(255) unique not null comment ‘微信‘ after id;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc oldchen;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| wechat | varchar(255) | NO | UNI | NULL | |
| name | varchar(255) | NO | | NULL | |
| qq | varchar(255) | NO | UNI | NULL | |
| age | tinyint(3) unsigned | NO | | 0 | |
| gender | enum(‘M‘,‘F‘,‘N‘) | NO | | N | |
| telnum | char(11) | NO | UNI | NULL | |
| state | tinyint(3) unsigned | NO | | 1 | |
+--------+---------------------+------+-----+---------+----------------+
8 rows in set (0.00 sec)
#在首列添加学生号
mysql> alter table oldchen add sid varchar(255) unique not null comment ‘学生号‘ first;
Query OK, 0 rows affected (0.31 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc oldchen;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| sid | varchar(255) | NO | UNI | NULL | |
| id | int(11) | NO | PRI | NULL | auto_increment |
| wechat | varchar(255) | NO | UNI | NULL | |
| name | varchar(255) | NO | | NULL | |
| qq | varchar(255) | NO | UNI | NULL | |
| age | tinyint(3) unsigned | NO | | 0 | |
| gender | enum(‘M‘,‘F‘,‘N‘) | NO | | N | |
| telnum | char(11) | NO | UNI | NULL | |
| state | tinyint(3) unsigned | NO | | 1 | |
+--------+---------------------+------+-----+---------+----------------+
9 rows in set (0.00 sec)
#修改列属性（modify）将sid列的varchar(255)改成varchar（128）
mysql> alter table oldchen modify sid varchar(128) unique not null;
Query OK, 0 rows affected, 1 warning (1.77 sec)
Records: 0 Duplicates: 0 Warnings: 1
mysql> desc oldchen;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| sid | varchar(128) | NO | UNI | NULL | |
| id | int(11) | NO | PRI | NULL | auto_increment |
| wechat | varchar(255) | NO | UNI | NULL | |
| name | varchar(255) | NO | | NULL | |
| qq | varchar(255) | NO | UNI | NULL | |
| age | tinyint(3) unsigned | NO | | 0 | |
| gender | enum(‘M‘,‘F‘,‘N‘) | NO | | N | |
| telnum | char(11) | NO | UNI | NULL | |
| state | tinyint(3) unsigned | NO | | 1 | |
+--------+---------------------+------+-----+---------+----------------+
9 rows in set (0.01 sec)
#修改列名（change）将sid改成SID
mysql> alter table oldchen change sid SID varchar(128) not null;
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc oldchen;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| SID | varchar(128) | NO | UNI | NULL | |
| id | int(11) | NO | PRI | NULL | auto_increment |
| wechat | varchar(255) | NO | UNI | NULL | |
| name | varchar(255) | NO | | NULL | |
| qq | varchar(255) | NO | UNI | NULL | |
| age | tinyint(3) unsigned | NO | | 0 | |
| gender | enum(‘M‘,‘F‘,‘N‘) | NO | | N | |
| telnum | char(11) | NO | UNI | NULL | |
| state | tinyint(3) unsigned | NO | | 1 | |
+--------+---------------------+------+-----+---------+----------------+
9 rows in set (0.00 sec)
#再改回来
mysql> alter table oldchen change SID sid varchar(128) not null comment ‘学生号‘;
Query OK, 0 rows affected (0.30 sec)
Records: 0 Duplicates: 0 Warnings: 0
#删除列（drop）
mysql> alter table oldchen drop sid;
Query OK, 0 rows affected (0.30 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc oldchen;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| wechat | varchar(255) | NO | UNI | NULL | |
| name | varchar(255) | NO | | NULL | |
| qq | varchar(255) | NO | UNI | NULL | |
| age | tinyint(3) unsigned | NO | | 0 | |
| gender | enum(‘M‘,‘F‘,‘N‘) | NO | | N | |
| telnum | char(11) | NO | UNI | NULL | |
| state | tinyint(3) unsigned | NO | | 1 | |
+--------+---------------------+------+-----+---------+----------------+
8 rows in set (0.00 sec)
#添加回来
mysql> alter table oldchen add sid varchar(128) unique not null comment ‘学生号‘ first;
Query OK, 0 rows affected (0.31 sec)
Records: 0 Duplicates: 0 Warnings: 0
#删除表
mysql> show create table chenhj.oldchen;
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| oldchen | CREATE TABLE `oldchen` (
`id` int(11) NOT NULL AUTO_INCREMENT COMMENT ‘学号‘,
`name` varchar(255) NOT NULL COMMENT ‘姓名‘,
`age` tinyint(3) unsigned NOT NULL DEFAULT ‘0‘ COMMENT ‘年龄‘,
`gender` enum(‘M‘,‘F‘,‘N‘) NOT NULL COMMENT ‘性别‘,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 |
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> drop table chenhj.oldchen;
Query OK, 0 rows affected (0.31 sec)
mysql> show create table chenhj.oldchen;
ERROR 1146 (42S02): Table ‘chenhj.oldchen‘ doesn‘t exist
- 创建一个已经存在且属性结构相同的空表（link）
mysql> create table chen like oldchen ;
Query OK, 0 rows affected (0.30 sec)
mysql> desc chen;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| sid | varchar(128) | NO | UNI | NULL | |
| id | int(11) | NO | PRI | NULL | auto_increment |
| wechat | varchar(255) | NO | UNI | NULL | |
| name | varchar(255) | NO | | NULL | |
| qq | varchar(255) | NO | UNI | NULL | |
| age | tinyint(3) unsigned | NO | | 0 | |
| gender | enum(‘M‘,‘F‘,‘N‘) | NO | | N | |
| telnum | char(11) | NO | UNI | NULL | |
| state | tinyint(3) unsigned | NO | | 1 | |
+--------+---------------------+------+-----+---------+----------------+
9 rows in set (0.00 sec)
mysql> desc oldchen;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| sid | varchar(128) | NO | UNI | NULL | |
| id | int(11) | NO | PRI | NULL | auto_increment |
| wechat | varchar(255) | NO | UNI | NULL | |
| name | varchar(255) | NO | | NULL | |
| qq | varchar(255) | NO | UNI | NULL | |
| age | tinyint(3) unsigned | NO | | 0 | |
| gender | enum(‘M‘,‘F‘,‘N‘) | NO | | N | |
| telnum | char(11) | NO | UNI | NULL | |
| state | tinyint(3) unsigned | NO | | 1 | |
+--------+---------------------+------+-----+---------+----------------+
9 rows in set (0.00 sec)
- 清空表数据(删除内容、释放空间但不删除定义)
mysql> truncate tbale 表名
truncate table oldchen;
三、MySQL DML语句操作
3.1.1 MySQL DML语句表操作
- MySQL DML语句表操作
#删除不必要的列
mysql> alter table oldchen drop sid;
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table oldchen drop wechat;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table oldchen drop state;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table oldchen drop gender;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table oldchen drop telnum;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc oldchen;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(255) | NO | | NULL | |
| qq | varchar(255) | NO | UNI | NULL | |
| age | tinyint(3) unsigned | NO | | 0 | |
+-------+---------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
- insert 插入数据语句的使用
mysql> desc oldchen;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(255) | NO | | NULL | |
| qq | varchar(255) | NO | UNI | NULL | |
| age | tinyint(3) unsigned | NO | | 0 | |
+-------+---------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
mysql> insert into oldchen values(1,‘chen‘,‘1010235677‘,18);
Query OK, 1 row affected (0.00 sec)
mysql> select id ,name, qq, age from oldchen;
+----+------+------------+-----+
| id | name | qq | age |
+----+------+------------+-----+
| 1 | chen | 1010235677 | 18 |
+----+------+------------+-----+
1 row in set (0.00 sec)
#生产规范插入
mysql> insert into oldchen(name,qq,age) values(‘liu‘,‘577397908‘,25);
Query OK, 1 row affected (0.00 sec)
mysql> select id ,name, qq, age from oldchen;
+----+------+------------+-----+
| id | name | qq | age |
+----+------+------------+-----+
| 1 | chen | 1010235677 | 18 |
| 2 | liu | 577397908 | 25 |
+----+------+------------+-----+
2 rows in set (0.00 sec)
#id是自增长可加可不加
mysql> insert into oldchen (name ,qq,age) values(‘wang‘,‘1314‘,18);
Query OK, 1 row affected (0.00 sec)
mysql> select id ,name, qq, age from oldchen;
+----+------+------------+-----+
| id | name | qq | age |
+----+------+------------+-----+
| 1 | chen | 1010235677 | 18 |
| 2 | liu | 577397908 | 25 |
| 3 | wang | 1314 | 18 |
+----+------+------------+-----+
3 rows in set (0.00 sec)
mysql> insert into oldchen (name ,qq,age) values(‘qiuan‘,‘553208‘,18);
Query OK, 1 row affected (0.00 sec)
mysql> select id ,name ,qq,age from oldchen;
+----+-------+------------+-----+
| id | name | qq | age |
+----+-------+------------+-----+
| 1 | chen | 1010235677 | 18 |
| 2 | liu | 577397908 | 25 |
| 3 | wang | 1314 | 18 |
| 4 | qiuan | 553208 | 18 |
+----+-------+------------+-----+
4 rows in set (0.00 sec)
- update 更新数据语句使用 (update set) #这条命令相当危险，注意一定要加where条件
mysql> select id ,name ,qq,age from oldchen;
+----+-------+------------+-----+
| id | name | qq | age |
+----+-------+------------+-----+
| 1 | chen | 1010235677 | 18 |
| 2 | liu | 577397908 | 25 |
| 3 | wang | 1314 | 18 |
| 4 | qiuan | 553208 | 18 |
+----+-------+------------+-----+
4 rows in set (0.00 sec)
mysql> update oldchen set qq=‘1314520‘ where id=4;
Query OK, 1 row affected (1.10 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select id ,name ,qq,age from oldchen;
+----+-------+------------+-----+
| id | name | qq | age |
+----+-------+------------+-----+
| 1 | chen | 1010235677 | 18 |
| 2 | liu | 577397908 | 25 |
| 3 | wang | 1314 | 18 |
| 4 | qiuan | 1314520 | 18 |
+----+-------+------------+-----+
4 rows in set (0.00 sec)
mysql> update oldchen set name=‘wanglin‘ where id=3;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select id ,name ,qq,age from oldchen;
+----+---------+------------+-----+
| id | name | qq | age |
+----+---------+------------+-----+
| 1 | chen | 1010235677 | 18 |
| 2 | liu | 577397908 | 25 |
| 3 | wanglin | 1314 | 18 |
| 4 | qiuan | 1314520 | 18 |
+----+---------+------------+-----+
4 rows in set (0.00 sec)
- delete 删除行数据语句使用(delete from ) 这条命令相当危险，注意一定要加where条件
mysql> select id ,name ,qq,age from oldchen;
+----+---------+------------+-----+
| id | name | qq | age |
+----+---------+------------+-----+
| 1 | chen | 1010235677 | 18 |
| 2 | liu | 577397908 | 25 |
| 3 | wanglin | 1314 | 18 |
| 4 | qiuan | 1314520 | 18 |
+----+---------+------------+-----+
4 rows in set (0.00 sec)
mysql> delete from oldchen where id=4;
Query OK, 1 row affected (0.01 sec)
mysql> select id ,name ,qq,age from oldchen;
+----+---------+------------+-----+
| id | name | qq | age |
+----+---------+------------+-----+
| 1 | chen | 1010235677 | 18 |
| 2 | liu | 577397908 | 25 |
| 3 | wanglin | 1314 | 18 |
+----+---------+------------+-----+
3 rows in set (0.00 sec)
mysql> delete from oldchen where name=‘wanglin‘;
Query OK, 1 row affected (0.00 sec)
mysql> select id ,name ,qq,age from oldchen;
+----+------+------------+-----+
| id | name | qq | age |
+----+------+------------+-----+
| 1 | chen | 1010235677 | 18 |
| 2 | liu | 577397908 | 25 |
+----+------+------------+-----+
2 rows in set (0.00 sec)
#清空表数据而不改变表结构属性
delete from oldchen; (逻辑逻辑删除，语句一条一条执行，效率慢)
truncate table oldchen (物理清空 ，效率高)
DELETE 和 TRUNCATE 区别
1、 DELETE 逻辑逐行删除，不会降低自增长的起始值。
效率很低，碎片较多，会影响到性能
2、TRUNCATE ，属于物理删除，将表段中的区进行清空，不会产生碎片。性能较高
- 生产需求：使用update替代delete，进行伪删除
#首先添加一个状态列（1代表存在，0代表删除）
mysql> alter table oldchen add state tinyint unsigned not null default 1 comment ‘状态‘;
Query OK, 0 rows affected (0.39 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> select id ,name ,qq,age ,state from oldchen;
+----+------+------------+-----+-------+
| id | name | qq | age | state |
+----+------+------------+-----+-------+
| 1 | chen | 1010235677 | 18 | 1 |
| 2 | liu | 577397908 | 25 | 1 |
+----+------+------------+-----+-------+
2 rows in set (0.00 sec)
#利用update将某一列的状态改为0 模拟设置伪删除
mysql> update oldchen set state=0 where id=2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select id ,name ,qq,age ,state from oldchen;
+----+------+------------+-----+-------+
| id | name | qq | age | state |
+----+------+------------+-----+-------+
| 1 | chen | 1010235677 | 18 | 1 |
| 2 | liu | 577397908 | 25 | 0 |
+----+------+------------+-----+-------+
2 rows in set (0.00 sec)
#业务语句修改，利用select条件查询
mysql> select id,name,age,qq,state from oldchen where state=1;
+----+------+-----+------------+-------+
| id | name | age | qq | state |
+----+------+-----+------------+-------+
| 1 | chen | 18 | 1010235677 | 1 |
+----+------+-----+------------+-------+
1 row in set (0.00 sec)
#这样你就看不到状态为0的数据，这就是伪删除
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