1 数据库相关
-- --------------------表相关的----------
-- 查看字符集
show variables like '%character%';
show databases;
# 创建数据库
create database test2;
# 删除数据库
drop database test2;
show databases;
#查看当前使用的数据库
select database();2 用户相关
-- --------------------用户授权相关的----------
#切换数据库
use test;
select user();
# 会报错：[42000][1064] You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'identified by 'user12345'' at line
# mysql8.0版本为新版本，需要先创建用户再分配权限，不能和旧版本一样
# MySQL5.7版本可以使用
grant all on test.* to 'user1'@'%' identified by 'user12345';
FLUSH PRIVILEGES;
# mysql8.0版本上需要使用：
# 1、创建新用户
create user 'user1'@'%' identified by 'user12345';
# 2、刷新权限
flush privileges;
# 3、赋予权限，%标识不限制主机
grant all on test.* to 'user1'@'%';
# 4、刷新权限
flush privileges;
# 5、查看用户
select host,user,plugin,authentication_string from mysql.user;
# 收回权限
revoke insert on test.* from 'user1'@'%';
# 删除用户
drop user 'user1'@'%';
# 查看权限，all包含的权限有：SELECT,INSERT, UPDATE, DELETE, CREATE, DROP, REFERENCES, INDEX, ALTER, CREATE TEMPORARY TABLES, LOCK TABLES, EXECUTE, CREATE VIEW, SHOW VIEW, CREATE ROUTINE, ALTER ROUTINE, EVENT, TRIGGER
show grants for 'user1'@'%';3 数据表相关
-- --------------------表相关的----------
CREATE TABLE business_logs (
id BIGINT PRIMARY KEY AUTO_INCREMENT,
timestamp DATETIME NOT NULL,
user_id INT NOT NULL,
action VARCHAR(255) NOT NULL,
description TEXT,
ip_address VARCHAR(50),
log_level VARCHAR(20)
);
-- 查看建表语句
show create table business_logs;
-- 查看所有表
show tables;
-- 查看表结构
desc business_logs;
select *
from business_logs;
-- 插入数据到表中
insert into business_logs (timestamp, user_id, action, description) values (sysdate(), 1, 'cap', '查询成功');
insert into business_logs (timestamp, user_id, action, description) values (sysdate(), 2, 'del', '删除成功');
insert into business_logs (timestamp, user_id, action, description) values (sysdate(), 1, 'add', '添加成功');
-- 更新表
update business_logs set action = 'delete', description = '删除成功' where id = 1;
-- 删除表中的数据
delete from business_logs where id = 1;
-- 查询语句
/*
查询关键字：
select
字段列表
from
表名
where
查询条件
group by
分组字段
having
分组后条件字段
order by
排序字段
limit
分页参数
*/
3.1 聚合函数
聚合函数（count、sum、max、min、avg）使用
-- 删除表
drop table if exists user;
-- 创建表
create table user (
id int primary key auto_increment,
name varchar(20) not null,
age int not null,
city varchar(20) default null
);
-- 插入数据
insert into user (name, age, city) values ('张三', 20, '北京'), ('李四', 22, '上海'), ('王五', 20, '北京');
select * from user;
-- 查询各个城市的人数
select city, count(*) as usr_num from user group by city;
-- 查询各个城市最小的年龄
select city, min(age) as age from user group by city;
-- 查询平均年龄
select avg(age) as age from user;
-- 查询年龄总和
select sum(age) as sum_age from user;
注意：所有的null值不参与聚合函数的运算
3.2 分组查询
基本语法
select 字段 from 表名 [where 条件] group by 分组字段 [having 分组后过滤条件]
where和having的区别
- 执行时机不同：where是分组之前进行过滤，不满足where条件的数据不参与分组，而having是分组之后对结果进行过滤
- 判断条件不同：where不能对聚合函数进行判断，而having可以
insert into user (name, age, city) values ('张三2', 20, '北京'), ('李四2', 22, '上海'), ('王五2', 20, '北京');
insert into user (name, age, city) values ('夏明', 30, '北京'), ('李红', 32, '北京'), ('张飞', 56, '西安');
-- 查询年龄小于等于30的人员
select * from user where age <= 30;
-- 查询各个城市中，年龄小于30的人数
select city, count(*) as user_num from user where age <= 30 group by city;
-- 查询年龄小于等于30的人员，按照城市进行分组，获取人员数量大于3的城市
select city, count(*) as user_num from user where age <= 30 group by city having user_num > 3;
注意：
- 执行顺序：where > 聚合函数 > having
- 分组之后，查询的字段一般为聚合函数和分组字段，其他字段不能查询
3.3 排序查询
语法
select 字段 from 表名 order by 字段1 排序1, 字段2 排序2;
- 支持多字段排序
- 排序方式
-
- ASC：升序（默认值）
- DESC：降序
-- 根据年龄进行排序
select * from user order by age;
-- 根据年龄进行排序，年龄相同时根据id倒序排序
select * from user order by age, id desc;3.4 分页查询
使用 limit 关键字
语法
select 字段 from 表名 limit 起始索引, 查询记录数量;
-- 查询第1页，每页显示5条
select * from user limit 0,5;
-- 查询第2页，每页显示5条
select * from user limit 5,5;4 其他命令
4.1 查看是否忽略大小写，1为忽略
show global variables like '%lower_case%';4.2 查看主从状态
查看主服务状态，在主从配置时，查看主服务信息时使用
show master status;
查看从服务状态，在主从配置时，查看从服务状态时使用
show slave status;使用示例：
mysql> show slave status \G
*************************** 1. row ***************************
Slave_IO_State: Waiting for source to send event
Master_Host: 10.0.24.10
Master_User: slave
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: binlog.000007
Read_Master_Log_Pos: 2050
Relay_Log_File: 5d5ab079a223-relay-bin.000004
Relay_Log_Pos: 546
Relay_Master_Log_File: binlog.000007
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Replicate_Do_DB:
Replicate_Ignore_DB:
Replicate_Do_Table:
Replicate_Ignore_Table:
Replicate_Wild_Do_Table:
Replicate_Wild_Ignore_Table:
Last_Errno: 0
Last_Error:
Skip_Counter: 0
Exec_Master_Log_Pos: 2050
Relay_Log_Space: 1574
Until_Condition: None
Until_Log_File:
Until_Log_Pos: 0
Master_SSL_Allowed: No
Master_SSL_CA_File:
Master_SSL_CA_Path:
Master_SSL_Cert:
Master_SSL_Cipher:
Master_SSL_Key:
Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
Last_IO_Errno: 0
Last_IO_Error:
Last_SQL_Errno: 0
Last_SQL_Error:
Replicate_Ignore_Server_Ids:
Master_Server_Id: 1
Master_UUID: 60a0cb5e-1a2f-11ee-8244-0242ac110003
Master_Info_File: mysql.slave_master_info
SQL_Delay: 0
SQL_Remaining_Delay: NULL
Slave_SQL_Running_State: Replica has read all relay log; waiting for more updates
Master_Retry_Count: 86400
Master_Bind:
Last_IO_Error_Timestamp:
Last_SQL_Error_Timestamp:
Master_SSL_Crl:
Master_SSL_Crlpath:
Retrieved_Gtid_Set:
Executed_Gtid_Set:
Auto_Position: 0
Replicate_Rewrite_DB:
Channel_Name:
Master_TLS_Version:
Master_public_key_path:
Get_master_public_key: 0
Network_Namespace:
1 row in set, 1 warning (0.00 sec)4.3 查看字符集
-- 查看字符集
show variables like '%character%';
4.4 查看连接数
-- 获取最大数据库连接数
show variables like 'max_connections';
-- 获取当前数据库连接数
show status like 'threads_connected';
-- 获取最大数据库连接数和当前数据库连接数
select @@max_connections as maxconnections, count(*) as currentconnections from information_schema.processlist;4.5 查看当前进程
show processlist;
# 找出所有执行时间超过 5 分钟的线程，拼凑出 kill 语句，方便后面查杀
select concat('kill ', id, ';')
from information_schema.processlist
where Command != 'Sleep' and Time > 300 order by Time desc;关注我，我们一起学习。
