文章目录
【1】子查询(sub query)
举例如下:
先来摆出我们的两张表(一张员工表emp,一张部门表dept)
mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+--------+-----------+------+------------+---------+---------+--------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-07-13 | 3000.00 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 |
| 7876 | ADAMS | CLERK | 7788 | 1987-07-13 | 1100.00 | NULL | 20 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |
+-------+--------+-----------+------+------------+---------+---------+--------+
14 rows in set (0.00 sec)
mysql> select * from dept;
+--------+------------+----------+
| deptno | dname | loc |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
4 rows in set (0.08 sec)
情况1:把子查询当做一个值:分解问题
求员工表内的最高工资的员工信息
mysql> select max(sal) from emp;
+----------+
| max(sal) |
+----------+
| 5000.00 |
+----------+
1 row in set (0.23 sec)
mysql> select * from emp where sal=5000;
+-------+-------+-----------+------+------------+---------+------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+-------+-----------+------+------------+---------+------+--------+
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
+-------+-------+-----------+------+------------+---------+------+--------+
1 row in set (0.00 sec)
首先我们经过找出最大工资值5000 找的其实是一个值。之后由最大值5000来找的对应的员工
用一句sql 语句来写就是:
mysql> select * from emp where sal=(select max(sal) from emp);
+-------+-------+-----------+------+------------+---------+------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+-------+-----------+------+------------+---------+------+--------+
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
+-------+-------+-----------+------+------------+---------+------+--------+
1 row in set (0.04 sec)
情况2:把子查询当做一张表
求每个部门的最高工资的员工
//首先求出的是每个部门的最高工资
mysql> select max(sal) from emp group by deptno;
+----------+
| max(sal) |
+----------+
| 5000.00 |
| 3000.00 |
| 2850.00 |
+----------+
3 rows in set (0.07 sec)
//按照内连接的方法,将emp表和生成的新表进行连接,根据条件获取对应的值
mysql> select * from emp e inner join (select max(sal) msal,deptno from emp group by deptno) a on e.deptno=a.deptno and e.sal=a.msal;
+-------+-------+-----------+------+------------+---------+------+--------+---------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno | msal | deptno |
+-------+-------+-----------+------+------------+---------+------+--------+---------+--------+
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 | 2850.00 | 30 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-07-13 | 3000.00 | NULL | 20 | 3000.00 | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 | 5000.00 | 10 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | 3000.00 | 20 |
+-------+-------+-----------+------+------------+---------+------+--------+---------+--------+
4 rows in set (0.06 sec)
【2】case when
可以配合select 使用,把一列的取值根据不同的条件进行翻译,类似于java 中的if else if
== 语法 :
case
when 条件1 then 结果1
when 条件2 then 结果2
…
else 结果n
end
==
举例如下:
mysql> select sc.sid,sc.cid,sc.score,
-> (case
-> when sc.score<50 then '不及格'
-> when sc.score>50 and sc.score<80 then '良好'
-> else '优秀'
-> end
-> )等级
-> from sc;
+------+-----+-------+--------+
| sid | cid | score | 等级 |
+------+-----+-------+--------+
| 1001 | 1 | 50 | 优秀 |
| 1002 | 1 | 90 | 优秀 |
| 1003 | 1 | 59 | 良好 |
| 1004 | 1 | 80 | 优秀 |
| 1005 | 1 | 50 | 优秀 |
| 1006 | 1 | 60 | 良好 |
| 1007 | 1 | 100 | 优秀 |
| 1001 | 2 | 70 | 良好 |
| 1003 | 2 | 70 | 良好 |
| 1004 | 2 | 70 | 良好 |
| 1005 | 2 | 50 | 优秀 |
| 1001 | 3 | 80 | 优秀 |
| 1005 | 3 | 100 | 优秀 |
| 1001 | 4 | 90 | 优秀 |
| 1002 | 4 | 55 | 良好 |
| 1005 | 4 | 30 | 不及格 |
| 1007 | 4 | 58 | 良好 |
| 1001 | 5 | 80 | 优秀 |
| 1001 | 6 | 80 | 优秀 |
| 1002 | 6 | 90 | 优秀 |
| 1007 | 6 | 90 | 优秀 |
| 1001 | 7 | 100 | 优秀 |
| 1002 | 7 | 80 | 优秀 |
| 1007 | 7 | 80 | 优秀 |
+------+-----+-------+--------+
24 rows in set (0.07 sec)
【3】DCL
grant授权
语法:
创建一个新用户:
create user 用户名 identified by ‘密码’
mysql> create user 'user1' identified by 'user1';
Query OK, 0 rows affected (0.27 sec)
此时这个用户就可以使用了,只不过他没有操作我们数据库的权限:
这个账号和密码是,可以登录的:
C:\Windows\system32>mysql -uuser1 -p
Enter password: *****
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 10
Server version: 5.7.24 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.
我们来试着查询一下:可以看到除了一个mysql自带的数据库其他都不显示
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
+--------------------+
1 row in set (0.00 sec)
授权举例如下:比如将查询student库中的查询权限授权给新用户:
grant select on school.* to user1;
Query OK, 0 rows affected (0.00 sec)
意思就是说,将student表下所有数据的查询权限都授予user1表
我们这个时候再来登录user1的账号来查看一下:
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| school |
+--------------------+
2 rows in set (0.00 sec)
mysql> use school;
Database changed
mysql> show tables;
+------------------+
| Tables_in_school |
+------------------+
| course |
| dept |
| emp |
| fs |
| sc |
| student |
| teacher |
| test |
| test2 |
| user |
+------------------+
10 rows in set (0.00 sec)
mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+--------+-----------+------+------------+---------+---------+--------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-07-13 | 3000.00 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 |
| 7876 | ADAMS | CLERK | 7788 | 1987-07-13 | 1100.00 | NULL | 20 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |
+-------+--------+-----------+------+------------+---------+---------+--------+
14 rows in set (0.00 sec)
但是如果没有给user1插入权限的话,执行insert into的话就会报错
mysql> insert into student(sid,sname,birthday,sex) values(1,'孙梅','1999-9-9','女');
ERROR 1142 (42000): INSERT command denied to user 'user1'@'localhost' for table 'student'
revoke回收权限
语法
revoke 权限 from 用户名
mysql> revoke select on school.* from user1;
Query OK, 0 rows affected (0.00 sec)
然后我们在user1里面再次查看
C:\Windows\system32>mysql -u user1 -p
Enter password: *****
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 19
Server version: 5.7.24 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> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
+--------------------+
1 row in set (0.00 sec)
最终发现已经没有查询的权限了
【4】事务和锁(TCL事物控制语言)
事务
首先我们来了解一下事务和锁的引入
transaction事务:把多余sql语句视为一个整体执行
举例:两个人同时在一个表中操作同一条数据
mysql的事务控制:默认情况下是一条语句一个事务
如果要多条语句一个事务,需要begin commit rollback进行控制事务
举例如下:
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into student(sid,sname,birthday,sex) values(1,'筱筱','1999-09-08','女');
Query OK, 1 row affected (0.08 sec)
mysql> select * from student;
+------+-------+------------+------+
| sid | sname | birthday | sex |
+------+-------+------------+------+
| 1 | 筱筱 | 1999-09-08 | 女 |
| 1001 | 张三 | 1990-10-10 | 男 |
| 1002 | 李四 | 1981-10-10 | 男 |
| 1003 | 王五 | 1981-11-10 | 女 |
| 1004 | 赵六 | 1988-10-10 | 男 |
| 1005 | 孙七 | 1989-01-10 | 女 |
| 1006 | 周八 | 1990-10-10 | 男 |
| 1007 | 张三 | 1990-06-10 | 女 |
| 1010 | 夏木 | 1998-08-08 | 女 |
+------+-------+------------+------+
9 rows in set (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.08 sec)
mysql> select * from student;
+------+-------+------------+------+
| sid | sname | birthday | sex |
+------+-------+------------+------+
| 1 | 筱筱 | 1999-09-08 | 女 |
| 1001 | 张三 | 1990-10-10 | 男 |
| 1002 | 李四 | 1981-10-10 | 男 |
| 1003 | 王五 | 1981-11-10 | 女 |
| 1004 | 赵六 | 1988-10-10 | 男 |
| 1005 | 孙七 | 1989-01-10 | 女 |
| 1006 | 周八 | 1990-10-10 | 男 |
| 1007 | 张三 | 1990-06-10 | 女 |
| 1010 | 夏木 | 1998-08-08 | 女 |
+------+-------+------------+------+
9 rows in set (0.00 sec)
大家发现了什么????????为什么在没有commit之前表里都已经有了我们插入的数据???????难道commit没什么用???????当然有用,大家请看下面
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into student(sid,sname,birthday,sex) values(2,'刘允','1999-09-18','男');
Query OK, 1 row affected (0.00 sec)
mysql> select * from student;
+------+-------+------------+------+
| sid | sname | birthday | sex |
+------+-------+------------+------+
| 1 | 筱筱 | 1999-09-08 | 女 |
| 2 | 刘允 | 1999-09-18 | 男 |
| 1001 | 张三 | 1990-10-10 | 男 |
| 1002 | 李四 | 1981-10-10 | 男 |
| 1003 | 王五 | 1981-11-10 | 女 |
| 1004 | 赵六 | 1988-10-10 | 男 |
| 1005 | 孙七 | 1989-01-10 | 女 |
| 1006 | 周八 | 1990-10-10 | 男 |
| 1007 | 张三 | 1990-06-10 | 女 |
| 1010 | 夏木 | 1998-08-08 | 女 |
+------+-------+------------+------+
10 rows in set (0.00 sec)
mysql> rollback;
Query OK, 0 rows affected (0.11 sec)
mysql> select * from student;
+------+-------+------------+------+
| sid | sname | birthday | sex |
+------+-------+------------+------+
| 1 | 筱筱 | 1999-09-08 | 女 |
| 1001 | 张三 | 1990-10-10 | 男 |
| 1002 | 李四 | 1981-10-10 | 男 |
| 1003 | 王五 | 1981-11-10 | 女 |
| 1004 | 赵六 | 1988-10-10 | 男 |
| 1005 | 孙七 | 1989-01-10 | 女 |
| 1006 | 周八 | 1990-10-10 | 男 |
| 1007 | 张三 | 1990-06-10 | 女 |
| 1010 | 夏木 | 1998-08-08 | 女 |
+------+-------+------------+------+
9 rows in set (0.00 sec)
这下清楚了吧,只要我们没有commit,之前的所有数据都是可以通过rollback反悔的
锁
分类:
【1】InnoDB行级锁:以行为单位的锁,只要两个客户端更新的是不同行的数据,则互不干扰
【2】MyISAM表锁:锁住的是整个表
这里我们要了解的是mysql对查询有特别的优化:不用锁也能实现并发访问,即多版本并发访问
这里举加共享锁的例子:
首先加的是一个共享锁,,别人也能查,能加共享锁。但是不能再加排他锁
mysql> select * from student lock in share mode;
+------+-------+------------+------+
| sid | sname | birthday | sex |
+------+-------+------------+------+
| 1 | 筱筱 | 1999-09-08 | 女 |
| 1001 | 张三 | 1990-10-10 | 男 |
| 1002 | 李四 | 1981-10-10 | 男 |
| 1003 | 王五 | 1981-11-10 | 女 |
| 1004 | 赵六 | 1988-10-10 | 男 |
| 1005 | 孙七 | 1989-01-10 | 女 |
| 1006 | 周八 | 1990-10-10 | 男 |
| 1007 | 张三 | 1990-06-10 | 女 |
| 1010 | 夏木 | 1998-08-08 | 女 |
+------+-------+------------+------+
9 rows in set (0.00 sec)
接下来举一个加排它锁的例子
添加排他锁,别人就不能加共享锁和排他锁了
mysql> select * from student for update;
+------+-------+------------+------+
| sid | sname | birthday | sex |
+------+-------+------------+------+
| 1 | 筱筱 | 1999-09-08 | 女 |
| 1001 | 张三 | 1990-10-10 | 男 |
| 1002 | 李四 | 1981-10-10 | 男 |
| 1003 | 王五 | 1981-11-10 | 女 |
| 1004 | 赵六 | 1988-10-10 | 男 |
| 1005 | 孙七 | 1989-01-10 | 女 |
| 1006 | 周八 | 1990-10-10 | 男 |
| 1007 | 张三 | 1990-06-10 | 女 |
| 1010 | 夏木 | 1998-08-08 | 女 |
+------+-------+------------+------+
9 rows in set (0.00 sec)
【5】事务隔离性
不同的事物有不同的隔离级别:
隔离级别越低,并发性越好,但数据的一致性就差
隔离级别越高,并发性越差,但数据的一致性就高
有四种级别:读未提交<读提交<可重复读<序列化读
错误级别(由高到低):脏读,不可重复读,幻读
【6】其他sql总结
查看建表语句:show create table 表名;
把表行转列:\G结束可以取代“;”
查看系统变量的值:举例:select @@port;查看的是端口号