MySQL学习笔记第二周(一)
目录
1、简单的查询语句(DQL)
1.1 查询一个字段
1.2 列起别名
2、条件查询
2.1 语法结构
2.2 大于,小于,等于及变式使用
2.3 between and 使用
2.4 is null和is not null
2.5 and和or
2.6 in
2.7 模糊查询like
3、排序(升序、降序)
4、Limit分页操作
4.1 limit的使用:
4.2 limit执行顺序
上次对MySQL进行了一个简单的入门了解,现在开始进行深入学习。
1、简单的查询语句(DQL)
1.1 查询一个字段
语法格式:select 字段名1,字段名2,字段名3,....from 表名字
提示: 1、任何一条sql语句以";"结尾
2、sql语句不区分大小写。
3、字段可以进行数学运算
1、查询员工的年薪? select ename,sal*12 from emp;
mysql> select ename,sal*12 from emp;
+--------+----------+
| ename | sal*12 |
+--------+----------+
| SMITH | 9600.00 |
| ALLEN | 19200.00 |
| WARD | 15000.00 |
| JONES | 35700.00 |
| MARTIN | 15000.00 |
| BLAKE | 34200.00 |
| CLARK | 29400.00 |
| SCOTT | 36000.00 |
| KING | 60000.00 |
| TURNER | 18000.00 |
| ADAMS | 13200.00 |
| JAMES | 11400.00 |
| FORD | 36000.00 |
| MILLER | 15600.00 |
+--------+----------+
1.2 列起别名
使用as来进行起别名
1、给查询结果的列重命名?
select ename,sal*12 as yearsal from emp;
2、别名中有中文?
select ename,sal*12 as 年薪 from emp; //错误
select ename,sal*12 as '年薪' from emp; //字符串用单引号括起来,在MySQL中可以用双引号,但是在其他数据库管理系统中不通用,因此尽量别用。
mysql> select ename,sal*12 as yearsal from emp;
+--------+----------+
| ename | yearsal |
+--------+----------+
| SMITH | 9600.00 |
| ALLEN | 19200.00 |
| WARD | 15000.00 |
| JONES | 35700.00 |
| MARTIN | 15000.00 |
| BLAKE | 34200.00 |
| CLARK | 29400.00 |
| SCOTT | 36000.00 |
| KING | 60000.00 |
| TURNER | 18000.00 |
| ADAMS | 13200.00 |
| JAMES | 11400.00 |
| FORD | 36000.00 |
| MILLER | 15600.00 |
+--------+----------+mysql> select ename,sal*12 as '年薪' from emp;
+--------+----------+
| ename | 年薪 |
+--------+----------+
| SMITH | 9600.00 |
| ALLEN | 19200.00 |
| WARD | 15000.00 |
| JONES | 35700.00 |
| MARTIN | 15000.00 |
| BLAKE | 34200.00 |
| CLARK | 29400.00 |
| SCOTT | 36000.00 |
| KING | 60000.00 |
| TURNER | 18000.00 |
| ADAMS | 13200.00 |
| JAMES | 11400.00 |
| FORD | 36000.00 |
| MILLER | 15600.00 |
+--------+----------+
3、如果别名有空格?
select deptno,dname 'dept naem' from dept//也要加单引号
在mysql中,所有的字符串都要加单引号
4、as关键字可以省略?
select empon,ename,sal*12 yearsal from emp;
mysql> select empno,ename,sal*12 yearsal from emp;
+-------+--------+----------+
| empno | ename | yearsal |
+-------+--------+----------+
| 7369 | SMITH | 9600.00 |
| 7499 | ALLEN | 19200.00 |
| 7521 | WARD | 15000.00 |
| 7566 | JONES | 35700.00 |
| 7654 | MARTIN | 15000.00 |
| 7698 | BLAKE | 34200.00 |
| 7782 | CLARK | 29400.00 |
| 7788 | SCOTT | 36000.00 |
| 7839 | KING | 60000.00 |
| 7844 | TURNER | 18000.00 |
| 7876 | ADAMS | 13200.00 |
| 7900 | JAMES | 11400.00 |
| 7902 | FORD | 36000.00 |
| 7934 | MILLER | 15600.00 |
+-------+--------+----------+
5、查询全部字段?
select * from emp; //实际开发中不建议使用*,效率较低。
2、条件查询
2.1 语法结构
执行顺序:先from,然后where,最后select
2.2 大于,小于,等于及变式使用
1、查询工资等于5000的员工姓名?
select ename from emp where sal=5000;
mysql> select ename from emp where sal=5000;
+-------+
| ename |
+-------+
| KING |
+-------+
2、查询SMITH的工资?
select sal from emp where ename='SMITH';
mysql> select sal from emp where ename='SMITH';
+--------+
| sal |
+--------+
| 800.00 |
+--------+
3、找到工资高于3000的员工?
select ename,sal from emp where sal>3000;
mysql> select ename,sal from emp where sal>3000;
+-------+---------+
| ename | sal |
+-------+---------+
| KING | 5000.00 |
+-------+---------+
4、变式: select ename,sal from emp where sal>=3000;
select ename,sal from emp where sal<=3000;
select ename,sal from emp where sal<>3000;(大于小于就是不等于,也可以用!=)
mysql> select ename,sal from emp where sal<>3000;
+--------+---------+
| ename | sal |
+--------+---------+
| SMITH | 800.00 |
| ALLEN | 1600.00 |
| WARD | 1250.00 |
| JONES | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| KING | 5000.00 |
| TURNER | 1500.00 |
| ADAMS | 1100.00 |
| JAMES | 950.00 |
| MILLER | 1300.00 |
+--------+---------+
12 rows in set (0.00 sec)
5、找到工资在1100和3000的员工,包括1100和3000?
第一种写法:select ename,sal from emp where sal>=1100 and sal<=3000;
第二种写法:select ename,sal from emp where sal between 1100 and 3000;(一定是左边是小数字,右边是大数字)
//下面两者是等效的mysql> select ename,sal from emp where sal>=1100 and sal<=3000;
+--------+---------+
| ename | sal |
+--------+---------+
| ALLEN | 1600.00 |
| WARD | 1250.00 |
| JONES | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| TURNER | 1500.00 |
| ADAMS | 1100.00 |
| FORD | 3000.00 |
| MILLER | 1300.00 |
+--------+---------+//一定是左边是小数字,右边是大数字
mysql> select ename,sal from emp where sal between 1100 and 3000;
+--------+---------+
| ename | sal |
+--------+---------+
| ALLEN | 1600.00 |
| WARD | 1250.00 |
| JONES | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| TURNER | 1500.00 |
| ADAMS | 1100.00 |
| FORD | 3000.00 |
| MILLER | 1300.00 |
+--------+---------+
2.3 between and 使用
between and除了可以使用在数字方面以外,还可以使用在字符串方面
select ename from emp where ename between 'A’ and 'C';(首字母范围)
注意,使用时是左闭右开。
mysql> select ename from emp where ename between 'A' and 'C';
+-------+
| ename |
+-------+
| ALLEN |
| BLAKE |
| ADAMS |
+-------+
select ename from emp where ename between 'A’ and 'D';
mysql> select ename from emp where ename between 'A' and 'D';
+-------+
| ename |
+-------+
| ALLEN |
| BLAKE |
| CLARK |
| ADAMS |
+-------+
2.4 is null和is not null
1、找出哪些人津贴为NULL:
在数据库当中NULL不是一个值,代表什么也没有,为空。
空不是一个值,因此不能使用“=”来衡量。
必须使用is null和is not null;
select ename,sal,comm from emp where comm = null; //错误,NULL不是一个值
select ename,sal,comm from emp where comm is null;
mysql> select ename,sal,comm from emp where comm is null;
+--------+---------+------+
| ename | sal | comm |
+--------+---------+------+
| SMITH | 800.00 | NULL |
| JONES | 2975.00 | NULL |
| BLAKE | 2850.00 | NULL |
| CLARK | 2450.00 | NULL |
| SCOTT | 3000.00 | NULL |
| KING | 5000.00 | NULL |
| ADAMS | 1100.00 | NULL |
| JAMES | 950.00 | NULL |
| FORD | 3000.00 | NULL |
| MILLER | 1300.00 | NULL |
+--------+---------+------+
2、找出哪些人津贴不为NULL:
select ename,sal,comm from emp where comm is not null;
mysql>select ename,sal,comm from emp where comm is not null;
+--------+---------+---------+
| ename | sal | comm |
+--------+---------+---------+
| ALLEN | 1600.00 | 300.00 |
| WARD | 1250.00 | 500.00 |
| MARTIN | 1250.00 | 1400.00 |
| TURNER | 1500.00 | 0.00 |
3、找出哪些人没有津贴?
select ename,sal,comm from emp where comm is null or comm=0;
mysql> select ename,sal,comm from emp where comm is null or comm=0;
+--------+---------+------+
| ename | sal | comm |
+--------+---------+------+
| SMITH | 800.00 | NULL |
| JONES | 2975.00 | NULL |
| BLAKE | 2850.00 | NULL |
| CLARK | 2450.00 | NULL |
| SCOTT | 3000.00 | NULL |
| KING | 5000.00 | NULL |
| TURNER | 1500.00 | 0.00 |
| ADAMS | 1100.00 | NULL |
| JAMES | 950.00 | NULL |
| FORD | 3000.00 | NULL |
| MILLER | 1300.00 | NULL |
+--------+---------+------+
2.5 and和or
and:并且 or:或者
1、找出工作岗位是MANAGER和SALESMAN的员工
select ename,job from emp where job='MANAGER' or job='SALESMAN';
mysql> select ename,job from emp where job='MANAGER' or job='SALESMAN';
+--------+----------+
| ename | job |
+--------+----------+
| ALLEN | SALESMAN |
| WARD | SALESMAN |
| JONES | MANAGER |
| MARTIN | SALESMAN |
| BLAKE | MANAGER |
| CLARK | MANAGER |
| TURNER | SALESMAN |
+--------+----------+
2、and和or联合起来用:找出薪资大于1000的并且部门编号是20或30部门的员工
select ename,sal,deptno from emp where sal>1000 and deptno=20 or deptno=30;
mysql> select ename,sal,deptno from emp where sal>1000 and deptno=20 or deptno=30;
+--------+---------+--------+
| ename | sal | deptno |
+--------+---------+--------+
| ALLEN | 1600.00 | 30 |
| WARD | 1250.00 | 30 |
| JONES | 2975.00 | 20 |
| MARTIN | 1250.00 | 30 |
| BLAKE | 2850.00 | 30 |
| SCOTT | 3000.00 | 20 |
| TURNER | 1500.00 | 30 |
| ADAMS | 1100.00 | 20 |
| JAMES | 950.00 | 30 |//不是我们所想要的
| FORD | 3000.00 | 20 |
+--------+---------+--------+
注意:and的优先级比or高,上面的代码会先结合sal>1000 and deptno=20并查找出来,然后再进行or deptno=30;把所有deptno为30的全部查找出来。
select ename,sal,deptno from emp where sal>1000 and (deptno=20 or deptno=30);
mysql> select ename,sal,deptno from emp where sal>1000 and (deptno=20 or deptno=30);
+--------+---------+--------+
| ename | sal | deptno |
+--------+---------+--------+
| ALLEN | 1600.00 | 30 |
| WARD | 1250.00 | 30 |
| JONES | 2975.00 | 20 |
| MARTIN | 1250.00 | 30 |
| BLAKE | 2850.00 | 30 |
| SCOTT | 3000.00 | 20 |
| TURNER | 1500.00 | 30 |
| ADAMS | 1100.00 | 20 |
| FORD | 3000.00 | 20 |
+--------+---------+--------+
Tips:以后运算符优先级不确定的时候加小括号,别去背优先级。
2.6 in
in等同于or
1、找出工作岗位是MANAGER和SALESMAN的员工?
select ename,job from emp where job in('SALESMAN','MANAGER');
mysql> select ename,job from emp where job in('SALESMAN','MANAGER');
+--------+----------+
| ename | job |
+--------+----------+
| ALLEN | SALESMAN |
| WARD | SALESMAN |
| JONES | MANAGER |
| MARTIN | SALESMAN |
| BLAKE | MANAGER |
| CLARK | MANAGER |
| TURNER | SALESMAN |
+--------+----------+
select ename,job from emp where sal in(800,5000); //in后面的每一个值不是区间是具体的值,这里的意思是工资等于800和5000的
mysql> select ename,job from emp where sal in(800,5000);
+-------+-----------+
| ename | job |
+-------+-----------+
| SMITH | CLERK |
| KING | PRESIDENT |
+-------+-----------+
not in:不在这几个值当中
select ename,job from emp where sal not in(800,5000);
2.7 模糊查询like
1、找出名字中带有o的?
在模糊查询当中,必须掌握两个特殊的符号,一个是%,一个_%代表任意多个字符,_代表任意一个字符。(注意,使用的时候整体都要被' '包括)
select ename from emp where ename like '%o%'; //%是任意多个字符,前后都是任意多个字符,只要有o就查找到
mysql> select ename from emp where ename like '%o%';
+-------+
| ename |
+-------+
| JONES |
| SCOTT |
| FORD |
+-------+
2、找到名字中第二个字母是A的?//第二个是A,第一个应该是任意一个字符
select ename from emp where ename like '_A%';
mysql> select ename from emp where ename like '_A%';
+--------+
| ename |
+--------+
| WARD |
| MARTIN |
| JAMES |
+--------+
3、找出名字中有下划线的?(这里创建了新的表,后面会学习建表)
mysql> select * from t_user;
+------+----------+
| id | name |
+------+----------+
| 1 | zhangsan |
| 2 | lisi |
| 3 | wang_wu |
+------+----------+
select name from t_user where name like '%\_%';
mysql> select name from t_user where name like '%\_%';
+---------+
| name |
+---------+
| wang_wu |
+---------+
4、找出名字中最后一个字母是T的?
select ename from emp where ename like '%T';
mysql> select ename from emp where ename like '%T';
+-------+
| ename |
+-------+
| SCOTT |
+-------+
3、排序(升序、降序)
语法格式:
select xxx from xxx order by a; 根据a的升序来排列,默认是升序
如果要指定升序和降序,asc表示升序,desc表示降序
1、按照工资升序,找出员工名字和薪资
select ename sal from emp order by sal;(默认升序)
mysql> select ename sal from emp order by sal;
+--------+
| sal |
+--------+
| ADAMS |
| ALLEN |
| BLAKE |
| CLARK |
| FORD |
| JAMES |
| JONES |
| KING |
| MARTIN |
| MILLER |
| SCOTT |
| SMITH |
| TURNER |
| WARD |
+--------+
select ename,sal from emp order by sal asc;(指定升序)
mysql> select ename,sal from emp order by sal asc;
+--------+---------+
| ename | sal |
+--------+---------+
| SMITH | 800.00 |
| JAMES | 950.00 |
| ADAMS | 1100.00 |
| WARD | 1250.00 |
| MARTIN | 1250.00 |
| MILLER | 1300.00 |
| TURNER | 1500.00 |
| ALLEN | 1600.00 |
| CLARK | 2450.00 |
| BLAKE | 2850.00 |
| JONES | 2975.00 |
| SCOTT | 3000.00 |
| FORD | 3000.00 |
| KING | 5000.00 |
+--------+---------+
select ename,sal from emp order by sal desc;(指定降序)
mysql> select ename,sal from emp order by sal desc;
+--------+---------+
| ename | sal |
+--------+---------+
| KING | 5000.00 |
| SCOTT | 3000.00 |
| FORD | 3000.00 |
| JONES | 2975.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| ALLEN | 1600.00 |
| TURNER | 1500.00 |
| MILLER | 1300.00 |
| WARD | 1250.00 |
| MARTIN | 1250.00 |
| ADAMS | 1100.00 |
| JAMES | 950.00 |
| SMITH | 800.00 |
+--------+---------+
多个字段进行排序:用逗号隔开然后,再指定根据xx进行升序降序。
注意:越靠前的字段越能起到主导作用。
按照工资的降序排列,当工资相同的时候再按照名字的升序排列。
select ename,sal from emp order by sal desc,ename asc;
mysql> select ename,sal from emp order by sal desc,ename asc;
+--------+---------+
| ename | sal |
+--------+---------+
| KING | 5000.00 |
| FORD | 3000.00 |
| SCOTT | 3000.00 |
| JONES | 2975.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| ALLEN | 1600.00 |
| TURNER | 1500.00 |
| MILLER | 1300.00 |
| MARTIN | 1250.00 |
| WARD | 1250.00 |
| ADAMS | 1100.00 |
| JAMES | 950.00 |
| SMITH | 800.00 |
+--------+---------+
指定第几列进行排序:(如果表格格式更改这个就不好用了,因此建议还是使用单词写死)
select ename,sal from emp order by 1;//指第一列进行排列
//名字进行了排序
mysql> select ename,sal from emp order by 1;
+--------+---------+
| ename | sal |
+--------+---------+
| ADAMS | 1100.00 |
| ALLEN | 1600.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| FORD | 3000.00 |
| JAMES | 950.00 |
| JONES | 2975.00 |
| KING | 5000.00 |
| MARTIN | 1250.00 |
| MILLER | 1300.00 |
| SCOTT | 3000.00 |
| SMITH | 800.00 |
| TURNER | 1500.00 |
| WARD | 1250.00 |
+--------+---------+
select ename,sal from emp order by 2; //指第二列进行排列
mysql> select ename,sal from emp order by 2;
+--------+---------+
| ename | sal |
+--------+---------+
| SMITH | 800.00 |
| JAMES | 950.00 |
| ADAMS | 1100.00 |
| WARD | 1250.00 |
| MARTIN | 1250.00 |
| MILLER | 1300.00 |
| TURNER | 1500.00 |
| ALLEN | 1600.00 |
| CLARK | 2450.00 |
| BLAKE | 2850.00 |
| JONES | 2975.00 |
| SCOTT | 3000.00 |
| FORD | 3000.00 |
| KING | 5000.00 |
+--------+---------+
找出工作岗位是SALESMAN的员工,并且要求按照薪资的降序排列
select ename,job,sal from emp where job='SLAEMAN' order by sal desc;
语法结构:select * (3)
from tablename (1)
where 条件 (2)
order by..... (4)------order by最后执行
4、Limit分页操作
limit 偏移量,每一页的长度
4.1 limit的使用:
mysql> select employee_id,last_name from employees limit 0,20;
+-------------+------------+
| employee_id | last_name |
+-------------+------------+
| 100 | K_ing |
| 101 | Kochhar |
| 102 | De Haan |
| 103 | Hunold |
| 104 | Ernst |
| 105 | Austin |
| 106 | Pataballa |
| 107 | Lorentz |
| 108 | Greenberg |
| 109 | Faviet |
| 110 | Chen |
| 111 | Sciarra |
| 112 | Urman |
| 113 | Popp |
| 114 | Raphaely |
| 115 | Khoo |
| 116 | Baida |
| 117 | Tobias |
| 118 | Himuro |
| 119 | Colmenares |
+-------------+------------+mysql> select employee_id,last_name from employees limit 20,20;
+-------------+-------------+
| employee_id | last_name |
+-------------+-------------+
| 120 | Weiss |
| 121 | Fripp |
| 122 | Kaufling |
| 123 | Vollman |
| 124 | Mourgos |
| 125 | Nayer |
| 126 | Mikkilineni |
| 127 | Landry |
| 128 | Markle |
| 129 | Bissot |
| 130 | Atkinson |
| 131 | Marlow |
| 132 | Olson |
| 133 | Mallin |
| 134 | Rogers |
| 135 | Gee |
| 136 | Philtanker |
| 137 | Ladwig |
| 138 | Stiles |
| 139 | Seo |
+-------------+-------------+
当偏移量从0开始,0可以省略。
4.2 limit执行顺序
where,order by,limit 声明顺序如下
select employee_id,last_name,salary from employees where salary>6000 order by salary desc limit 0,10;
mysql> select employee_id,last_name,salary from employees where salary>6000 order by salary desc limit 0,10;
+-------------+-----------+----------+
| employee_id | last_name | salary |
+-------------+-----------+----------+
| 100 | K_ing | 24000.00 |
| 101 | Kochhar | 17000.00 |
| 102 | De Haan | 17000.00 |
| 145 | Russell | 14000.00 |
| 146 | Partners | 13500.00 |
| 201 | Hartstein | 13000.00 |
| 108 | Greenberg | 12000.00 |
| 147 | Errazuriz | 12000.00 |
| 205 | Higgins | 12000.00 |
| 168 | Ozer | 11500.00 |
+-------------+-----------+----------+mysql> select employee_id,last_name,salary from employees where salary>6000 order by salary desc limit 10,10;
+-------------+-----------+----------+
| employee_id | last_name | salary |
+-------------+-----------+----------+
| 174 | Abel | 11000.00 |
| 114 | Raphaely | 11000.00 |
| 148 | Cambrault | 11000.00 |
| 162 | Vishney | 10500.00 |
| 149 | Zlotkey | 10500.00 |
| 156 | K_ing | 10000.00 |
| 169 | Bloom | 10000.00 |
| 204 | Baer | 10000.00 |
| 150 | Tucker | 10000.00 |
| 170 | Fox | 9600.00 |
+-------------+-----------+----------+
1、表里有107条数据,我们只想要显示32,33条数据?
select employee_id,last_name from employees limit 31,2;
select employee_id,last_name from employees limit 2 offset 31;(8.0新特性)
mysql> select employee_id,last_name from employees limit 2 offset 31;
+-------------+-----------+
| employee_id | last_name |
+-------------+-----------+
| 131 | Marlow |
| 132 | Olson |
+-------------+-----------+mysql> select employee_id,last_name from employees limit 31,2;
+-------------+-----------+
| employee_id | last_name |
+-------------+-----------+
| 131 | Marlow |
| 132 | Olson |
+-------------+-----------+
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