一.单表查询
(一). 查询字段
I.单个字段
格式： select 字段名 from 表名；
说明：
① select 和from都是关键字
② 字段名和表名都是标识符
③ SQL语句不区分大小写，并且都是以“ ；”结尾
演示：
mysql> desc dept;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| DEPTNO | int | NO | PRI | NULL | |
| DNAME | varchar(14) | YES | | NULL | |
| LOC | varchar(13) | YES | | NULL | |
+--------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql> select dname from dept;
+------------+
| dname |
+------------+
| ACCOUNTING |
| RESEARCH |
| SALES |
| OPERATIONS |
+------------+
4 rows in set (0.00 sec)
II.查询多个字段
方法： 在I的基础上通过“ ，”将多个字段名隔开即可
演示：
mysql> select dname,deptno from dept;
+------------+--------+
| dname | deptno |
+------------+--------+
| ACCOUNTING | 10 |
| RESEARCH | 20 |
| SALES | 30 |
| OPERATIONS | 40 |
+------------+--------+
4 rows in set (0.00 sec)
III.查询所有字段
格式：
(1) select 字段一，字段二，字段三 ... from tablename; (所有字段)
(2)select * from tablename;
说明：
方式（2）特点：
①效率低（需要先将 * 转化为 字段 ，然后再执行 ）
②可读性差
应用场景：需要快速查看表中所有数据时可以使用
演示：
方式一：
mysql> select dname,deptno,loc from dept;
+------------+--------+----------+
| dname | deptno | loc |
+------------+--------+----------+
| ACCOUNTING | 10 | NEW YORK |
| RESEARCH | 20 | DALLAS |
| SALES | 30 | CHICAGO |
| OPERATIONS | 40 | BOSTON |
+------------+--------+----------+
4 rows in set (0.00 sec)
方式二：
mysql> select* from dept;
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
4 rows in set (0.00 sec)
(二). 列起别名
方法：通过使用as 关键字来实现的
说明：
① 使用as 关键字起别名只是将结果列名显示为别名，原表列名是不变的
② select 语句永远不会进行修改操作的（只负责查询）
演示：
方式一（不省略关键字as）：
mysql> select* from dept;
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
4 rows in set (0.00 sec)
mysql> select deptno,dname as deptname from dept; #deptname为dname的别名
+--------+------------+
| deptno | deptname |
+--------+------------+
| 10 | ACCOUNTING |
| 20 | RESEARCH |
| 30 | SALES |
| 40 | OPERATIONS |
+--------+------------+
4 rows in set (0.00 sec)
方式二(省略关键字as)：
mysql> select deptno,dname deptname from dept;
+--------+------------+
| deptno | deptname |
+--------+------------+
| 10 | ACCOUNTING |
| 20 | RESEARCH |
| 30 | SALES |
| 40 | OPERATIONS |
+--------+------------+
4 rows in set (0.00 sec)
mysql> select deptno,dname deptname name from dept;
ERROR 1064 (42000): 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 'name from dept' at line 1
补充，如果别名中存在空格，可以将这整个别名使用 ‘’ 或 “ ” 包括起来
mysql> select deptno,dname 'deptname name' from dept;
+--------+---------------+
| deptno | deptname name |
+--------+---------------+
| 10 | ACCOUNTING |
| 20 | RESEARCH |
| 30 | SALES |
| 40 | OPERATIONS |
+--------+---------------+
4 rows in set (0.00 sec)
注意：
① 在所有的数据库当中，字符串统一使用单引号（标准）括起来
②双引号在oracle数据库中不能使用，但是在mysql数据库中是可以使用的
字段可以使用数学表达式：
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 |
+--------+----------+
14 rows in set (0.00 sec)
说明：如果 别名 为中文，需要使用 单引号 ‘ ’ 括起来
(三) 条件查询
介绍：根据条件查询出符合要求的。
语法格式：
select 字段1，字段2，字段3 ...from tablename where 条件；
常见条件如下表：
运算符 | 说明 |
= | 等于 |
<> 或！= | 不等于 |
< | 小于 |
<= | 小于等于 |
> | 大于 |
>= | 大于等于 |
between… and …. | 两个值之间，等同于 **>= and <=** |
is null | 为 null（is not null 不为空） |
and | 并且 |
or | 或者 |
in | 包含，相当于多个 o（r not in 不在这个范围中） |
not | not 可以取非，主要用在 is 或 in 中 |
like | like 称为模糊查询，支持 % 或下划线匹配 % 匹配任意个字符 下划线，一个下划线只匹配一个字符 |
应用实例：
（1）= （查询薪资等于 1600的员工姓名和编号）
mysql> select empno,ename from emp where sal=1600 ;
+-------+-------+
| empno | ename |
+-------+-------+
| 7499 | ALLEN |
+-------+-------+
1 row in set (0.01 sec)
（2）！= （查询薪资不等于 1600的员工姓名和编号）
mysql> select empno,ename from emp where sal!=1600 ;
+-------+--------+
| empno | ename |
+-------+--------+
| 7369 | SMITH |
| 7521 | WARD |
| 7566 | JONES |
| 7654 | MARTIN |
| 7698 | BLAKE |
| 7782 | CLARK |
| 7788 | SCOTT |
| 7839 | KING |
| 7844 | TURNER |
| 7876 | ADAMS |
| 7900 | JAMES |
| 7902 | FORD |
| 7934 | MILLER |
+-------+--------+
13 rows in set (0.00 sec)
（3） < （查询薪资小于1600的员工姓名和编号）
mysql> select empno,ename,sal from emp where sal<1600 ;
+-------+--------+---------+
| empno | ename | sal |
+-------+--------+---------+
| 7369 | SMITH | 800.00 |
| 7521 | WARD | 1250.00 |
| 7654 | MARTIN | 1250.00 |
| 7844 | TURNER | 1500.00 |
| 7876 | ADAMS | 1100.00 |
| 7900 | JAMES | 950.00 |
| 7934 | MILLER | 1300.00 |
+-------+--------+---------+
7 rows in set (0.00 sec)
（4） = （查询SMITH的编号和薪资）
mysql> select empno,ename,sal from emp where ename='SMITH' ;
+-------+-------+--------+
| empno | ename | sal |
+-------+-------+--------+
| 7369 | SMITH | 800.00 |
+-------+-------+--------+
1 row in set (0.00 sec)
说明：字符串使用单引号
（5）betweent ... and ... (查询薪资在2450到 4000之间的员工姓名、编号、薪资)
mysql> select empno,ename,sal from emp where sal between 2450 and 4000 ;
+-------+-------+---------+
| empno | ename | sal |
+-------+-------+---------+
| 7566 | JONES | 2975.00 |
| 7698 | BLAKE | 2850.00 |
| 7782 | CLARK | 2450.00 |
| 7788 | SCOTT | 3000.00 |
| 7902 | FORD | 3000.00 |
+-------+-------+---------
说明：必须遵循左小右大原则
（6）is null (查询哪些员工的津贴/补助为null)
mysql> select empno,ename,sal,comm from emp where comm is null;
+-------+--------+---------+------+
| empno | ename | sal | comm |
+-------+--------+---------+------+
| 7369 | SMITH | 800.00 | NULL |
| 7566 | JONES | 2975.00 | NULL |
| 7698 | BLAKE | 2850.00 | NULL |
| 7782 | CLARK | 2450.00 | NULL |
| 7788 | SCOTT | 3000.00 | NULL |
| 7839 | KING | 5000.00 | NULL |
| 7876 | ADAMS | 1100.00 | NULL |
| 7900 | JAMES | 950.00 | NULL |
| 7902 | FORD | 3000.00 | NULL |
| 7934 | MILLER | 1300.00 | NULL |
+-------+--------+---------+------+
10 rows in set (0.00 sec)
注意：在数据库当中null不能使用等号来进行衡量。需要使用is null，因为数据库中的null代表什么也没有，它不是一个值，所以不能使用等号衡量。
如果查询员工的津贴不是null，通过如下命令：
mysql>select empno ,ename,sal,comm from emp where comm is not null;
mysql> select empno ,ename,sal,comm from emp where comm is not null;
+-------+--------+---------+---------+
| empno | ename | sal | comm |
+-------+--------+---------+---------+
| 7499 | ALLEN | 1600.00 | 300.00 |
| 7521 | WARD | 1250.00 | 500.00 |
| 7654 | MARTIN | 1250.00 | 1400.00 |
| 7844 | TURNER | 1500.00 | 0.00 |
+-------+--------+---------+---------+
4 rows in set (0.00 sec)
(7) and 并且（查询工作岗位是MANAGER并且工资大于2500的员工信息）
mysql> select empno,ename,job,sal from emp where job ='MANAGER' and sal > 2500;
+-------+-------+---------+---------+
| empno | ename | job | sal |
+-------+-------+---------+---------+
| 7566 | JONES | MANAGER | 2975.00 |
| 7698 | BLAKE | MANAGER | 2850.00 |
+-------+-------+---------+---------+
2 rows in set (0.00 sec)
（8）or 或则 （查询岗位是MANAGER和 SALESMAN的员工）
mysql> select empno,ename,job from emp where job = 'MANAGER' or job = 'SALESMAN';
+-------+--------+----------+
| empno | ename | job |
+-------+--------+----------+
| 7499 | ALLEN | SALESMAN |
| 7521 | WARD | SALESMAN |
| 7566 | JONES | MANAGER |
| 7654 | MARTIN | SALESMAN |
| 7698 | BLAKE | MANAGER |
| 7782 | CLARK | MANAGER |
| 7844 | TURNER | SALESMAN |
+-------+--------+----------+
7 rows in set (0.00 sec)
混合应用：
查询工资大于2500,并且部门编号为10或20部门的员工
#错误写法：and的优先级高于or(原因)
mysql> select * from emp where sal >2500 and deptno =10 or deptno =20;
+-------+-------+-----------+------+------------+---------+------+--------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |
+-------+-------+-----------+------+------------+---------+------+--------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
| 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
+-------+-------+-----------+------+------------+---------+------+--------+
6 rows in set (0.00 sec)
#正确写法
mysql> select * from emp where sal >2500 and( deptno =10 or deptno =20);
+-------+-------+-----------+------+------------+---------+------+--------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |
+-------+-------+-----------+------+------------+---------+------+--------+
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
+-------+-------+-----------+------+------------+---------+------+--------+
4 rows in set (0.00 sec)
说明：and 与 or同时出现，and 的优先级高于 or
(9) in (查询工作岗位是 MANAGER 和 SALESMAN)
#方式一
mysql> select empno,ename,job from emp where job = 'MANAGER' or job = 'SALESMAN';
+-------+--------+----------+
| empno | ename | job |
+-------+--------+----------+
| 7499 | ALLEN | SALESMAN |
| 7521 | WARD | SALESMAN |
| 7566 | JONES | MANAGER |
| 7654 | MARTIN | SALESMAN |
| 7698 | BLAKE | MANAGER |
| 7782 | CLARK | MANAGER |
| 7844 | TURNER | SALESMAN |
+-------+--------+----------+
7 rows in set (0.00 sec)
#方式二
mysql> select empno,ename,job from emp where job in('MANAGER','SALESMAN');
+-------+--------+----------+
| empno | ename | job |
+-------+--------+----------+
| 7499 | ALLEN | SALESMAN |
| 7521 | WARD | SALESMAN |
| 7566 | JONES | MANAGER |
| 7654 | MARTIN | SALESMAN |
| 7698 | BLAKE | MANAGER |
| 7782 | CLARK | MANAGER |
| 7844 | TURNER | SALESMAN |
+-------+--------+----------+
7 rows in set (0.00 sec)
注意：in后面跟的内容是不是一个区间 ，而是一个具体的值
(10)like (模糊查询，支持%或下划线 匹配)
说明： %
演示：
mysql> select ename from emp where ename like '%O%';//查询名字中有 字母 ‘O’的
+-------+
| ename |
+-------+
| JONES |
| SCOTT |
| FORD |
+-------+
3 rows in set (0.00 sec)
//查询以 字母 ‘T’ 结尾的名字
mysql> select ename from emp where ename like'%T';
+-------+
| ename |
+-------+
| SCOTT |
+-------+
1 row in set (0.00 sec)
//查询以字母 ‘K’开头的名字
mysql> select ename from emp where ename like'K%';
+-------+
| ename |
+-------+
| KING |
+-------+
1 row in set (0.00 sec)
//查询第二个是字母‘A’的名字
mysql> select ename from emp where ename like'_A%';
+--------+
| ename |
+--------+
| WARD |
| MARTIN |
| JAMES |
+--------+
3 rows in set (0.00 sec)
//查询第三个字母是‘R’的名字
mysql> select ename from emp where ename like'__R%';
+--------+
| ename |
+--------+
| WARD |
| MARTIN |
| TURNER |
| FORD |
+--------+
4 rows in set (0.00 sec)
拓展：如果要查询带有下划线‘_’的名字,可以使用反斜线 ‘ \ ’进行转义
(四)排序
格式：select 字段1，字段2，字段3... from tablename order by 字段；
解释：根据 字段 将字段1，字段2，字段3...进行升序
I.单个字段
升序：
mysql> select ename,sal from emp order by sal;//默认升序
+--------+---------+
| 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 |
+--------+---------+
14 rows in set (0.00 sec)
mysql> select ename,sal from emp order by sal asc;//指定升序
+--------+---------+
| 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 |
+--------+---------+
14 rows in set (0.00 sec)
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 |
+--------+---------+
14 rows in set (0.00 sec)
II.多个字段
格式：select 字段1，字段2，字段3... from tablename order by 字段1，字段2...；
解释：先通过字段1进行排序，然后如果字段1中存在相等的，就根据字段2的方式进行排序.....
演示：
mysql> select ename,sal from emp order by sal asc,ename asc;
+--------+---------+
| ename | sal |
+--------+---------+
| SMITH | 800.00 |
| JAMES | 950.00 |
| ADAMS | 1100.00 |
| MARTIN | 1250.00 |
| WARD | 1250.00 |
| MILLER | 1300.00 |
| TURNER | 1500.00 |
| ALLEN | 1600.00 |
| CLARK | 2450.00 |
| BLAKE | 2850.00 |
| JONES | 2975.00 |
| FORD | 3000.00 |
| SCOTT | 3000.00 |
| KING | 5000.00 |
+--------+---------+
14 rows in set (0.00 sec)
综合案例
找出工资在1250到3000之间的员工信息，要求按照薪资降序排列
mysql> select
-> ename,sal
-> from
-> emp
-> where
-> sal between 1250 and 3000
-> order by
-> sal desc;
+--------+---------+
| ename | sal |
+--------+---------+
| 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 |
+--------+---------+
10 rows in set (0.00 sec)
注意：执行顺序
from --> where -->select --> order by (排序总是在最后才执行)
(五)函数
介绍：
①单行处理函数：指的是一个输入对应一个输出的函数
②多行处理函数：指的是多个输入对应一个输出的函数
单行处理函数
lower | 转换小写 |
upper | 转换大写 |
substr | 取子串（substr (被截取的字符串，起始下标，截取的长度)） |
length | 取长度 |
trim | 去除字符串中的空格 |
str_to_date | 将字符串转换成日期 |
date_format | 格式化日期 |
format | 设置千分位 |
round | 四舍五入 |
rand() | 生成随机数 |
Ifnull | 可以将 null 转换成一个具体值 |
使用演示：
lower
mysql> select ename from emp;
+--------+
| ename |
+--------+
| SMITH |
| ALLEN |
| WARD |
| JONES |
| MARTIN |
| BLAKE |
| CLARK |
| SCOTT |
| KING |
| TURNER |
| ADAMS |
| JAMES |
| FORD |
| MILLER |
+--------+
14 rows in set (0.00 sec)
//全部转化为小写
mysql> select lower(ename) from emp;//通过 lower(字段名)方式，将字段名全部以小写的形式输出
+--------------+
| lower(ename) |
+--------------+
| smith |
| allen |
| ward |
| jones |
| martin |
| blake |
| clark |
| scott |
| king |
| turner |
| adams |
| james |
| ford |
| miller |
+--------------+
14 rows in set (0.01 sec)
upper
mysql> select (ename) from emp;
substr
找出员工名字第一个字母是A的员工信息
mysql> select ename from emp where ename like 'A%';//方式一
+-------+
| ename |
+-------+
| ALLEN |
| ADAMS |
+-------+
2 rows in set (0.00 sec)
mysql> select ename from emp where substr(ename,1,1)='A';//方式二
+-------+
| ename |
+-------+
| ALLEN |
| ADAMS |
+-------+
2 rows in set (0.00 sec)
length
mysql> select length(ename) enamelength from emp;
+-------------+
| enamelength |
+-------------+
| 5 |
| 5 |
| 4 |
| 5 |
| 6 |
| 5 |
| 5 |
| 5 |
| 4 |
| 6 |
| 5 |
| 5 |
| 4 |
| 6 |
+-------------+
14 rows in set (0.00 sec)
trim
mysql> select * from emp where ename =trim(' KING');
+-------+-------+-----------+------+------------+---------+------+--------+
| 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)
str_to_date
dete_format
format
round
功能：四舍五入
格式：select 字段名 from 表名；
说明：如果select后面直接跟‘‘字面量/字面值’’，会根据表的结构形成重复的字面量数据
mysql> select 'abx' from emp;
+-----+
| abx |
+-----+
| abx |
| abx |
| abx |
| abx |
| abx |
| abx |
| abx |
| abx |
| abx |
| abx |
| abx |
| abx |
| abx |
| abx |
+-----+
14 rows in set (0.00 sec)
演示：
mysql> select round('1220.234',0) as result from emp;//参数0表示保留的小数位
+--------+
| result |
+--------+
| 1220 |
| 1220 |
| 1220 |
| 1220 |
| 1220 |
| 1220 |
| 1220 |
| 1220 |
| 1220 |
| 1220 |
| 1220 |
| 1220 |
| 1220 |
| 1220 |
+--------+
14 rows in set (0.00 sec)
mysql> select round('1220.234',-2) as result from emp;//保留到十位
+--------+
| result |
+--------+
| 1200 |
| 1200 |
| 1200 |
| 1200 |
| 1200 |
| 1200 |
| 1200 |
| 1200 |
| 1200 |
| 1200 |
| 1200 |
| 1200 |
| 1200 |
| 1200 |
+--------+
14 rows in set (0.00 sec)
如果参数是 0：保留整数位 1：保留一位小数 2：保留两位小数 3：保留三位小数 ....
-1: 保留到十位 -2: 保留到百位
rand()
mysql> select rand() from emp;
+---------------------+
| rand() |
+---------------------+
| 0.16059188839103272 |
| 0.1701412919630681 |
| 0.3689308253758874 |
| 0.3342302817238777 |
| 0.5643589632253712 |
| 0.8191040016888678 |
| 0.40244314950187055 |
| 0.5549037731763942 |
| 0.5671894481100044 |
| 0.17123595175448428 |
| 0.15461144517605327 |
| 0.25934940135045853 |
| 0.8329127019577778 |
| 0.3865153364711584 |
+---------------------+
14 rows in set (0.01 sec)
mysql> select round(rand()*100,0) from emp;//生成100以内的随机数
+---------------------+
| round(rand()*100,0) |
+---------------------+
| 43 |
| 1 |
| 75 |
| 70 |
| 28 |
| 30 |
| 66 |
| 39 |
| 97 |
| 68 |
| 51 |
| 48 |
| 90 |
| 4 |
+---------------------+
14 rows in set (0.00 sec)
ifnull
介绍：空处理函数(将null转化为一个具体值)。
用法：isfull(数据，具体值)
解释： 如果数据为null,就使用该 具体值 来进行 参与运算
说明：在所有的数据库中，有null参与的数学运算，最终结果就是null。（使用该函数就可以避免）
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 | 1987-04-19 | 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 | 1987-05-23 | 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)
//将sal与comm段相加，别名为 salcomm
mysql> select ename ,sal+ comm as salcomm from emp;
+--------+---------+
| ename | salcomm |
+--------+---------+
| SMITH | NULL |
| ALLEN | 1900.00 |
| WARD | 1750.00 |
| JONES | NULL |
| MARTIN | 2650.00 |
| BLAKE | NULL |
| CLARK | NULL |
| SCOTT | NULL |
| KING | NULL |
| TURNER | 1500.00 |
| ADAMS | NULL |
| JAMES | NULL |
| FORD | NULL |
| MILLER | NULL |
+--------+---------+
14 rows in set (0.00 sec)
使用演示：
mysql> select ename ,(sal + ifnull(comm,0))*12 as yearsal from emp;//计算年薪
+--------+----------+
| ename | yearsal |
+--------+----------+
| SMITH | 9600.00 |
| ALLEN | 22800.00 |
| WARD | 21000.00 |
| JONES | 35700.00 |
| MARTIN | 31800.00 |
| 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 |
+--------+----------+
14 rows in set (0.00 sec)
concat
功能：字符串的拼接
格式：select contace(字段1，字段2，字段3....) from tablename;
演示：
mysql> select from emp;
ERROR 1064 (42000): 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 'from emp' at line 1
mysql> select concat(empno,ename) from emp;
+---------------------+
| concat(empno,ename) |
+---------------------+
| 7369SMITH |
| 7499ALLEN |
| 7521WARD |
| 7566JONES |
| 7654MARTIN |
| 7698BLAKE |
| 7782CLARK |
| 7788SCOTT |
| 7839KING |
| 7844TURNER |
| 7876ADAMS |
| 7900JAMES |
| 7902FORD |
| 7934MILLER |
+---------------------+
14 rows in set (0.00 sec)
结论：select后面可以跟某个表的字段名（可以看做是变量名），也可以跟字面量/字面值（数据）
case ...when ,,,then...when...then...else...end
当员工的工作岗位是MANAGER时，工资上调10%。当工作岗位是SALESMAN的时候，工资上调50%,其他不变
说明：不修改数据库，只是将查询结果显示为工资上调
演示：
mysql> select ename,job,sal from emp;
+--------+-----------+---------+
| ename | job | sal |
+--------+-----------+---------+
| SMITH | CLERK | 800.00 |
| ALLEN | SALESMAN | 1600.00 |
| WARD | SALESMAN | 1250.00 |
| JONES | MANAGER | 2975.00 |
| MARTIN | SALESMAN | 1250.00 |
| BLAKE | MANAGER | 2850.00 |
| CLARK | MANAGER | 2450.00 |
| SCOTT | ANALYST | 3000.00 |
| KING | PRESIDENT | 5000.00 |
| TURNER | SALESMAN | 1500.00 |
| ADAMS | CLERK | 1100.00 |
| JAMES | CLERK | 950.00 |
| FORD | ANALYST | 3000.00 |
| MILLER | CLERK | 1300.00 |
+--------+-----------+---------+
14 rows in set (0.00 sec)
mysql> select ename,job,sal as oldsal,(case job when 'MANAGER' then sal*1.1 when 'SALESMAN' then sal*1.5 else sal end) as newsal from emp;
+--------+-----------+---------+---------+
| ename | job | oldsal | newsal |
+--------+-----------+---------+---------+
| SMITH | CLERK | 800.00 | 800.00 |
| ALLEN | SALESMAN | 1600.00 | 2400.00 |
| WARD | SALESMAN | 1250.00 | 1875.00 |
| JONES | MANAGER | 2975.00 | 3272.50 |
| MARTIN | SALESMAN | 1250.00 | 1875.00 |
| BLAKE | MANAGER | 2850.00 | 3135.00 |
| CLARK | MANAGER | 2450.00 | 2695.00 |
| SCOTT | ANALYST | 3000.00 | 3000.00 |
| KING | PRESIDENT | 5000.00 | 5000.00 |
| TURNER | SALESMAN | 1500.00 | 2250.00 |
| ADAMS | CLERK | 1100.00 | 1100.00 |
| JAMES | CLERK | 950.00 | 950.00 |
| FORD | ANALYST | 3000.00 | 3000.00 |
| MILLER | CLERK | 1300.00 | 1300.00 |
+--------+-----------+---------+---------+
14 rows in set (0.00 sec)
多行处理函数（分组函数）
特点：输入多行，输出一行
说明：分组函数在使用时必须先进行分组，然后才能使用
若未对数据进行分组，默认整个表为一个分组
常见函数
count | 取得记录数(记数) |
sum | 求和 |
avg | 取平均（平均值） |
max | 取最大的数（最大值） |
min | 取最小的数（最小值） |
cout
mysql> select count(ename) from emp;
+--------------+
| count(ename) |
+--------------+
| 14 |
+--------------+
1 row in set (0.00 sec)
sum
mysql> select sum(sal) as totalsum from emp;
+----------+
| totalsum |
+----------+
| 29025.00 |
+----------+
1 row in set (0.00 sec)
avg
mysql> select avg(sal) as totalsum from emp;
+-------------+
| totalsum |
+-------------+
| 2073.214286 |
+-------------+
1 row in set (0.01 sec)
max
mysql> select max(sal) as totalsum from emp;
+----------+
| totalsum |
+----------+
| 5000.00 |
+----------+
1 row in set (0.00 sec)
min
mysql> select min(sal) as totalsum from emp;
+----------+
| totalsum |
+----------+
| 800.00 |
+----------+
1 row in set (0.01 sec)
注意事项：
（1）分组函数自动忽略null，不需要提前对null进行处理
（2）分组函数中count(*)与count(具体字段)的区别？
mysql> select count(*) from emp;
+----------+
| count(*) |
+----------+
| 14 |
+----------+
1 row in set (0.00 sec)
mysql> select count(comm) from emp;
+-------------+
| count(comm) |
+-------------+
| 4 |
+-------------+
1 row in set (0.00 sec)
说明：
count(具体字段)： 表示统计该字段下所有不为null的元素的总数
count(*) :统计表当中的总行数。（只要有一行数据count就加1）
每一行记录不可能都为null，一行数据中有一列不为null，则这行数据就是有效的。
（3）分组函数不能够直接使用在where子句中
找出比最低工资高的员工信息
mysql> select ename ,sal from emp where sal > min(sal);
ERROR 1111 (HY000): Invalid use of group function
错误原因分析：
在where执行的时候，还没有进行分组,所以where的后面不能出现分组函数
在select后面出现分组函数是可以的，因为在执行select之前已经分好组
（六）分组查询（重点）
基本介绍：
在实际应用中·，将需要先分组然后再查询的一种查询方式称为分组查询
格式：
select ... from ... group by ...
实例：
找出每个岗位的工资和 ？？
思路：先根据工作岗位进行分组，然后在求每一组的工作和
mysql> select job,sum(sal) from emp group by job;
+-----------+----------+
| job | sum(sal) |
+-----------+----------+
| CLERK | 4150.00 |
| SALESMAN | 5600.00 |
| MANAGER | 8275.00 |
| ANALYST | 6000.00 |
| PRESIDENT | 5000.00 |
+-----------+----------+
5 rows in set (0.01 sec)
//执行顺序： 先从emp表中查询数据-->根据job字段进行分组 --> 然后对每一个分组数据进行求和（sum）
mysql> select ename,job,sum(sal) from emp group by job;
+-------+-----------+----------+
| ename | job | sum(sal) |
+-------+-----------+----------+
| SMITH | CLERK | 4150.00 |
| ALLEN | SALESMAN | 5600.00 |
| JONES | MANAGER | 8275.00 |
| SCOTT | ANALYST | 6000.00 |
| KING | PRESIDENT | 5000.00 |
+-------+-----------+----------+
5 rows in set (0.00 sec)
//以上写法在mysql可以执行，但是在oracle中执行报错（语法更加严格）---->无意义
找出每个部门的最高薪资？
思路：按照部门编号分组，然后求每一个分组的最大值
mysql> select deptno,max(sal) from emp group by deptno;
+--------+----------+
| deptno | max(sal) |
+--------+----------+
| 20 | 3000.00 |
| 30 | 2850.00 |
| 10 | 5000.00 |
+--------+----------+
3 rows in set (0.00 sec)
找出每个部门 ， 不同工作岗位的最高薪资？？
mysql> select ename,job,sal,deptno from emp order by deptno;
+--------+-----------+---------+--------+
| ename | job | sal | deptno |
+--------+-----------+---------+--------+
| CLARK | MANAGER | 2450.00 | 10 |
| KING | PRESIDENT | 5000.00 | 10 |
| MILLER | CLERK | 1300.00 | 10 |
| SMITH | CLERK | 800.00 | 20 |
| JONES | MANAGER | 2975.00 | 20 |
| SCOTT | ANALYST | 3000.00 | 20 |
| ADAMS | CLERK | 1100.00 | 20 |
| FORD | ANALYST | 3000.00 | 20 |
| ALLEN | SALESMAN | 1600.00 | 30 |
| WARD | SALESMAN | 1250.00 | 30 |
| MARTIN | SALESMAN | 1250.00 | 30 |
| BLAKE | MANAGER | 2850.00 | 30 |
| TURNER | SALESMAN | 1500.00 | 30 |
| JAMES | CLERK | 950.00 | 30 |
+--------+-----------+---------+--------+
14 rows in set (0.00 sec)
//技巧：两个字段联合成一个字段（两个字段联合分组）
mysql> select deptno,job,max(sal) from emp group by deptno,job;
+--------+-----------+----------+
| deptno | job | max(sal) |
+--------+-----------+----------+
| 20 | CLERK | 1100.00 |
| 30 | SALESMAN | 1600.00 |
| 20 | MANAGER | 2975.00 |
| 30 | MANAGER | 2850.00 |
| 10 | MANAGER | 2450.00 |
| 20 | ANALYST | 3000.00 |
| 10 | PRESIDENT | 5000.00 |
| 30 | CLERK | 950.00 |
| 10 | CLERK | 1300.00 |
+--------+-----------+----------+
9 rows in set (0.00 sec)
找出每个部门的最高薪资，要求显示最高薪资大于3000？
思路：（1）找出每个部门的最高薪资（按照部门编号分组，求每一组的最大值）
（2）要求显示最高薪资大于3000
//方式一
mysql> select deptno,max(sal) from emp group by deptno;
+--------+----------+
| deptno | max(sal) |
+--------+----------+
| 20 | 3000.00 |
| 30 | 2850.00 |
| 10 | 5000.00 |
+--------+----------+
3 rows in set (0.00 sec)
mysql> select deptno,max(sal) from emp group by deptno having max(sal)>3000;
+--------+----------+
| deptno | max(sal) |
+--------+----------+
| 10 | 5000.00 |
+--------+----------+
1 row in set (0.01 sec)
//方式二
mysql> select deptno,max(sal) from emp where sal>3000 group by deptno;//where进行过滤
+--------+----------+
| deptno | max(sal) |
+--------+----------+
| 10 | 5000.00 |
+--------+----------+
1 row in set (0.00 sec)
说明：可以使用having来对分组后的数据进一步过滤，having不能够单独使用，having不能代替where，having必须和group by 联合使用。（优先使用where）
找出每个部门平均薪资，要求显示平均薪资高于2500的
思路：（1）找出每个部门平均薪资
（2）要求显示平均薪资高于2500的
mysql> select deptno,avg(sal) from emp group by deptno;
+--------+-------------+
| deptno | avg(sal) |
+--------+-------------+
| 20 | 2175.000000 |
| 30 | 1566.666667 |
| 10 | 2916.666667 |
+--------+-------------+
3 rows in set (0.00 sec)
mysql> select deptno ,avg(sal) from emp group by deptno having avg(sal)>2500;
+--------+-------------+
| deptno | avg(sal) |
+--------+-------------+
| 10 | 2916.666667 |
+--------+-------------+
1 row in set (0.01 sec)
conclusion:
在执行一条select语句中，如果含有group语句，select后面只能跟：参加分组的字段或分组函数。
关键字组合：
select ... from ...where... group by ... order by ....
说明：关键字的使用先后顺序不能颠倒
执行顺序：
① from ② where ③ group by ④ select ⑤ order by
总结：
select ... where ... group by ... having ...order by ...
执行顺序：
① from ② where ③ group by ④ having ⑤ select ⑥ order by
说明（执行过程）：
过程一：通过where条件筛选出有价值的数据
过程二：对有价值的数据进行分组
过程三：分组后通过having来及进行进一步的筛选
过程四： select 查询
过程五：直接排序输出
找出每个岗位的平均薪资，要求显示平均薪资大于1500，除MANAGER岗位之外（薪资按照降序排列）
mysql> select job,avg(sal) as avgsal from emp where job <> 'MANAGER' group by job having avg(sal) >1500 order by avgsal desc;
+-----------+-------------+
| job | avgsal |
+-----------+-------------+
| PRESIDENT | 5000.000000 |
| ANALYST | 3000.000000 |
+-----------+-------------+
2 rows in set (0.00 sec)
