select 字段列表 | 表达式 函数
1、数学函数
PI()：圆周率;跟book表没关系但表必须有数据因为圆周率函数，跟我这张表里面的任何字段都没有关系，所以在查询过程当中是不需要传入任何参数的
mysql> select * from book; +------+--------+------------+-------+ | num | name | datel | price | +------+--------+------------+-------+ | 2 | chenxi | NULL | 90 | | 1 | linux | 2020-04-04 | 99 | +------+--------+------------+-------+ 2 rows in set (0.00 sec) mysql> select distinct pi() from book; +----------+ | pi() | +----------+ | 3.141593 | +----------+ 1 row in set (0.00 sec)
FLOOR(x)：返回小于x的最大整数值（去掉小树取整）
mysql> insert into emp(empmo,ename,sal,comm) values(777,'zhang',2356.54,234.45); Query OK, 1 row affected (0.01 sec) mysql> select * from emp; +-------+--------+-----------+------+---------------------+---------+---------+--------+ | empmo | ename | job | mgr | hiredate | sal | comm | deptno | +-------+--------+-----------+------+---------------------+---------+---------+--------+ | 7369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | | 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | | 7521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | | 7566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | 7782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | | 7788 | SCOTT | ANALYST | 7566 | 1982-12-09 00:00:00 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | | 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | | 7876 | ADAMS | CLERK | 7788 | 1983-01-12 00:00:00 | 1100.00 | NULL | 20 | | 7900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | | 7934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | | 777 | zhang | NULL | NULL | NULL | 2356.54 | 234.45 | NULL | +-------+--------+-----------+------+---------------------+---------+---------+--------+ 15 rows in set (0.00 sec) mysql> select floor(sal) from emp where empmo = 777; +------------+ | floor(sal) | +------------+ | 2356 | +------------+ 1 row in set (0.00 sec) mysql> select floor(sal) from emp; +------------+ | floor(sal) | +------------+ | 800 | | 1600 | | 1250 | | 2975 | | 1250 | | 2850 | | 2450 | | 3000 | | 5000 | | 1500 | | 1100 | | 950 | | 3000 | | 1300 | | 2356 | +------------+ 15 rows in set (0.00 sec)
CEILING(x)：返回大于x的最小整数值（进一取整）
mysql> select ceiling(comm) from emp where empmo = 777; +---------------+ | ceiling(comm) | +---------------+ | 235 | +---------------+ 1 row in set (0.00 sec)
ROUND(x)：返回参数x的四舍五入的有y位小数的值（四舍五入）
mysql> select comm from emp where empmo = 777; +--------+ | comm | +--------+ | 234.45 | +--------+ 1 row in set (0.00 sec) mysql> select round(comm) from emp where empmo = 777; +-------------+ | round(comm) | +-------------+ | 234 | +-------------+ 1 row in set (0.00 sec) mysql> select round(comm,1) from emp where empmo = 777; +---------------+ | round(comm,1) | +---------------+ | 234.5 | +---------------+ 1 row in set (0.02 sec)
TRUNCATE(x,y):返回数字x剪短为y位小数的结果
mysql> select comm from emp where empmo=777; +--------+ | comm | +--------+ | 234.45 | +--------+ 1 row in set (0.00 sec) mysql> truncate(comm,1)ct comm from emp where empmo=777; +------------------+ | truncate(comm,1) | +------------------+ | 234.4 | +------------------+ 1 row in set (0.00 sec)
聚合函数，也称之为分组函数
AVG(col)：返回指定列的平均值
mysql> select avg(sal) from emp; +-------------+ | avg(sal) | +-------------+ | 2092.102667 | +-------------+ 1 row in set (0.00 sec)
COUNT(x,y)：返回指定列中非 null 值/行的个数（当函数参数为星号*时不会忽略）
mysql> select comm from emp; +---------+ | comm | +---------+ | NULL | | 300.00 | | 500.00 | | NULL | | 1400.00 | | NULL | | NULL | | NULL | | NULL | | 0.00 | | NULL | | NULL | | NULL | | NULL | | 234.45 | +---------+ 15 rows in set (0.00 sec) mysql> select count(comm) from emp; 统计comm列值不为空的记录 +-------------+ | count(comm) | +-------------+ | 5 | +-------------+ 1 row in set (0.01 sec) mysql> select empmo from emp; +-------+ | empmo | +-------+ | 7369 | | 7499 | | 7521 | | 7566 | | 7654 | | 7698 | | 7782 | | 7788 | | 7839 | | 7844 | | 7876 | | 7900 | | 7902 | | 7934 | | 777 | +-------+ 15 rows in set (0.01 sec) mysql> select count(empmo) from emp;找出多少员工 +--------------+ | count(empmo) | +--------------+ | 15 | +--------------+ 1 row in set (0.01 sec)
MIN(col)：返回指定列的最小值
mysql> select * from emp; +-------+--------+-----------+------+---------------------+---------+---------+--------+ | empmo | ename | job | mgr | hiredate | sal | comm | deptno | +-------+--------+-----------+------+---------------------+---------+---------+--------+ | 7369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | | 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | | 7521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | | 7566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | 7782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | | 7788 | SCOTT | ANALYST | 7566 | 1982-12-09 00:00:00 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | | 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | | 7876 | ADAMS | CLERK | 7788 | 1983-01-12 00:00:00 | 1100.00 | NULL | 20 | | 7900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | | 7934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | | 777 | zhang | NULL | NULL | NULL | 2356.54 | 234.45 | NULL | +-------+--------+-----------+------+---------------------+---------+---------+--------+ 15 rows in set (0.00 sec) mysql> select min(sal) from emp; +----------+ | min(sal) | +----------+ | 800.00 | +----------+ 1 row in set (0.03 sec)
MAX(col)：返回指定列的最大值
mysql> select max(sal) from emp; +----------+ | max(sal) | +----------+ | 5000.00 | +----------+ 1 row in set (0.00 sec)
SUM(col)：返回指定列的所有之和（聚合函数）
mysql> select SUM(sal) from emp; +----------+ | SUM(sal) | +----------+ | 31381.54 | +----------+ 1 row in set (0.00 sec)
字符串函数
更新一条数据
mysql> update emp set ename= ' zhang ' where empmo = 777; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> update emp set empmo= '7777'rom emp where empmo = 777; Query OK, 1 row affected (0.01 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> select ename from emp where empmo = 7777; +-----------+ | ename | +-----------+ | zhang | +-----------+ 1 row in set (0.01 sec)
LTRIM(str)：去掉字符串str开头的空格
mysql> select ltrim(ename) from emp where empmo = 7777; +--------------+ | ltrim(ename) | +--------------+ | zhang | +--------------+ 1 row in set (0.00 sec)
RTRIM(str)：去掉字符串str尾部的空格
mysql> select rtrim (ename) from emp where empmo = 7777; +---------------+ | rtrim (ename) | +---------------+ | zhang | +---------------+ 1 row in set (0.00 sec)
通过length验证长度。查看是否去掉成功。
mysql> select length(rtrim(ename)) from emp where empmo = 7777; +----------------------+ | length(rtrim(ename)) | +----------------------+ | 7 | +----------------------+ 1 row in set (0.00 sec) mysql> select length(ename) from emp where empmo = 7777; +---------------+ | length(ename) | +---------------+ | 10 | +---------------+ 1 row in set (0.01 sec)
TRIM(str)：去掉字符串首部和尾部的所有空格
mysql> select trim(ename) from emp where empmo = 7777; +-------------+ | trim(ename) | +-------------+ | zhang | +-------------+ 1 row in set (0.00 sec)
通过length验证长度。查看是否去掉成功。
mysql> select length(ename) from emp where empmo = 7777; +---------------+ | length(ename) | +---------------+ | 10 | +---------------+ 1 row in set (0.01 sec) mysql> select length(trim(ename)) from emp where empmo = 7777; +---------------------+ | length(trim(ename)) | +---------------------+ | 5 | +---------------------+ 1 row in set (0.00 sec)
INSERT(str,x,y,instr)：将字符串str从第x位置开始，y个字符长的子字符串instr，返回结果
SUBSTRING(str,x,y)：截取字符串x开始y个；从第一个截取，截取1个。
mysql> select substring(ename,1,1) from emp; +----------------------+ | substring(ename,1,1) | +----------------------+ | S | | A | | W | | J | | M | | B | | C | | S | | K | | T | | A | | J | | F | | M | | | +----------------------+ 15 rows in set (0.00 sec) mysql> select ename from emp; +------------+ | ename | +------------+ | SMITH | | ALLEN | | WARD | | JONES | | MARTIN | | BLAKE | | CLARK | | SCOTT | | KING | | TURNER | | ADAMS | | JAMES | | FORD | | MILLER | | zhang | +------------+ 15 rows in set (0.00 sec) mysql> select substring(ename,1,2) from emp; +----------------------+ | substring(ename,1,2) | +----------------------+ | SM | | AL | | WA | | JO | | MA | | BL | | CL | | SC | | KI | | TU | | AD | | JA | | FO | | MI | | | +----------------------+
后面的长度可以不写。截取到结尾。
mysql> select substring(ename,1) from emp; +--------------------+ | substring(ename,1) | +--------------------+ | SMITH | | ALLEN | | WARD | | JONES | | MARTIN | | BLAKE | | CLARK | | SCOTT | | KING | | TURNER | | ADAMS | | JAMES | | FORD | | MILLER | | zhang | +--------------------+ 15 rows in set (0.00 sec)
日期函数
YEAR(date)：返回日期date的年份(1000~9999)
mysql> select year(hiredate) from emp; +----------------+ | year(hiredate) | +----------------+ | 1980 | | 1981 | | 1981 | | 1981 | | 1981 | | 1981 | | 1981 | | 1982 | | 1981 | | 1981 | | 1983 | | 1981 | | 1981 | | 1982 | | NULL | +----------------+ 15 rows in set (0.03 sec)
MONTH(date)：返回date的月份值(1~12)
mysql>select month(hiredate) from emp; +-----------------+ | month(hiredate) | +-----------------+ | 12 | | 2 | | 2 | | 4 | | 9 | | 5 | | 6 | | 12 | | 11 | | 9 | | 1 | | 12 | | 12 | | 1 | | NULL | +-----------------+ 15 rows in set (0.28 sec)
DAY(date)：返回date 的天数部分
mysql> select day(hiredate) from emp; +---------------+ | day(hiredate) | +---------------+ | 17 | | 20 | | 22 | | 2 | | 28 | | 1 | | 9 | | 9 | | 17 | | 8 | | 12 | | 3 | | 3 | | 23 | | NULL | +---------------+ 15 rows in set (0.00 sec)
HOUR(time)： 返回time的小时值(0~23)
mysql> select hour(hiredate) from emp; +----------------+ | hour(hiredate) | +----------------+ | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | NULL | +----------------+ 15 rows in set (0.00 sec)
MINUTE(time)：返回time的分钟值(0~59)
mysql> select minute(hiredate) from emp; +------------------+ | minute(hiredate) | +------------------+ | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | NULL | +------------------+ 15 rows in set (0.00 sec)
DATE(datetime）：返回datetime的日期值
mysql> select date(hiredate) from emp; +----------------+ | date(hiredate) | +----------------+ | 1980-12-17 | | 1981-02-20 | | 1981-02-22 | | 1981-04-02 | | 1981-09-28 | | 1981-05-01 | | 1981-06-09 | | 1982-12-09 | | 1981-11-17 | | 1981-09-08 | | 1983-01-12 | | 1981-12-03 | | 1981-12-03 | | 1982-01-23 | | NULL | +----------------+ 15 rows in set (0.00 sec)
TIME(datetime）：返回datetime的时间值
mysql> select time(hiredate) from emp; +----------------+ | time(hiredate) | +----------------+ | 00:00:00 | | 00:00:00 | | 00:00:00 | | 00:00:00 | | 00:00:00 | | 00:00:00 | | 00:00:00 | | 00:00:00 | | 00:00:00 | | 00:00:00 | | 00:00:00 | | 00:00:00 | | 00:00:00 | | 00:00:00 | | NULL | +----------------+ 15 rows in set (0.00 sec)
MySQL分组函数
别名:可以给字段表达式和函数还有表起别名
字段别名一个字段
mysql> select ename from emp; +------------+ | ename | +------------+ | SMITH | | ALLEN | | WARD | | JONES | | MARTIN | | BLAKE | | CLARK | | SCOTT | | KING | | TURNER | | ADAMS | | JAMES | | FORD | | MILLER | | zhang | +------------+ 15 rows in set (0.00 sec) mysql> select ename name from emp; +------------+ | name | +------------+ | SMITH | | ALLEN | | WARD | | JONES | | MARTIN | | BLAKE | | CLARK | | SCOTT | | KING | | TURNER | | ADAMS | | JAMES | | FORD | | MILLER | | zhang | +------------+ 15 rows in set (0.00 sec) mysql> select ename nam from emp; +------------+ | nam | +------------+ | SMITH | | ALLEN | | WARD | | JONES | | MARTIN | | BLAKE | | CLARK | | SCOTT | | KING | | TURNER | | ADAMS | | JAMES | | FORD | | MILLER | | zhang | +------------+ 15 rows in set (0.00 sec)
求多字段
mysql> select ename nam,sal w from emp; +------------+---------+ | nam | w | +------------+---------+ | SMITH | 800.00 | | ALLEN | 1600.00 | | WARD | 1250.00 | | JONES | 2975.00 | | MARTIN | 1250.00 | | BLAKE | 2850.00 | | CLARK | 2450.00 | | SCOTT | 3000.00 | | KING | 5000.00 | | TURNER | 1500.00 | | ADAMS | 1100.00 | | JAMES | 950.00 | | FORD | 3000.00 | | MILLER | 1300.00 | | zhang | 2356.54 | +------------+---------+ 15 rows in set (0.00 sec)
表达式别名
mysql> select ename name,sal*12 from emp; +------------+----------+ | name | 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 | | zhang | 28278.48 | +------------+----------+ 15 rows in set (0.00 sec)
函数别名
mysql> select avg(sal) from emp; +-------------+ | avg(sal) | +-------------+ | 2092.102667 | +-------------+ 1 row in set (0.00 sec) mysql> select avg(sal) avg_sal from emp; +-------------+ | avg_sal | +-------------+ | 2092.102667 | +-------------+ 1 row in set (0.00 sec) mysql> select avg(sal) 'avg wer' from emp; +-------------+ | avg wer | +-------------+ | 2092.102667 | +-------------+ 1 row in set (0.01 sec) mysql> select avg(sal) '平均值' from emp; +-------------+ | 平均值 | +-------------+ | 2092.102667 | +-------------+ 1 row in set (0.00 sec)
as关键字可以省略
mysql> select avg(sal) as '平均值' from emp; +-------------+ | 平均值 | +-------------+ | 2092.102667 | +-------------+ 1 row in set (0.29 sec)
--查询出对应的平均工资
mysql> select avg(sal) avg_sal,deptno from emp group by deptno; --查询出对应部门的平均工资group分组 bye那一个字段 +-------------+--------+ | avg_sal | deptno | +-------------+--------+ | 2356.540000 | NULL | | 2916.666667 | 10 | | 2175.000000 | 20 | | 1566.666667 | 30 | +-------------+--------+ 4 rows in set (0.00 sec)
分组工资总和
mysql> select sum(sal) total_sal,deptno from emp group by deptno; +-----------+--------+ | total_sal | deptno | +-----------+--------+ | 2356.54 | NULL | | 8750.00 | 10 | | 10875.00 | 20 | | 9400.00 | 30 | +-----------+--------+ 4 rows in set (0.00 sec)
求每一个部门中的最大值
mysql> select max(sal) max_sal,deptno from emp group by deptno; +---------+--------+ | max_sal | deptno | +---------+--------+ | 2356.54 | NULL | | 5000.00 | 10 | | 3000.00 | 20 | | 2850.00 | 30 | +---------+--------+ 4 rows in set (0.00 sec)
求每一个部门中的最小值
mysql> select min(sal) min_sal,deptno from emp group by deptno; +---------+--------+ | min_sal | deptno | +---------+--------+ | 2356.54 | NULL | | 1300.00 | 10 | | 800.00 | 20 | | 950.00 | 30 | +---------+--------+ 4 rows in set (0.00 sec)
求每一个部门中的人数；如果使用分组函数，那么不在分组函数中的字段必须存在于 group by 后面。
mysql> select count(*) count,deptno from emp group by deptno; +-------+--------+ | count | deptno | +-------+--------+ | 1 | NULL | | 3 | 10 | | 5 | 20 | | 6 | 30 | +-------+--------+ 4 rows in set (0.00 sec)
补充having
mysql> select avg(sal),deptno from emp group by deptno; +-------------+--------+ | avg(sal) | deptno | +-------------+--------+ | 2356.540000 | NULL | | 2916.666667 | 10 | | 2175.000000 | 20 | | 1566.666667 | 30 | +-------------+--------+ 4 rows in set (0.01 sec) mysql> select avg(sal) avg_sal,deptno from emp group by deptno where avg_sal > 2000; 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 'where avg_sal > 2000' at line 1 这个时候注意一下：按照以前的方式，查询的过程当中后面可以做一个筛选条件，但是现在用where做筛选条件的时候出现了上面的错误，原因是因为from是emp这张表，但emp这张表里面并没有avg_sal这一个字段，所以说会出现对应的错误。。。那么这个时候，当我们进行分组查询的时候，再分组之后要做筛选条件的时候，一定要用having mysql> select avg(sal) avg_sal,deptno from emp group by deptno having avg_sal > 2000; +-------------+--------+ | avg_sal | deptno | +-------------+--------+ | 2356.540000 | NULL | | 2916.666667 | 10 | | 2175.000000 | 20 | +-------------+--------+ 3 rows in set (0.04 sec)
HAVING 子句允许指定条件来过滤将出现在最终结果中的分组结果。
WHERE 子句在所选列上设置条件，而 HAVING 子句则在由 GROUP BY 子句创建的分组上设置条件。
Having：当它把整个的表格进行分组查询完之后，会生成一个新的结果。Having语句的avg_sal是新结果的里面的字段。
所以说在使用的时候如果使用where会去emp表里面查询字段，emp表里面并没有avg_sal字段，而having会去新的里面去查询，这时候有了这个字段就可以进行查询的操作了。