分组函数
group bySELECT * FROM studetail;
group by stusex 根据性别进行分组
count() 统计个数
SELECT stusex, count(stusex) FROM studetail group by stusex ;根据年龄进行分组统计个数
SELECT stuage,count(*) FROM studetail group by stuage ;根据性别进行分组求平均年龄
SELECT stusex,avg(stuage) FROM studetail group by stusex ;根据性别分组 求最大值 / min 求最小年龄
SELECT stusex,max(stuage) FROM studetail group by stusex ;sum 求年龄的总和
SELECT stusex,sum(stuage) FROM studetail group by stusex ;
HAVING WHERE
条件的意思
SELECT * FROM studetail WHERE stuage >18;分组后 还有其他条件
根据性别进行分组 ，求分组后的平均年龄，获取平均年龄大于18岁的信息
SELECT stusex,avg(stuage) FROM studetail group by stusex HAVING avg(stuage) = 18;根据工作岗位进行分组，求每一个组的平均工资，筛选平均工资大于5000的
SELECT job_position,avg(salary),count(*) FROM emp group by job_position HAVING avg(salary) > 5000 ;SELECT sum(salary) FROM emp ;
SELECT avg(salary) FROM emp ;
SELECT max(salary) FROM emp ;HAVING
根据工作岗位进行分组，求每一个组的平均工资，筛选平均工资大于5000的 从大到小排序
order by desc asc
SELECT job_position,avg(salary),count(*) FROM emp
group by job_position
HAVING avg(salary) > 5000
order by avg(salary) desc;SELECT job_position,avg(salary),count(*) FROM emp
group by job_position
order by avg(salary) desc;
limit 限制行号
SELECT job_position,avg(salary),count(*) FROM emp
group by job_position
HAVING avg(salary) > 5000
order by avg(salary) desc
limit 3;SELECT id,empname,salary FROM emp order by salary desc;SELECT id,empname,salary FROM emp order by salary desc limit 3;
IFNULL
WITH ROLLUP 分组之后进行job_position求和IFNULL job_position
SELECT ifnull(job_position,"部门总和"),count(job_position) FROM emp
group by job_position WITH ROLLUP;SELECT deptid FROM students;
SELECT IFNULL(deptid,"1011") FROM students;
DISTINCT 去重
SELECT DISTINCT empname FROM emp;
left 截取
SELECT left(salary,4) FROM emp;
SELECT right(salary,3) FROM emp;
SELECT substring(salary,1,4) FROM emp; # 取第一个到第四个
CONCAT
SELECT CONCAT(salary,empname) as namesalry FROM emp;
SELECT NOW(); # 查询当前时间
SELECT * FROM department;
+-----+--------------------+
| did | dname |
+-----+--------------------+
| 102 | 上海交通大学 |
| 103 | 浙江大学 |
| 101 | 清华大学 |
| 105 | 理工学院 |
+-----+--------------------+
SELECT * FROM department join students on ;
+-----+---------+--------+
| sid | sname | deptid |
+-----+---------+--------+
| 1 | 张三 | 101 |
| 2 | 李四 | 102 |
| 3 | 王五 | 103 |
| 4 | 赵六 | 101 |
| 5 | 孙琪 | 102 |
| 6 | 小花 | 103 |
| 8 | 小测 | 105 |
| 9 | c俄式 | NULL |
+-----+---------+--------+
＃建立约束的时候 会创建索引
show keys from students;索引 书籍目录id
模糊查询 like _一个字符 % 任意给字符
SELECT empname from emp WHERE empname like "_花";
SELECT empname from emp WHERE empname like "%花%";
