Oracle table query statement optimization

In practical applications, it is often necessary to perform complex data statistics, and it is often necessary to display data from multiple tables.

1) Data grouping - max, min, avg, sum, count
? How to display the highest salary and lowest salary among all employees
SQL> select max(sal),min(sal) from emp;

? How to display the owner with the highest salary among all employees
SQL> select ename,sal from emp where sal=(select max(sal) from emp);

? How to display all the information of the owner with the highest salary among all employees
SQL> select * from emp where sal=(select max(sal) from emp);

? How to display all the information of the owner of the highest salary and lowest salary among all employees
SQL> select * from emp where sal=(select max(sal) from emp) or sal=(select min(sal) from emp);

? Display the average salary and salary sum of all employees
SQL> select avg(sal),sum(sal) from emp;

? Count how many employees there are
SQL> select count(*) from emp;

? Calculate how many employees have a superior
SQL> select count(mgr) from emp;



Extended requirements:
? Please show the name of the highest paid employee, job title
SQL> select ename,job from emp where sal=(select max(sal) from emp);

? Please display the information of employees whose salary is higher than the average salary
SQL> select * from emp where sal>(select avg(sal) from emp);



1) group by and having clause
group by is used to group the results of the query and count the
having clause Used to limit the group display results

? How to display the average salary and maximum salary of each department
SQL> select avg(sal), max(sal), deptno from emp group by deptno;
means to perform related calculations by department

? Display each The average salary and minimum salary of each position in each department
SQL> select avg(sal), min(sal), deptno, job from emp group by deptno, job order by deptno;

? Display the average of each position in each department Salary, minimum salary, maximum salary, number of positions
SQL> select avg(sal),min(sal),max(sal),count(sal),deptno,job from emp group by deptno,job order by deptno;

?Display Department number with average salary below 2000 and its average salary (using having)
SQL> select avg(sal), deptno from emp group by deptno having avg(sal)<2000;

The role of having is to do secondary screening

? Display the department with MANAGER and its maximum salary and minimum salary
SQL> select avg( sal),min(sal),max(sal),deptno,job from emp group by deptno,job having job='MANAGER';

? Display the department number with an average salary higher than 2000 and a PRESIDENT job
SQL> select deptno from emp group by deptno, job having avg(sal)>2000 and job='PRESIDENT';