Oracle Advanced query

Group query

The concept of grouping function

Grouping functions act on a set of data , And return a value for a set of data .

Use of grouping functions

The grouping function automatically ignores null values , Can pass nvl() Go for special treatment .

AVG and SUM

//  Calculate the average salary and total salary of employees 
select avg(sal), sum(sal) from emp;

MIN and MAX

//  Find out the maximum and minimum wage of employees 
select  MAX(sal),MIN(sal) from emp;

COUNT

//  Find out the total number of employees 
select count(*) from emp;

WM_CONCAT( Transfer line column )

select deptno,wm_concat(ename) from emp group by deptno;

Use Group by Clause data grouping

//  Calculate the average salary of each department 
select deptno,avg(sal) from emp group by deptno;

partition by Partition function

select deptno,row_number() over(partition by ename order deptno) from emp group by deptno;

Partition function partition by Usage of

Use Having Sentence filter grouping result set

Can't be in Where Use group function in clause , Can be in Having Use group function in clause

**where and having If it can be used universally , Use as much as possible where **,having First group and then filter ,where First filter and then group .

GROUP BY Sentence enhancement ( Make reports )

effect

Realization

select deptno,job,sum(sal) from emp group by deptno,job;
+
select deptno,sum(sal) from emp group by deptno;
+
select sum(sal) from emp;

 The above three statements are equal to the following one 
====
select deptno,job,sum(sal) from emp group by rollup(deptno,job);

Multiple table joins

Cartesian collection ( Multi-table query )

Equivalent connection

Query employee information , Ask to show ： Employee number , full name , a monthly salary , Department name

select e.empno,e.ename,e.sal,d.dname from emp e ,dept d where e.deptno = d.deptno;

Unequal value connection

Query employee information , Ask to show ： Employee number , full name , a monthly salary , The salary level of employees

select e.empno,e.ename,e.sal,s.grade from emp,salgrade s where e.sal  between s.losal and s.hisal;

External connection

Count the number of employees by department , Ask to show ： Department number , Department name , The number of

//  Right connection (dept  Main table ) , If you can understand it, you can select d.deptno,d.dname,count(e.empno)  The number of from emp e, dept dwhere e.deptno(+)=d.deptnogroup by d.deptno,d,dname;//  I suggest you write this select d.deptno,d.dname,count(e.empno)  The number of from emp e left join dept don e.deptno=d.deptnogroup by d.deptno,d,dname;//  The left outer join （emp  Main table ）select d.deptno,d.dname,count(e.empno)  The number of from emp e, dept dwhere e.deptno=d.deptno(+)group by d.deptno,d,dname;select d.deptno,d.dname,count(e.empno)  The number of from emp e right join dept don e.deptno=d.deptnogroup by d.deptno,d,dname;

Self join

Query the employee's name and the employee's boss's name

select e.ename  Employee name ,b.ename  The name of the boss from emp e,emp bwhere e.mgr = b.empno

• Not suitable for large table operation

select level,empno,ename,sal,mgrfrom empconnect by piror empno = mgrstart with mgr is nullorder by 1;

• Single table query , There is no obvious result of self connection

Subquery

You can use the location of the subquery where,select ,from,having

select Location use subquery

stay having Location use subquery

stay from Location use subquery

Cannot use the location of subquery group by

Top-N Query questions

// Query the information of the top three employees with the highest wages select rownum, empno,enmame,sal from （select * from emp order by sal desc） where rownum <= 3

Two problems need to be paid attention to in line number

• Line numbers are always generated in the default order
• The line number can only be used <.,<=; Out of commission >,>=

Sub query null Value problem （ Many lines ）

Case study

Paging query

// Query the first 5 To 8 The record of the line 
select r,empno,ename,sal
(select rownum r,empno,ename,sal
from (select  rownum ,empno,ename,sal from emp order by sal desc) e1
where rownum <=8) e2
where r >=5

Find the employee whose salary in the employee list is higher than the average salary of the Department

select empno,ename,sal,(select avg(sal) from emp where deptno = e.deptno) avgsal
from emp e
where sal > (select avg(sal) from emp where deptno = e.deptno)



select e.empno,e.ename,e.sal,d.avgsal
from emp e, (select deptno,avg(sal) avgsal from emp group by deptno) d
where e.deptno = d.deptno and e.sal > d.avgsal;

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