There are several ways to query multiple tables

There are three methods for multi-table query, which are: 1. Traditional method, including left outer join query, right outer join query, and complete outer link query; 2. Subquery method, including single row query and multi-row query. Query; 3. Aggregation query methods, including summation, average query, and total number of records.

There are three methods for multi-table query, namely:

1. Traditional method

/*-- 传统连接方式 --*/
 
select e.ename,d.dname
 
from dept d , emp e
 
where d.deptno = e.deptno
 
 
 
/*--natural join 两张表中只有一个同名栏位时行连接 --*/
 
select e.ename,d.dname
 
from dept d 
 
 
natural join emp e
 
 
 
/*--using 两个表中有多个同名栏位时 --*/
 
select e.ename,d.dname
 
from dept d join emp e
 
using(deptno)

1. Left outer join query

/*--左外连结 左表的全部记录（部门表中的所有记录：demp）-- */
 
/*--方式１--*/
 
select e.ename,d.dname from dept d left outer join emp e
 
on d.deptno = e.deptno
 
/*--方式２--*/ 
 
 
select e.ename,d.dname
 
from dept d , emp e
 
where d.deptno = e.deptno(+)

2. Right outer join query

/*--左外连结 左表的全部记录（部门表中的所有记录：demp）-- */
 
/*--方式１--*/
 
select e.ename,d.dname from dept d left outer join emp e
 
on d.deptno = e.deptno
 
/*--方式２--*/ 
 
 
select e.ename,d.dname
 
from dept d , emp e
 
where d.deptno = e.deptno(+)

3. Complete outer link query

/*--完全外连结(匹配与不匹配)-- (包括部门表及员工表中的所有记录)*/
 
select e.ename,d.dname from dept d full outer join emp e
 
on d.deptno = e.deptno

2. Sub-query (single or multiple rows)

1. Single-row query

/*--子查询（单行，多行）--*/
 
select * from emp
 
where sal > (select avg(sal) from emp)
 
/*--单行子查询可以引用（＝，＞，＞＝，＜，＜＝，＜＞）操作符 --*/
 
select * from emp e
 
where e.deptno=(select d.deptno from dept d where d.deptno=1 )

2. Multi-row query

/*--多行子查询可以引用(in,any,all)操作符--*/
 
select * from emp e
 
where e.deptno in(select d.deptno from dept d)
 
/*-- any 比较返回值中的任何一个,其中一个满足，则返回true --*/
 
select * from emp e
 
where e.sal < any(select sal from emp where deptno=2)
 
/*-- all 比较返回值中的所有,全部满足，则返回true --*/
 
select * from emp e
 
where e.sal < all(select sal from emp where deptno=2)

3. Aggregation query (sum, average, total number of records)

1. Sum, average query

/*--求和，平均 --*/
 
select sum(sal),avg(sal) from emp
 
select avg(nvl(sal,0)) from emp

2. Total number of records

/*--记录总数 --*/
 
select count(*) from emp
 
select count(memo) from emp /*--统计该栏非空记录 --*/
 
select count(distinct(sex)) from emp /*--去掉重复记录 --*/
 
/*-- group by --*/
 
select e.deptno,sum(e.sal)
 
from emp e
 
group by e.deptno
 
/*-- group by having --*/
 
select e.deptno,avg(e.sal)
 
from emp e
 
group by e.deptno
 
having avg(e.sal) > 4500

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