Oracle database-row query

The filter conditions are:

• 比较：=、>、<、>=、<=、!=、<>、between and
• 逻辑：and、or、not、union、union all、intersect、minus
• null：is null、is not null、not is null
• Fuzzy query: like:% _
• in、exists
• Subquery

Statement:

where 条件

Implementation process:

from→where→select

1. Comparison conditions

1、=

-- 查询部门编号为30的员工
select * from emp where deptno = 30

operation result

2、>

-- 查询工资大于1500的员工
select * from emp where sal > 1500

operation result

3、<

-- 查询工资小于1500的员工
select * from emp where sal < 1500

operation result

4、>=

-- 查询工资大于等于1600的员工
select * from emp where sal >= 1600

operation result

5、<=

-- 查询工资小于等于1600的员工
select * from emp where sal <= 1600

operation result

6、!=|<>

!=And <>the effect is the same

-- 查询不是30部门的员工 !=
select * from emp where deptno != 30
-- 查询不是30部门的员工 <>
select * from emp where deptno <> 30

operation result

7、between and

between andIs greater than or equal to a certain value, and less than or equal to a certain value

-- 大于等于并且小于等于
select * from emp where sal between 1250 and 1500

operation result

Two, logical operations

1、and

And, satisfy the left and satisfy the right

-- and
select * from emp where sal >= 1250 and sal<=1500

operation result

2、or

Or, meet the left or right

-- or
select * from emp where deptno = 20 or deptno = 30

operation result

3、not

Negate

-- 不在部门30的员工
select * from emp where not deptno = 30

operation result

4、union

Union, remove duplication, union of two result sets, sort by default rules

select * from emp where deptno = 30
union
select * from emp where deptno = 20

operation result

5、union all

Same as above, but does not remove duplicate record values ​​and does not sort

select * from emp where mgr = 7839
union all
select * from emp where deptno = 20

operation result

6、intersect

Intersection, find duplicate records, intersection of two result sets, default sort

select * from emp where mgr = 7839
intersect
select * from emp where deptno = 20

operation result

7、minus

Difference set, remove duplicate records, difference set of two result sets, default sort

select * from emp where sal >=1500
minus
select * from emp where sal <=3000

operation result

Three, null

1、is null

Is null

-- 没有奖金的员工
select * from emp where comm is null

operation result

2、is not null|not is null

Not null, is not nulland not is nullconsistent

-- 有奖金的员工
select * from emp where not comm is null
select * from emp where comm is not null

operation result

Four, fuzzy query

Tips: fuzzy query efficiency is low

grammar:like 通配符

1、%

%: any number of symbols, zero to any number of symbols

-- 名字中带N的员工
select * from emp where ename like '%N%'

operation result

2、_

_: one character

-- 名字的第一个字母是A的员工
select * from emp where ename like '_A%'

operation result

Five, in and exists

1、in

Equivalent to multipleor

-- 20部门和30部门的员工
select * from emp where deptno in (20,30)

operation result

2、exists

If exists has a result, the outer layer retains the data, and the inner query uses the external data

select * from emp e1 where exists(select * from emp e2 where e1.deptno = e2.deptno)

operation result

Six, subqueries

-- 有员工的部门的部门名称
select dname from dept where deptno in (select distinct deptno from emp)

operation result