[Oracle database] mammy tutorial day04 Sorting Query

Sort groups

1 Sort

select Name , Name ,.., Name from Table name where Conditional expression order by Column name of sorting column asc|desc;

asc|desc: asc Expressing ascending order ,desc Representation of descending order .

give an example ： Query employee information , Sort by employee's salary in ascending order

SELECT * FROM emp ORDER BY sal ASC;  --- If it's in ascending order ,asc It can be omitted 
SELECT * FROM emp ORDER BY sal;

give an example ： Query employee information , Sort by employee number in descending order

SELECT * FROM emp ORDER BY empno DESC;

order by ： It can be followed by multiple rows , With multiple sort Columns , What it means ： In the first row （ Follow closely order by Columns of keywords ） Sort , If the value of the first sorting field is the same , It will sort by the second sort field .

give an example ： Query employee information , Sort by department number in ascending order , If the Department numbers are the same , Sort by salary in ascending order

SELECT * FROM emp ORDER BY deptno ASC,sal ASC;

give an example ： Query employee information , Sort by department number in descending order , If the Department numbers are the same , Sort by employee number in ascending order

SELECT * FROM emp ORDER BY deptno DESC,empno;

give an example ： Check the employee's number 、 full name 、 Work 、 Wages 、 Department number , Sort by department number in ascending order , If the departments are the same , Sort by salary in descending order

SELECT empno,ename,job,sal,deptno FROM emp ORDER BY deptno,sal DESC;

Be careful ：order by It can be followed by the column names in the table , You can follow select and from Sequence number of query results between

SELECT empno,ename,job,sal,deptno FROM emp ORDER BY 5,4 DESC;

SQL Statement execution order

FROM
WHERE
GROUP BY
HAVING
SELECT
ORDER BY

2 Aggregate functions

Aggregate functions ： Perform calculations on a set of values , And return a single value .

count( Name |*| constant ): It counts the number of records （ Number of data ）

max( Name ): Taking the maximum

min( Name ): Minimum value

avg( Name ): Average.

sum( Name ): Sum up

give an example ： Query the number of employees in the employee table

SELECT COUNT(empno) FROM emp;
SELECT COUNT(*) FROM emp;
SELECT COUNT(1) FROM emp;

count( Primary key column or index column )
count(*)
count( constant )
count( Common column )
 They perform efficiently from top to bottom , Lower in turn .
null Value does not participate in  count Count .

give an example ： Inquire about emp Number of departments in the table

select count(deptno) from emp;

give an example ：distinct: Remove keywords , Follow at the top of the column

select distinct deptno from emp;

select count(distinct deptno) from emp;

select empno,distinct deptno from emp;  -- This sql The statement is wrong 

select distinct empno,deptno from emp;  

Be careful ：distinct When followed by more than one column , Judge duplicate data , When the values of all columns are exactly the same , It will be considered as duplicate data

give an example ： Check the total wage

select sum(sal) from emp;

give an example ： Query the average salary of employees

select avg(sal) from emp;

give an example ： Inquire about 10 The highest salary in department No

select max(sal) from emp where deptno=10;

give an example ： Query minimum wage

select min(sal) from emp;

3 grouping

select Name , Name ,..., Name from Table name

where Conditions

group by Group columns

order by Sort columns asc|desc

group by: Grouped keywords , Followed by the group column name , It can be a grouping column , It can also be multiple columns

give an example ： Query the department number and average salary of each department

select deptno,avg(sal) from emp group by deptno;

give an example ： Query the number of employees in each department

select deptno,count(empno) from emp group by deptno;

select job,deptno,count(*) from emp group by job,deptno;
 Be careful ：group by  When followed by more than one column , Only when the values of multiple columns are equal at the same time , It will be divided into the same group ;

4 HAVING

select col_name,col_name,...,col_name

from Table name

where Conditions

group by Group columns

having Conditions

order by Sort columns

having: It is used to filter the grouped data , Aggregate functions can be used in conditional expressions

give an example ： Query the department number and average salary of each department

select deptno,avg(sal) from emp group by deptno;

give an example ： The average wage is greater than 2000 Department number and average wage of

select deptno,avg(sal) from emp group by deptno having avg(sal)>2000;

where and having Similarities and differences

where: Conditions ,where The following conditions are better than having The latter condition is executed first , Aggregate functions... Are not allowed in conditions .

having： You can use aggregate functions in conditions , commonly having and group by In combination with .