Database SQL language 04 subquery and grouping function

Subquery

What is subquery ?
In a query statement WHERE In the conditional clause , Another query statement is nested
Nested queries can consist of multiple subqueries , The solution is from inside to outside ;
The results returned by subqueries are generally collections , Therefore, it is recommended to use IN keyword ;

– Ask for the employee information of the Department where the highest paid employee works

analysis :
First of all :
Ask for the highest salary first :

second ： Ask for the department number with the highest salary :

Third ： The Department is required to be 10 Employee information :

Aggregate functions

– In a sense ,count(1) And count() Both represent queries for all data rows .
– count( Field ) Count the number of times the field appears in the table , The ignored field is null The situation of . That is to say, do not count the fields as null
The record of .
– count() It includes all the columns , Equivalent to the number of lines , When it comes to statistical results , The containing field is null The record of ;
– count(1) use 1 Represents the line of code , When it comes to statistical results , The containing field is null The record of .

subject :

Ask the average salary of the Department to be greater than 2000 The department in charge of the

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

where Written in group by front .
If you put the filter after the group
To use HAVING…
because having It's to filter from the previous fields , and where It's from the data table > Fields are filtered directly