——— aggregation function and grouping of relational database

Count: Count rows

Select COUNT (*) from student

Select count (age) from student/* will not count null values */

Sum: Returns the sum of the values in the column and automatically skips the null value (numeric +null=null)

Select SUM (age) from student

Avg: Returns the average of all values in the result set

Select AVG (age) from student

Max/min: Returns the maximum/small value in all values

Select Max (age) from student

GROUP BY: Data Group query

Select Age,name,count (*) from the student group by age/* Group queries with the ages and count how many people are in each age range */

Having: Filtering the data again after grouping

Select Age,name,count (*) from student Group by age have age=21

——— aggregation function and grouping of relational database