MySQL database basic usage - aggregation - grouping

polymerization

• In order to get statistics quickly, 5 aggregation functions are provided
• count(*) means to calculate the total number of rows, write asterisk and column name in parentheses, the result is the same
• Query the total number of students

select count(*) from students;

　　

• max(column) means to find the maximum value of this column
• Query the maximum number of girls

select max(id) from students where gender=0;

　　

• min(column) means to find the minimum value of this column
• Query the minimum number of undeleted students

select min(id) from students where isdelete=0;

 

• sum(column) means to find the sum of this column
• After querying the boy's number

select sum(id) from students where gender=1;

　　

• avg(column) means to find the average of this column
• Query the average number of undeleted girls

select sum(id) from students where gender=1;

　　

grouping

• Group by field, indicating that data with the same field will be put into a group
• After grouping, only the same data column can be queried, and the different data columns cannot appear in the result set
• The grouped data can be counted and aggregated
• grammar

select column 1, column 2, aggregation... from table name group by column 1, column 2, column 3....

• 　　Query the total number of men and women

select gender as gender,count(*)

from students

group by gender;

　　Query the number of people in each city