Oracle group statistics query

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One 、 Group function

Sum up ：sum()
Average ：avg()
minimum value ：min()
Maximum ：max()

1、 Query the average age of students

select avg(age) from student;

2、 Query the maximum age of students

select max(age) from student;

3、 Query the maximum and minimum age of students

select max(age),min(age) from student;

4、 Query the total age of students

select sum(age) from student;

Two 、COUNT() function

count() Functions can be written in three ways , They are as follows
count(*)： It can accurately return the total number of records in the table .
count( Field )： Statistics are not null All data volume of .
count(distinct Field )： The amount of data after de duplication .

1、 Query the amount of data in the student table

select count(*) from student;

2、 The name in the query student table is not null The amount of data

select count(name) from student;

3、 Query the amount of data with no duplicate names in the student table

select count(distinct name) from student;

3、 ... and 、 Grouping statistics

group by： Group statistical functions

1、 Query the average age of students in each class in the student table , Group by class

select avg(age) from student group by class;

2、 Query the average age of students in each region in the student table , Group by Region

select avg(age) from student group by add;

Be careful ： If you don't use group by sentence , that select Only statistical functions are allowed in clause , Other fields are not allowed . For example, query the number of all students , And it is not allowed to find out the student's name .
error sql：select name,count() from student;
correct sel：select count() from student;

Be careful ： If you use group by sentence , that select Only... Is allowed in clause group by Statement field , Other fields are not allowed . For example, query the total number of students according to the class , And all student names are not allowed .
error sql：select name,class,count() from student group by class;
correct sql：select class,count() from student group by class;

Four 、 Multi table query and grouping statistics

Grouping statistics can be used with multi table queries .

1、 Check the student list id And teacher table sid Same student name , And count the number of students in groups according to their names

select s.name,count(*) from student s,teacher t where s.id = t.sid group by s.name;

5、 ... and 、HAVING Clause

having Is in group by After the statement , So go ahead group by After grouping, it can be based on having Clause to filter

1、 Query the class and average age of students in the student table , And group according to class , Then the average age is greater than 10 Class and grade check

select class,avg(age) from student group by class having avg(age) > 10;

6、 ... and 、 summary

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The commands involved in the article must be typed several times each like me , Only in the process of knocking can you find out whether you really master the command .

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