按日期分组查询

Question

I'm working on the following user table, where role = 2 means the user is an instructor, and role = 3 means that the user is a student.

+--------+------+---------------+
|  name  | role | creation_date |
+--------+------+---------------+
| Tom    |    2 | 2020-07-01    |
| Diana  |    3 | 2020-07-01    |
| Rachel |    3 | 2020-07-01    |
| Michel |    3 | 2020-08-01    |
+--------+------+---------------+

My goal is to select the sum value of all instructors and students, grouped by date. The result should look like this:

+------------------+---------------+---------------+
| totalInstructors | totalStudents | creation_date |
+------------------+---------------+---------------+
|                1 |             2 | 2020-07-01    |
|                0 |             1 | 2020-08-01    |
+------------------+---------------+---------------+

In this case, on 2020-07-01, I had 1 instructor and 2 students registered and on 2020-08-01, I had no instructors and I had 1 student registered.

My problem is that I am having difficulties in setting up this query, if someone can help me thank you very much!

Answer 1

Use conditional aggregation:

SELECT
    creation_date,
    COUNT(CASE WHEN role = 2 THEN 1 END) AS totalInstructors,
    COUNT(CASE WHEN role = 3 THEN 1 END) AS totalStudents
FROM yourTable
GROUP BY
    creation_date;

Demo

Answer 2

You would need count with a case statement as follows

select count(case when role=2 then 1 end) as totalInstructors 
      ,count(case when role=3 then 1 end) as totalStudents 
      ,creation_date 
  from tbl
group by creation_date

Answer 3

A simple GRoupBY and SUM helps

This works. because the compasison role = 2 gives 1 back it it tue and 0 if it is false

 CREATE TABLE table1 ( `name` VARCHAR(6), `role` INTEGER, `creation_date` VARCHAR(10) ); INSERT INTO table1 (`name`, `role`, `creation_date`) VALUES ('Tom', '2', '2020-07-01'), ('Diana', '3', '2020-07-01'), ('Rachel', '3', '2020-07-01'), ('Michel', '3', '2020-08-01');

 SELECT SUM(`role` = 2) totalInstructors, SUM(`role` = 3) totalStudents, `creation_date` FROM table1 GROUP BY `creation_date` ORDER BY `creation_date`

 totalInstructors | totalStudents | creation_date ---------------: | ------------: |:------------ 1 |  2 |  2020-07-01 0 |  1 |  2020-08-01 

db<>fiddle here

Answer 4

Please use below query,

select 
case when role = 2 then count(1) end as totalInstructors,
case when role = 3 then count(1) end as totalStudents,
creation_date
from table_name
group by  creation_date;

You can use COALESCE() to replace null with 0

select COALESCE(totalInstructors, 0) as totalInstructors, COALESCE(totalStudents, 0) as totalStudents,creation_date
from
(select 
case when role = 2 then count(1) end as totalInstructors,
case when role = 3 then count(1) end as totalStudents,
creation_date
from table_name
group by  creation_date) qry;