How to calculate average result for certain date?

Question

Could you help me? I have two tables.
Into the first table (activity) there are: user_id, sessions and login_time.
Into the second (payments) there's only one column - user_id.

Here's me query:

SELECT activity.login_time, activity.user_id, avg(activity.sessions) as 
user_sessions
FROM activity
inner JOIN payments ON payments.user_id = activity.user_id
WHERE activity.login_time ='2018-04-05' group by activity.user_id;

Using this query, I get such table:

+------------+---------+---------------
| login_time | user_id | user_sessions
+------------+---------+---------------
| 2018-04-05 |     107 |       12.0000
| 2018-04-05 |     110 |        1.0000
| 2018-04-05 |     112 |        5.0000
| 2018-04-05 |     115 |        5.0000
| 2018-04-05 |     117 |        7.0000
| 2018-04-05 |     120 |        1.0000
| 2018-04-05 |     123 |        1.0000
...

How should I make a query to get average:

+------------+------------
| login_time | avg_user_sessions 
+------------+---------
| 2018-04-05 | 4,57

Note: difficulty is in that user_id has duplicates

Tables

user_id login_time  sessions
107 2018-04-05  12
110 2018-04-05  1
112 2018-04-05  5
115 2018-04-05  5
117 2018-04-05  7
120 2018-04-05  1
123 2018-04-05  1



user_id
107
107
107
110
112
115
115
117
120
123

Answer 1

If there are many user_id duplicates in payments table, you can try to use DISTINCT in your user_id from payments table.

but in your case, You can only select activity directly, don't need to join with payments , because you didn't get any column from it.

CREATE TABLE activity(
   login_time date,
   user_id int,
   sessions float
);

CREATE TABLE payments (
   user_id INT
);



INSERT INTO payments VALUES (107);
INSERT INTO payments VALUES (107);
INSERT INTO payments VALUES (110);
INSERT INTO payments VALUES (112);
INSERT INTO payments VALUES (115);
INSERT INTO payments VALUES (115);
INSERT INTO payments VALUES (117);
INSERT INTO payments VALUES (120);
INSERT INTO payments VALUES (123);





INSERT INTO activity VALUES ('2018-04-05',107,12);
INSERT INTO activity VALUES ('2018-04-05',110,1);
INSERT INTO activity VALUES ('2018-04-05',112,5);
INSERT INTO activity VALUES ('2018-04-05',115,5);
INSERT INTO activity VALUES ('2018-04-05',117,7);
INSERT INTO activity VALUES ('2018-04-05',120,1);
INSERT INTO activity VALUES ('2018-04-05',123,1);

Query 1 :

SELECT a.login_time, avg(a.sessions) as 
user_sessions
FROM activity a
inner JOIN (SELECT DISTINCT user_id FROM payments) p ON p.user_id = a.user_id
WHERE a.login_time ='2018-04-05' 
group by a.login_time

Results :

| login_time |     user_sessions |
|------------|-------------------|
| 2018-04-05 | 4.571428571428571 |