Crosstable on multiple rows in PostgreSQL

Question

Hi I want to know how can do a pivot table with crosstable in a table like:

user_id	action	time
1	a	2022-01-01 12:30
1	b	2022-01-01 12:40
1	b	2022-01-01 12:50
1	c	2022-01-01 13:00
1	c	2022-01-01 13:10
1	c	2022-01-01 13:20
2	b	2022-01-01 13:11
2	c	2022-01-01 13:21

The expected result should look like this:

user_id	a	b	c
1	2022-01-01 12:30	2022-01-01 12:40	2022-01-01 13:00
1	NULL	2022-01-01 12:50	2022-01-01 13:10
1	NULL	NULL	2022-01-01 13:20
2	NULL	2022-01-01 13:11	2022-01-01 13:21

Following several tutorials, they gave me this result:

user_id	a	b	c
1	2022-01-01 12:30	2022-01-01 12:40	2022-01-01 13:00
2	NULL	2022-01-01 13:11	2022-01-01 13:21

However, as you can note, there is plenty of data loss by the single line result. Does any have an idea, please? Thanks in advance

Answer 1

Here is the SQL:

with action_a
as
(
 SELECT user_id,
       Row_number()
         OVER (
           ORDER BY a) id,
       a
FROM   (SELECT user_id,
               CASE
                 WHEN action = 'a' THEN time
               END "a"
        FROM   tabaction) x 
),
action_b
as
(
  SELECT user_id,
       Row_number()
         OVER (
           ORDER BY b) id,
       b
FROM   (SELECT user_id,
               CASE
                 WHEN action = 'b' THEN time
               END "b"
        FROM   tabaction) x     
),
action_c
as
(
SELECT user_id,
       Row_number()
         OVER (
           ORDER BY c) id,
       c
FROM   (SELECT user_id,
               CASE
                 WHEN action = 'c' THEN time
               END "c"
        FROM   tabaction) x 

)
SELECT action_a.user_id,
       action_a.a,
       action_b.b,
       action_c.c
FROM   action_a
       FULL OUTER JOIN action_b
               ON action_a.id = action_b.id
                  AND action_a.user_id = action_b.user_id
       FULL OUTER JOIN action_c
               ON action_b.id = action_c.id
                  AND action_b.user_id = action_c.user_id; 

Output:

 user_id |          a          |          b          |          c
---------+---------------------+---------------------+---------------------
       1 | 2022-01-01 12:30:00 | 2022-01-01 12:30:00 | 2022-01-01 12:00:00
       1 |                     | 2022-01-01 12:50:00 | 2022-01-01 13:10:00
       1 |                     |                     | 2022-01-01 13:20:00
       1 |                     |                     |
       1 |                     |                     |
       1 |                     |                     |
(6 rows)

Setup:

create table tabaction
( user_id int, action varchar(5) , time timestamp);

insert into tabaction values(1,'a','2022-01-01 12:30'),(1,'b','2022-01-01 12:30'),(1,'b','2022-01-01 12:50'),(1,'c','2022-01-01 12:00'),(1,'c','2022-01-01 13:10'),(1,'c','2022-01-01 13:20');

postgres=# select * from tabaction;
 user_id | action |        time
---------+--------+---------------------
       1 | a      | 2022-01-01 12:30:00
       1 | b      | 2022-01-01 12:50:00
       1 | c      | 2022-01-01 12:00:00
       1 | c      | 2022-01-01 13:10:00
       1 | c      | 2022-01-01 13:20:00
       1 | b      | 2022-01-01 12:30:00
(6 rows)

Ideal SQL should include user_id for action A, B, C so that those not missed out. Here the situation does not arise because USER_ID is only 1 and all actions are mapped to USER_ID. I believe might get different, in which case you might need this SQL:

with action_a
as
(
 SELECT user_id,
       Row_number()
         OVER (
           ORDER BY a) id,
       a
FROM   (SELECT user_id,
               CASE
                 WHEN action = 'a' THEN time
               END "a"
        FROM   tabaction) x 
),
action_b
as
(
  SELECT user_id,
       Row_number()
         OVER (
           ORDER BY b) id,
       b
FROM   (SELECT user_id,
               CASE
                 WHEN action = 'b' THEN time
               END "b"
        FROM   tabaction) x   
),
action_c
as
(
SELECT user_id,
       Row_number()
         OVER (
           ORDER BY c) id,
       c
FROM   (SELECT user_id,
               CASE
                 WHEN action = 'c' THEN time
               END "c"
        FROM   tabaction) x 

)
SELECT action_a.user_id a_userid,
       action_b.user_id b_userid,
       action_c.user_id c_userid,
       action_a.a,
       action_b.b,
       action_c.c
FROM   action_a
       FULL OUTER JOIN action_b
               ON action_a.id = action_b.id
                  AND action_a.user_id = action_b.user_id
       FULL OUTER JOIN action_c
               ON action_b.id = action_c.id
                  AND action_b.user_id = action_c.user_id;