Club two table based on certain condition in postgresql - sql

Question

Here's my sample input tables:
employee_id
project
effective_date**
1
A
2014-08-13
1
B
2016-12-21
1
C
2018-02-21
employee_id
designation
effective_date
1
trainee
2014-08-05
1
senior
2016-08-17
1
team leader
2018-02-05
Table1: describes an employee who undergoes different projects at different date's in an organization.
Table2: describes the same employee from Table1 who undergoes different designation in the same organisation.
Now I want an Expected output table like this:
employee_id
project
designation
effective_date
1
A
trainee
2014-08-13
1
A
senior
2016-08-17
1
B
Senior
2016-12-21
1
B
team leader
2018-02-05
1
C
team leader
2018-02-21
The fact is that whenever:
his project changes, I need to display project effective_date.
his designation changes, I need to display designation effective_date but with the project he worked on during this designation change

Answer 1

This problem falls into the gaps-and-islands taxonomy. This specific variant can be solved in three steps:
applying a UNION ALL of the two tables while splitting "tab1.project" and "tab2.role" in two separate fields within the same schema
compute the partitions, between a non-null value and following null values, with two running sums (one for the "designation" and one for "project")
apply two different aggregations on the two different fields, to remove the null values.
WITH cte AS (
SELECT employee_id, effective_date,
project AS project,
NULL AS role FROM tab1
UNION ALL
SELECT employee_id, effective_date,
NULL AS project,
designation AS role FROM tab2
), cte2 AS (
SELECT *,
COUNT(CASE WHEN project IS NOT NULL THEN 1 END) OVER(
PARTITION BY employee_id
ORDER BY effective_date
) AS project_partition,
COUNT(CASE WHEN role IS NOT NULL THEN 1 END) OVER(
PARTITION BY employee_id
ORDER BY effective_date
) AS role_partition
FROM cte
)
SELECT employee_id, effective_date,
MAX(project) OVER(PARTITION BY project_partition) AS project,
MAX(role) OVER(PARTITION BY role_partition) AS role
FROM cte2
ORDER BY employee_id, effective_date
Check the demo here.