Loop through rows and match values in SQL - sql

Question

appreciate any help with my problem! I have an org chart of all employees and then columns for their supervisors. I am trying to find the first in the org structure supervisor for each employee that has 3+ years' experience. So if supervisor 1 has only 1 year, I will need to move to the next column with super visor 2 and see if they have more experience. At the end, I would like to return a column of supervisors' ids [experienced_supervisor column]
Table: org_chart
id | experience | supervisor_id_1| supervisor_id_2 | experienced_supervisor
A | 2 | X | C | X
C | 5 | V | D | D
V | 1 | M | X | M
X | 3
D | 8
M | 11
I am new to SQL and not even sure if this is the best approach. But here is my thinking: I will use CASE to look though every row (employee) and compare their supervisor's experience.
SELECT CASE
WHEN experience >=3 THEN supervisor_id_1
ELSE
CASE WHEN experience >=3 THEN supervisor_id_2
ELSE 'not found'
END AS experienced_supervisor
FROM org_chart
Questions:
Is this the best way to tackle the problem?
Can I look up the value [experience years] of supervisors by matching supervisor_id_1, supervisor_id_2 to id? Or do I need to create a new column supervisor_id_1_experience and fill the years of experience by doing the join?
I am using Redshift.

Answer 1

You only need one case expression, but a lot of joins or subqueries. Perhaps
SELECT (CASE WHEN (SELECT oc2.experience >=3 FROM org_chart oc2 WHERE oc2.id = supervisor_id_1) >= 3
THEN supervisor_id_1
WHEN (SELECT oc2.experience >=3 FROM org_chart oc2 WHERE oc2.id = supervisor_id_2) >= 3
THEN supervisor_id_2
. . .
END) AS experienced_supervisor
FROM org_chart oc

Answer 2

After lots of trial and errors here is the result that worked for my problem. I am using Redshift in this case.
-- Use common table expression to find job level for each supervisor from reporting levels 8 to 2
WITH cte1 AS
(
SELECT B.id as employee
,B.experience as employee_experience
,B.supervisor_id_1 as manager_1
,A.experience as supervisor_1_experience
FROM org_chart
INNER JOIN org_chart B ON B.supervisor_id_1 = A.id
),
cte2 AS
(
SELECT B.id as employee2
,B.experience as employee_experience
,B.supervisor_id_2 as manager_2
,A.experience as supervisor_2_experience
FROM org_chart
INNER JOIN org_chart B ON B.supervisor_id_2 = A.id
),
........-- Write as many statements as I have columns with reporting levels
-- Join all tables above
cte3 AS
(
SELECT employee
,employee_experience
,manager_1
,supervisor_1_experience
,manager_2
,supervisor_2_experience
FROM cte1
JOIN cte2 ON cte2.employee2 = cte1.employee
....... -- Write as many statements as I have columns with reporting levels
)
-- Run through every row and evaluate if each supervisor has more than 3 years of experience
SELECT *
,CASE
WHEN cte3.supervisor_1_experience >= 3 THEN cte3.manager_1
WHEN cte3.supervisor_1_experience < 3
AND cte3.supervisor_2_experience >=3
THEN cte3.manager_2
........ -- Write as many statements as I have columns with reporting levels
END experienced_supervisor
FROM cte3