SQL | Count based on each column's date value

Question

I have 3 tables:

CustomerID | PromotionDate
1111       | 01-01-2020
2222       | 03-06-2021
3333       | 05-07-2021
4444       | 09-10-2020

CustomerID | WorkID   
1111       | aaaa
1111       | bbbb
2222       | cccc
2222       | dddd
2222       | eeee

WorkID | StartDate
aaaa   | 01-01-2020
bbbb   | 01-02-2021
cccc   | 05-07-2020
dddd   | 06-08-2021
eeee   | 03-07-2022

I want to find the count of WorkIDs for each CustomerIDs where StartDate >= PromotionDate for that CustomerID.

So the result for the above sample should be:

CustomerID  | Count
1111        | 2
2222        | 2
3333        | 0
4444        | 0

How to achieve this?

Answer 1

Replace the table names with the real names and that should work as you except.

SELECT table1.CustomerID, COALESCE(a.totalCount,0) counts FROM table1
LEFT JOIN (SELECT table1.CustomerID, COUNT(table3.WorkID) as totalCount
FROM table3
LEFT JOIN table2 ON table3.WorkID = table2.WorkID 
LEFT JOIN table1 ON table2.CustomerID = table1.CustomerID 
WHERE table3.StartDate >= table1.PromotionDate 
GROUP BY table1.CustomerID ORDER BY table1.CustomerID ASC)a ON table1.CustomerID = a.CustomerID GROUP BY table1.CustomerID

I am using aggregation function called COUNT() to calculate the number of similar occurrences of the same Customer. GROUP BY to span similar records into one and in order to use the COUNT() function. ORDER BY is used to sort it by the CustomerID

My data:

Results:

Answer 2

Disclaimer: I don't use PostgreSQL so there may be a better option. However a general approach is to use conditional SUM() to count the number of records where StartDate >= PromotionDate :

SELECT t1.CustomerId 
       , SUM( CASE WHEN t3.StartDate >= t1.PromotionDate THEN 1 ELSE 0 END ) AS "Count"
FROM   Table1 t1 
          LEFT JOIN Table2 t2 ON t1.CustomerID = t2.CustomerID 
          LEFT JOIN Table3 t3 ON t2.WorkId = t3.WorkId 
GROUP BY t1.CustomerId  

Results:

customerid	totalcount
1111	2
2222	2
3333	0
4444	0

db<>fiddle here

Answer 3

This should work in Redshift:

SELECT p.customerid
     , count(s.startdate >= p.promotiondate OR NULL) AS total_count
FROM   cust_promo      p
LEFT   JOIN cust_work  w ON w.customerid = p.customerid
LEFT   JOIN work_start s ON s.workid = w.workid
GROUP  BY p.customerid;

db<>fiddle here

Assuming workid is UNIQUE in table work_start (as seems reasonable).

The aggregate FILTER clause performs better in modern Postgres. But Redshift doesn't keep up since its fork long ago:

SELECT p.customerid
     , count(*) FILTER (WHERE s.startdate >= p.promotiondate) AS total_count
FROM   cust_promo      p
LEFT   JOIN cust_work  w ON w.customerid = p.customerid
LEFT   JOIN work_start s ON s.workid = w.workid
GROUP  BY p.customerid;

See:

• How to get the latest non-negative value in SQL?
• For absolute performance, is SUM faster or COUNT?