Introduction to Conditional Aggregation and Subtraction in SQL Queries

As a technical blogger, I often come across questions and queries that require creative solutions using SQL. In this article, we’ll explore how to use conditional aggregation to calculate the sum of certain values and then subtract these sums from another related value.

Background on Conditional Aggregation

Conditional aggregation is a powerful feature in SQL that allows you to perform calculations on rows based on conditions applied to the data. This technique can be used to group by multiple columns, apply different aggregations to each group, or even calculate the difference between two values within the same row.

In our example, we’ll use conditional aggregation to combine two queries and calculate a third value that represents their difference.

The Problem: Combining Two Queries with Conditional Aggregation

Suppose you have two similar queries that return payment data from different tables. Both queries use a date column (POST_DATE) as the primary filter and group by this date.

Query 1 calculates the total amount of payments for specific detail types (2, 5, 11, 20, 22, 32, 33) and multiplies these amounts by -1 to represent negative payments.

SELECT 
    TDL.POST_DATE,
    SUM(CASE WHEN TDL.DETAIL_TYPE IN(2,5,11,20,22,32,33) THEN TDL.AMOUNT*-1 ELSE 0 END) PAYMENTS

FROM 
STG_OJDT.STG_CL.CLARITY_TDL_TRAN TDL

WHERE 
TDL.POST_DATE = '2018-08-01 00:00:00'
AND TDL.SERV_AREA_ID = 10

GROUP BY 
TDL.POST_DATE

Query 2 calculates a specific amount of payments for the same detail types, but only for certain bill areas (810000020, 810000025, 810000030).

SELECT 
    TDL.POST_DATE,
    SUM(CASE WHEN TDL.DETAIL_TYPE IN(2,5,11,20,22,32,33) AND TDL.BILL_AREA_ID IN (810000020, 810000025, 810000030) THEN TDL.AMOUNT*-1 ELSE 0 END) CCP_PAYMENTS

FROM 
STG_OJDT.STG_CL.CLARITY_TDL_TRAN TDL

WHERE 
TDL.POST_DATE = '2018-08-01 00:00:00'
AND TDL.SERV_AREA_ID = 10    
GROUP BY  TDL.POST_DATE;

We want to combine these two queries and calculate a third value representing the difference between the total payments for each day.

The Solution: Using Conditional Aggregation and Subtracting Values

To achieve this, we can use conditional aggregation to combine the results of both queries into a single query. This involves applying different aggregations to specific conditions and then subtracting one aggregated value from another.

Query 1 with Conditional Aggregation (Payments)

SELECT 
    TDL.POST_DATE,
    SUM(CASE WHEN TDL.DETAIL_TYPE IN(2,5,11,20,22,32,33) THEN TDL.AMOUNT*-1 ELSE 0 END) as PAYMENTS,
    SUM(CASE WHEN TDL.DETAIL_TYPE IN (2, 5, 11, 20, 22, 32, 33) AND TDL.BILL_AREA_ID IN (810000020, 810000025, 810000030) THEN TDL.AMOUNT*-1 ELSE 0 END) as CCP_PAYMENTS
FROM STG_OJDT.STG_CL.CLARITY_TDL_TRAN TDL

WHERE 
TDL.POST_DATE = '2018-08-01 00:00:00' AND
      TDL.SERV_AREA_ID = 10    
GROUP BY  TDL.POST_DATE;

In this query, we’re using conditional aggregation to calculate the total payments and CCP payments for each day. We’re also grouping by the post date.

Query 2 with Conditional Aggregation (Difference)

To calculate the difference between the two sums, we can use a single query that combines both conditions:

SELECT 
    TDL.POST_DATE,
    SUM(CASE WHEN TDL.DETAIL_TYPE IN(2,5,11,20,22,32,33) THEN TDL.AMOUNT*-1 ELSE 0 END) as diff
FROM STG_OJDT.STG_CL.CLARITY_TDL_TRAN TDL

WHERE 
TDL.POST_DATE = '2018-08-01 00:00:00' AND
      TDL.SERV_AREA_ID = 10 AND 
      TDL.BILL_AREA_ID NOT IN (810000020, 810000025, 810000030) 
GROUP BY TDL.POST_DATE;

However, this query only calculates the difference when a specific bill area is not present. We want to calculate the difference for all dates.

Combining Queries and Calculating Difference

To combine both queries and calculate the difference, we can use the following approach:

SELECT 
    TDL.POST_DATE,
    SUM(CASE WHEN TDL.DETAIL_TYPE IN(2,5,11,20,22,32,33) THEN TDL.AMOUNT*-1 ELSE 0 END) as PAYMENTS,
    SUM(CASE WHEN TDL.DETAIL_TYPE IN (2, 5, 11, 20, 22, 32, 33) AND TDL.BILL_AREA_ID IN (810000020, 810000025, 810000030) THEN TDL.AMOUNT*-1 ELSE 0 END) as CCP_PAYMENTS,
    SUM(CASE WHEN TDL.DETAIL_TYPE IN(2,5,11,20,22,32,33) AND TDL.BILL_AREA_ID NOT IN (810000020, 810000025, 810000030) THEN TDL.AMOUNT*-1 ELSE 0 END) as diff
FROM STG_OJDT.STG_CL.CLARITY_TDL_TRAN TDL

WHERE 
TDL.POST_DATE = '2018-08-01 00:00:00' AND
      TDL.SERV_AREA_ID = 10    
GROUP BY  TDL.POST_DATE;

In this final query, we’re combining both queries and calculating the difference by using a conditional aggregation that checks for the presence of each bill area.

Conclusion

Conditional aggregation is a powerful feature in SQL that allows you to perform complex calculations on your data. By combining two similar queries with different aggregations, we can create a single query that calculates multiple values based on conditions applied to the data. This approach enables us to extract insights from our data and provide meaningful results.

I hope this explanation helps! Let me know if you have any questions or need further clarification.

Last modified on 2025-03-01