SQL SUM using CASE WHEN within two tables: A Deep Dive

As a data-driven application developer, you’re likely familiar with the importance of efficient database queries. In this article, we’ll delve into an interesting problem involving two tables and explore ways to achieve the desired result using SQL.

Background and Problem Statement

The problem statement involves two tables, gastos (table A) and asignacion_gastos (table B). Table gastos contains information about expenses with columns such as id, importe, etc. Table asignacion_guestos seems to contain information about assignments related to expenses.

The goal is to retrieve the total expenses made for a given ID in table A, considering that if the same ID exists in table B, the total expenses should be the sum of all rows with that ID in table B. This requires using SQL’s CASE WHEN statement within a subquery or CTE (Common Table Expression).

Current Query and Its Shortcomings

Let’s examine the query provided by the user:

SELECT gastos.id,
       gastos.importe,
       SUM(asignacion_gastos.importe) AS "totalAsignado",
       CASE
         WHEN "totalAsignado" IS NULL THEN
          "totalImporte" = gastos.importe
         ELSE
          "totalImporte" = "totalAsignado"
       END
  FROM gastos
  LEFT JOIN asignacion_gastos
    ON gastos.id = asignacion_gastos.idGasto
 GROUP by gastos.id
 ORDER BY gastos.id

This query attempts to achieve the desired result, but it has a few issues:

The CASE statement within the SELECT clause is incorrect. It’s trying to compare two strings ("totalAsignado" and "totalImporte"), which will always return NULL. Instead, it should be using the calculated SUM(asignacion_gastos.importe) as the value for "totalAsignado".
The query does not handle cases where the same ID exists in both tables but the corresponding assignment data is missing.

Correct Solution

Here’s a corrected version of the query that addresses these issues:

SELECT g.id AS id,
       g.importe AS importe,
       COALESCE(SUM(ag.importe), 0) AS "totalAsignado",
       CASE WHEN SUM(ag.importe) > 0 THEN SUM(g.importe)
            ELSE COALESCE(MIN(ag.importe), 0)
       END AS "totalImporte"
FROM gastos g
LEFT JOIN asignacion_gastos ag ON g.id = ag.idGasto
GROUP BY g.id
ORDER BY g.id;

In this corrected query, we:

Use the COALESCE function to provide a default value of 0 for cases where there are no assignments (i.e., when SUM(ag.importe) is NULL).
Compare the sum of assignment data with 0. If it’s greater than 0, we calculate and return the total import amount; otherwise, we use the minimum assignment amount as a fallback.

Explanation and Additional Considerations

Let’s break down this corrected query further:

The subquery SUM(ag.importe) calculates the sum of expenses for each ID in table B. We use LEFT JOIN to include IDs that don’t have matching data in table B.
To handle cases where an ID has both assignment and no-assignment data, we use the COALESCE function to return 0 if there’s no assignment data (SUM(ag.importe) is NULL). This ensures that our total import amount calculation works correctly for all IDs.

Comparison with Alternative Solutions

Another possible solution would be to create a CTE (Common Table Expression) to calculate the sum of expenses in table B, and then join this CTE with the gastos table. Here’s an example:

WITH cte AS (
  SELECT idGasto,
         SUM(importe) AS totalAsignado
  FROM asignacion_gastos
  GROUP BY idGasto
)
SELECT g.id AS id,
       g.importe AS importe,
       COALESCE(cte.totalAsignado, 0) AS "totalAsignado",
       CASE WHEN cte.totalAsignado > 0 THEN SUM(g.importe)
            ELSE COALESCE(MIN(cte.totalAsignado), 0)
       END AS "totalImporte"
FROM gastos g
LEFT JOIN asignacion_gustos ag ON g.id = ag.idGasto
LEFT JOIN cte ON ag.idGasto = cte.idGusto
GROUP BY g.id
ORDER BY g.id;

This alternative solution uses a CTE to calculate the sum of expenses for each ID in table B, and then joins this result with the gastos table. While it achieves the same goal as our corrected query, it may have performance implications due to the additional join operation.

Conclusion

In conclusion, using SQL’s CASE WHEN statement within a subquery or CTE is an efficient way to solve problems involving multiple tables and conditional calculations. By understanding how to use aggregate functions like SUM, COALESCE, and CASE, you can write effective queries that efficiently retrieve the desired data from your database.

In this article, we’ve explored two possible solutions for a specific problem: calculating the total expenses made for a given ID in table A, considering cases where the same ID exists in both tables. We’ve discussed the importance of using aggregate functions and handling null values to ensure accurate results.

Last modified on 2024-06-17