Merging Data from Two Tables with Current Year and Prior Year Records

As data engineers and analysts, we often encounter the challenge of merging data from multiple tables to extract specific insights. In this article, we’ll delve into a common scenario where we need to join two tables, one containing current year records and another containing prior year records, and merge them based on a common identifier.

Introduction

The problem statement involves joining TableA with the current year’s data from TableB, and then merging the results with the prior year’s data from TableB. The goal is to create an output table that contains both the current year and prior year records. In this article, we’ll explore a solution using SQL joins and subqueries.

Background

Let’s start by understanding the structure of our tables:

TableA

TableB

TableC

We’ll join TableA with the current year’s data from TableB, and then merge the results with the prior year’s data from TableC.

The Challenge: Self-Joining vs. Regular Join

The original question attempts to solve this problem using a self-join, but it doesn’t quite work as expected. A self-join involves joining a table with itself based on a common column. In this case, the intention is to join TableA with TableB, and then merge the results with TableC. However, the provided solution uses two separate joins: one for TableA and TableB, and another for TableB and TableC.

Solution Overview

To solve this problem, we’ll use a combination of SQL joins and subqueries. The steps are as follows:

1. Join TableA with the current year’s data from TableB.
2. Use a subquery to retrieve the prior year’s data from TableC.
3. Merge the results from step 1 with the results from step 2.

Step-by-Step Solution

Step 1: Join TableA with TableB

We’ll start by joining TableA with the current year’s data from TableB. This join will be based on the ID and YEAR columns.

SELECT a.*, b.amount, NULL AS prev_year_amount
FROM tablea a
LEFT JOIN tableb b ON a.id = b.id AND b.year = a.year;

Step 2: Retrieve Prior Year’s Data from TableC

Next, we’ll use a subquery to retrieve the prior year’s data from TableC. This will involve selecting only the rows with an ID matching the current row and a YEAR value one less than the current year.

SELECT id, amount AS prev_year_amount
FROM tablec
WHERE id IN (SELECT id FROM tablea WHERE year = b.year)
AND year = b.year - 1;

Note that we’re using a correlated subquery to filter rows based on the ID value from TableA.

Step 3: Merge Results

Now, we’ll merge the results from step 1 with the results from step 2. This will involve adding the prior year’s amount to the current row.

SELECT a.id, b.year AS curr_year, a.amount, b.amount AS prev_year_amount,
       COALESCE(prev_year_amount, NULL) AS prev_year_amount
FROM tablea a
LEFT JOIN tableb b ON a.id = b.id AND b.year = a.year
LEFT JOIN (
  SELECT id, amount AS prev_year_amount
  FROM tablec
  WHERE id IN (SELECT id FROM tablea WHERE year = b.year)
  AND year = b.year - 1
) p ON a.id = p.id;

In this final step, we’re using a left join to merge the current row with the prior year’s data. If there is no prior year record for an ID, the prev_year_amount column will be null.

Example Use Case

Suppose we have the following data in our tables:

TableA

TableB

TableC

Running the query above will produce the following output:

Note that the prior year’s amount is added for each ID, and the prev_year_amount column is calculated based on the available data.

Conclusion

In this article, we’ve solved a classic problem in database querying: joining a table with another table, and then merging the results with a separate table. We’ve used a combination of SQL joins and subqueries to achieve this goal, and have provided an example use case to illustrate the solution. With practice and experience, you’ll become proficient in solving complex queries like these and can tackle even more challenging problems.

Last modified on 2025-02-27