SQL Conditional Join Based on Rank

Introduction

In this article, we will explore a common SQL challenge where we need to perform a conditional join based on rank. We’ll discuss the problem statement, provide an example scenario, and finally, dive into the solution with sample code.

Problem Statement

Imagine you have two tables: Table1 and Table2. Each table has columns for Instrument, Qty, and Rank. You want to join these two tables based on Instrument and Rank, but with a twist. If the matching record in both tables doesn’t exist, you should seek the next available rank in order.

Example Scenario

To illustrate this concept, let’s consider an example:

Table 1:

Table 2:

We want to join Table1 and Table2 based on Instrument and Rank, with the above condition.

Solution

To solve this problem, we can use a combination of SQL techniques:

1. Joining tables using JOIN clause

First, we’ll perform a regular join between the two tables using the JOIN clause:

SELECT *
FROM Table1 AS t1
JOIN Table2 AS t2 ON t1.Instrument = t2.Instrument AND t1.Rank = t2.Rank;

This will give us all matching records, but we need to handle the case where no match exists.

2. Using ROW_NUMBER() function with OVER clause

To achieve our desired behavior, we can use the ROW_NUMBER() function in combination with an OVER clause:

SELECT *
FROM (
  SELECT *, ROW_NUMBER() OVER (PARTITION BY Instrument ORDER BY Rank) AS RankNum
  FROM Table1
)
AS t1
JOIN Table2 AS t2 ON t1.Instrument = t2.Instrument AND t1.RankNum <= t2.Rank;

Here’s what’s happening:

• We first partition the Table1 table by Instrument and then order it by Rank.
• We assign a unique rank to each row within this partition using the ROW_NUMBER() function.
• In our join condition, we use the RankNum column from Table1 and compare it with the corresponding Rank in Table2.

This will give us all matching records, but we need to handle the case where no match exists.

3. Using COALESCE() function

To achieve our final result, we can use the COALESCE() function:

SELECT *
FROM (
  SELECT *, COALESCE(
    (SELECT Quantity FROM Table2 AS t2 WHERE t1.Instrument = t2.Instrument AND t2.Rank = t1.Rank),
    NULL
  ) AS Quantity
  FROM (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY Instrument ORDER BY Rank) AS RankNum
    FROM Table1
  ) AS t1
) AS final_table;

Here’s what’s happening:

• We first assign a unique rank to each row within the partition using the ROW_NUMBER() function.
• In our subquery, we use the COALESCE() function to replace any NULL value with an empty string ('').
• The outer query then selects all columns from this final table.

Conclusion

By combining SQL techniques such as joining tables, using the ROW_NUMBER() function with an OVER clause, and the COALESCE() function, we’ve achieved our desired result: a conditional join based on rank.

Last modified on 2023-11-27