Using Case Inside the ON Clause of a Join: Efficient Solutions for Conditional Logic

Using Case Inside the ON Clause of a Join

Overview

In this article, we’ll explore the best practices for using case statements inside the ON clause of a join. We’ll delve into common pitfalls and alternative approaches to achieve similar results.

Introduction

When working with self joins or joining tables with conditional logic, it’s easy to get stuck on how to use a case statement effectively in the ON clause. In this article, we’ll provide guidance on how to write efficient and readable SQL queries using window functions, joins, and conditionals.

Problem Statement

The problem at hand is to find the balance for each customer on a specific date (20apr2020) and the base date (31dec2019 or 31mar2020 depending on the branch). We’ll explore different approaches to achieve this goal.

The Original Query

The original query attempts to use a case statement in the ON clause:

select 
B.branch_name,
B.customer_id,
B.balance as current_balance,
B1.balance as base_date_balance,
from 
balance B inner join balance B1
on B.customer_id=B1.customer_id
and B.date = '20apr2020'
and B1.date= (case when B.branch_name = 'X' then '31-mar-2020' else '31-dec-2019' end)

This query has several issues:

The use of a case statement in the ON clause can lead to performance issues and is generally considered bad practice.
The query does not handle the branching logic correctly, as it will only match when B.branch_name equals 'X'.

Solution 1: Using Window Functions

One efficient approach to solve this problem is by using window functions:

select b.*,
       max(case when branch = 'B' and date = date '2019-03-31' then balance
                when date = date '2019-12-31' then balance
           end) over (partition by customer_id, branch_name
                     ) as base_date_balance
from balance b;

This query uses the max window function to calculate the maximum value of balance for each partition group (customer_id, branch_name). The case statement inside the max function checks if the date matches the base dates for each branch.

Solution 2: Joining with Conditional Logic

Another approach is to join the table with itself and apply conditional logic:

from b join
     b bbase
     on bbase.customer_id = b.customer_id and
        bbase.branch_name = b.branch_name and
        ( (bbase.branch_name = 'B' and bbase.date = date '2019-03-31') or
          (bbase.branch_name &lt;&gt; 'B' and bbase.date = date '2019-12-31')
        )

This query uses a self join to create a new table b that includes both the original data and the data with the base dates applied. The ON clause filters the results based on the branch name and date.

Comparison of Approaches

Approach	Advantages	Disadvantages
Window Functions	Efficient, Scalable	Limited by query optimizer
Joining with Conditional Logic	Clear, Readable	Can be less efficient

Conclusion

When working with conditional logic in the ON clause of a join, it’s essential to consider both performance and readability. In this article, we’ve explored two approaches: using window functions and joining with conditional logic. By choosing the right approach for your specific use case, you can write efficient and effective SQL queries.

Additional Considerations

When working with self joins or joins with conditional logic:

Ensure that the query is well-documented and easy to understand.
Optimize the query by using indexes and partitioning if necessary.
Consider using alternative approaches, such as common table expressions (CTEs) or views, depending on your specific requirements.