Simplifying Complex Queries with Grouping and Conditional Logic in SQL

Subquery vs. Common Table Expressions (CTEs)

Simplifying Complex Queries with Grouping and Conditional Logic

When working with complex queries that involve multiple subqueries, it’s common to wonder if there’s a more efficient or elegant way to approach the problem. In this article, we’ll explore how to simplify a query by using grouping and conditional logic to eliminate the need for multiple subqueries.

Problem Statement

Trying to Write a Query to Specify if a Customer Has Completed Two Types of a Specific Column

The provided Stack Overflow question describes a scenario where a customer has information stored in a table called resedtl. The customer’s ID, type of “RES”, and status are all relevant. We want to determine whether the customer has completed both types of RES, only one type, or only two types.

Initial Query Attempt

Using Multiple Subqueries

The provided initial query attempts to solve this problem by using multiple subqueries within a CASE statement:

select distinct  t.CustId, 
case when (
 select top 1 rd.CustId from resedtl rd
 where rd.CustId = t.CustId
 and rd.CustId in (
   select rd.CustId from resedtl rd
  where rd.status ='done'
  and rd.RESType = 1
  and rd.CustId = t.CustId
 )
 and rd.CustId in (
    select rd.CustId from resedtl rd
  where rd.status ='done'
  and rd.RESType = 2
  and rd.CustId = t.CustId
 )
 ) = t.CustId then 'both'
 when

 (
  select top 1 rd.CustId from resedtl rd
 where rd.CustId = t.CustId
 and rd.CustId in (
   select rd.CustId from resedtl rd
  where rd.status ='done'
  and rd.RESType = 1
  and rd.CustId = t.CustId
 )
 and rd.CustId not in (
    select rd.CustId from resedtl rd
  where rd.status ='done'
  and rd.RESType = 2
  and rd.CustId = t.CustId
 )
 ) = t.CustId then 'just one'

 when

 (
  select top 1 rd.CustId from resedtl rd
 where rd.CustId = t.CustId
 and rd.CustId not in (
   select rd.CustId from resedtl rd
  where rd.status ='done'
  and rd.RESType = 1
  and rd.CustId = t.CustId
 )
 and rd.CustId  in (
    select rd.CustId from resedtl rd
  where rd.status ='done'
  and rd.RESType = 2
  and rd.CustId = t.CustId
 )
 ) = t.CustId then 'just two'
 else 'None'
 end as result from resedtl t
 where t.CustId in (123,124,125,126)

This query seems overly complicated, with multiple nested subqueries.

Simplified Solution

Using Grouping and Conditional Logic

A simpler solution can be achieved by using grouping and conditional logic to eliminate the need for multiple subqueries:

SELECT 
  CustId,
  CASE 
    WHEN MIN(CASE WHEN status = 'done' AND RESType = 1 THEN 1 ELSE NULL END) > 0 AND 
     MIN(CASE WHEN status = 'done' AND RESType = 2 THEN 1 ELSE NULL END) > 0 THEN 'both'
    WHEN MIN(CASE WHEN status = 'done' AND RESType = 1 THEN 1 ELSE NULL END) > 0 THEN 'just one'
    WHEN MIN(CASE WHEN status = 'done' AND RESType = 2 THEN 1 ELSE NULL END) > 0 THEN 'just two'
    ELSE 'none'
  END AS "result"
FROM 
  (SELECT * FROM resedtl) t
GROUP BY CustId

This simplified query achieves the same result as the original query but with significantly fewer subqueries.

How it Works

The subquery within the CASE statement uses conditional logic to check if a customer has completed both types of RES (either type 1 and type 2) or just one type.
The use of MIN(CASE WHEN ... THEN 1 ELSE NULL END) ensures that only non-NULL values are considered in the grouping process.
By using GROUP BY CustId, we group the results by customer ID, allowing us to easily determine if a customer has completed both types of RES or just one type.

Benefits and Implications

This simplified query offers several benefits over the original complex query:

Reduced Subquery Complexity: The new query eliminates the need for multiple nested subqueries, making it easier to read and maintain.
Improved Performance: Fewer subqueries can lead to improved performance, as SQL engines can optimize and execute fewer queries more efficiently.
Increased Readability: By using grouping and conditional logic, we make the query more straightforward and understandable, reducing the complexity of the original query.

However, there are some implications to consider:

Assumes Non-Null Status: The use of MIN(CASE WHEN ... THEN 1 ELSE NULL END) assumes that non-null values will always indicate a completed status. If this assumption is not valid for your data, you may need to adjust the query accordingly.
Grouping by CustId: The use of GROUP BY CustId ensures that the results are grouped correctly, but it also means that if two customers have the same ID and complete different types of RES, they will be treated as a single customer.

Conclusion

In this article, we explored how to simplify a complex query using grouping and conditional logic. By eliminating the need for multiple subqueries, we made the query more readable, maintainable, and efficient. While there are some assumptions and implications to consider, this simplified solution offers significant benefits over the original complex query.

References

Last modified on 2023-07-01