SQL Query to Find Profiles with More than 3 Photos but Not in Used Service Table

As a technical blogger, it’s essential to provide clear explanations and examples of complex queries. In this article, we’ll explore a SQL query that solves the given problem using EXISTS and NOT EXISTS clauses.

Understanding the Tables and Relationships

The problem statement provides four tables: profile, photo, service, and used. The relationships between these tables are as follows:

• Each profile has multiple photos, which is established through the foreign key profile_id in the photo table.
• Each photo belongs to a specific profile, establishing another relationship between the two tables.
• The used table contains information about profiles that have been used with certain services.

Understanding the Query

The answer provided uses two main SQL clauses: EXISTS and NOT EXISTS. These clauses are essential in this context because they allow us to filter out profiles based on specific conditions.

The Query

     select p.*
     from profile p
     where exists ( 
         select 1 from photo ph 
         where ph.profile_id =p.id 
             having count (distinct image_id )=3 
     )
    and not exists ( 
        select 1 from used u 
        where u.profile_id =p.id 
    )

How the Query Works

The query consists of two main parts:

• The EXISTS clause checks if there is at least one row in the photo table that meets the specified condition. This condition is having count (distinct image_id )=3, which means we’re looking for profiles with exactly 3 unique photos.
• The NOT EXISTS clause checks if there are no rows in the used table where the profile’s ID matches.

To understand how this works, let’s break it down step by step:

1. We start by selecting all columns (*) from the profile table and assign it a temporary alias p.
2. The first part of the query uses an EXISTS clause to check if there are any rows in the photo table where the profile_id matches the current profile’s ID.
3. Within this subquery, we use another SELECT statement with an aggregate function (COUNT) to count the number of unique photos for each profile.
4. We filter these results by only including profiles that have exactly 3 unique photos (using the HAVING clause).
5. The second part of the query uses a NOT EXISTS clause to check if there are any rows in the used table where the profile’s ID matches.
6. If the profile has more than 3 photos but is not in the used service table, it will be returned by the query.

The Importance of Using EXISTS and NOT EXISTS

The use of EXISTS and NOT EXISTS clauses provides a way to filter out rows based on conditions that exist or don’t exist elsewhere in the table. These clauses are particularly useful when you need to join multiple tables together, but still want to maintain the flexibility to exclude certain records.

Additional Considerations

When writing queries using EXISTS and NOT EXISTS, it’s essential to consider the following factors:

• Performance: The query performance can be affected by the number of rows in the subquery. If the subquery returns a large number of rows, it may impact overall query performance.
• Indexing: Proper indexing on columns used in the WHERE and HAVING clauses can improve query performance.
• Data Types: Ensure that data types are correct to avoid potential issues when performing calculations or comparisons.

Conclusion

In conclusion, the provided SQL query uses EXISTS and NOT EXISTS clauses to find profiles with more than 3 photos but not in the used service table. This approach provides a clear and efficient way to filter out records based on conditions that exist or don’t exist elsewhere in the table.

By understanding how these clauses work and when to use them, developers can write more effective queries that meet specific requirements while maintaining performance and efficiency.

Last modified on 2023-07-09