WHERE vs. HAVING: When Should You Use Each Clause to Find Aggregated Data?

SQL: HAVING vs. WHERE – Filtering Aggregated Data

This article demonstrates the key difference between WHERE and HAVING clauses in SQL, particularly when working with aggregated data. We'll use the example of finding the lecturer with the most specializations. A WHERE clause alone is insufficient for this task; a HAVING clause is necessary.

Understanding the Difference

The WHERE clause filters individual rows before grouping occurs. It applies conditions to individual records within a table. Conversely, the HAVING clause filters after grouping, operating on aggregate functions like COUNT, SUM, AVG, and MIN. This is critical when filtering based on the results of aggregation, as in our lecturer specialization example.

Best Practice

For clarity and efficiency, the general rule is to use WHERE before GROUP BY and HAVING after GROUP BY. This approach simplifies query construction and improves readability.

Optimized Query

The following query uses an ANSI join and a HAVING clause to efficiently identify the lecturer(s) with the maximum number of specializations:

SELECT  L.LectID, Fname, Lname
FROM Lecturers L
JOIN Lecturers_Specialization S ON L.LectID = S.LectID
GROUP BY L.LectID, Fname, Lname
HAVING COUNT(S.Expertise) >= ALL
(SELECT COUNT(Expertise) FROM Lecturers_Specialization GROUP BY LectID)

This revised query eliminates the need for a WHERE clause as a theta join condition, leading to a more streamlined and efficient query.

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