HAVING vs. WHERE in SQL: When Should I Use Each Clause for Aggregation?

Differences between HAVING and WHERE clauses in SQL aggregate queries

Understanding the difference between HAVING and WHERE clauses is crucial when working with aggregated data. This article will dive into a common query scenario and explain why one method returns the correct results and another doesn't.

Query context

Given the following table:

• Lecturers (Columns: LectID, Fname, Lname, degree)
• Lecturers_Specialization (Columns: LectID, Expertise)

The goal is to find the instructor with the most specialties.

Failed attempt to use WHERE clause

Failed query attempts to use the WHERE clause to filter the results based on the number of majors:

<code class="language-sql">SELECT
  L.LectID, 
  Fname, 
  Lname 
FROM Lecturers L, 
     Lecturers_Specialization S
WHERE L.LectID = S.LectID
AND COUNT(S.Expertise) >= ALL (SELECT
  COUNT(Expertise)
FROM Lecturers_Specialization
GROUP BY LectID);</code>

This query failed to produce the expected results because the WHERE clause is not suitable for applying conditions on aggregate values.

Successful query using HAVING clause

Successful queries use the HAVING clause instead:

<code class="language-sql">SELECT
  L.LectID,
  Fname,
  Lname 
FROM Lecturers L,
     Lecturers_Specialization S
WHERE L.LectID = S.LectID
GROUP BY L.LectID,
         Fname,
         Lname 
HAVING COUNT(S.Expertise) >= ALL (SELECT
  COUNT(Expertise)
FROM Lecturers_Specialization
GROUP BY LectID);</code>

The reasons why HAVING is successful

HAVING clause is specifically used to filter the results of aggregate queries. It applies a condition after the GROUP BY operation, allowing us to check the number of majors for each instructor.

Rule of thumb

Generally speaking, use WHERE before GROUP BY to filter individual rows, and use HAVING after GROUP BY to filter aggregate results.

Improved query using ANSI JOIN

An alternative query version using ANSI JOIN syntax is also provided:

<code class="language-sql">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)</code>

This version eliminates the use of WHERE for join conditions and uses JOIN instead.

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