SQL HAVING vs. WHERE: When to Use Each Clause for Data Filtering?

SQL data filtering: Differences between WHERE and HAVING clauses

The HAVING and WHERE clauses in SQL are both used to filter data, but their functions are different. The WHERE clause works on individual rows, while the HAVING clause works on aggregated results.

Difference example

Suppose we have two tables: Lecturers and Lecturers_Specialization, and we want to find the lecturers with the most areas of expertise.

The initial query using the WHERE clause does not give the expected results:

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);

This is because the WHERE clause filters a single row, and the COUNT(S.Expertise) >= ALL comparison cannot be applied to a single row.

The correct query uses the HAVING clause:

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); 

The

HAVING clause acts on the aggregated results to ensure that the comparison is applied to the number of areas of expertise for each instructor.

Rules of thumb and optimization

Generally speaking, the WHERE clause is used before the GROUP BY clause to filter individual rows; the HAVING clause is used after the GROUP BY clause to filter aggregate results.

Additionally, queries can be optimized using ANSI JOIN syntax instead of L, S θ join syntax:

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)

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