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

Understanding the WHERE and HAVING clauses in SQL queries

SQL developers often face the challenge of distinguishing between WHERE and HAVING clauses. This article aims to clarify their different usage in SQL queries.

When to use WHERE

• Filtering individual rows: The WHERE clause is used to impose conditions on individual rows in a table or join result set. It evaluates each row and selects only those that meet the specified criteria.

When to use HAVING

• Filter aggregate results: The HAVING clause applies a condition to a group of rows represented by an aggregate function (such as COUNT(), SUM(), AVG()). Use it when you need to set conditions on summarized data after an aggregation operation.

Example: Lecturer with the most majors

To illustrate this difference, consider the following table:

<code>1. Lecturers (LectID, Fname, Lname, degree)
2. Lecturers_Specialization (LectID, Expertise)</code>

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

WHERE query (error)

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 WHERE query does not work because it attempts to use WHERE to filter based on an aggregate function (COUNT(S.Expertise)). The WHERE clause is used to filter individual rows, not aggregate results.

HAVING query (correct)

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);

This HAVING query successfully retrieves the correct results because it sets conditions on the aggregated data (COUNT(S.Expertise)) after the GROUP BY operation.

ANSI connection version (recommended)

Using ANSI join syntax, the query can be rewritten as:

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 version eliminates the WHERE clause for theta join conditions, resulting in more efficient and standards-compliant queries.

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