What is the difference between the where and having sql clauses?

Answer 1

The short answer is that the WHERE clause is used for comparing values in the base table, whereas the HAVING clause can be used for filtering the results of aggregate functions in the result set of the query. SELECT * FROM tablename WHERE ID > 100 ...is an example of a WHERE clause. Here's a simple example of a HAVING clause that returns the count of workers with the same last names... SELECT WorkerLastName, COUNT(WorkerLastName) AS WorkerCount FROM tblWorker GROUP BY WorkerLastName HAVING COUNT(WorkerLastName) > 1 The HAVING clause allows you to filter the results of aggregate functions, such as COUNT() or AVG() or SUM(), or MAX() or MIN(), just to name a few. HAVING provides you a means to filter these results in the same query, as opposed to saving the results of a WHERE clause SQL statement to a temporary table and running another query on the temporary table results to extract the same results. To do the HAVING SQL above without the HAVING clause would require the following SQL: SELECT WorkerLastName, COUNT(WorkerLastName) AS WorkerCount INTO TempTable FROM tblWorker GROUP BY WorkerLastName SELECT * FROM TempTable WHERE WorkerCount > 1 DROP TABLE TempTable