What is the function of the HAVING clause in SQL?

The HAVING clause in SQL is used to filter the results of a GROUP BY operation based on a condition.

In more detail, the HAVING clause is a filter that acts similar to a WHERE clause, but operates on grouped data produced by a GROUP BY clause. It is used in the SELECT statement to filter the results of grouped rows, unlike the WHERE clause, which filters rows before they are grouped.

The HAVING clause is particularly useful when you want to filter the results of your query based on a condition that applies to the summary of a group of rows, not to individual rows. For example, if you have a table of sales data and you want to find out which departments have a total sales over a certain amount, you would use the HAVING clause.

The syntax of the HAVING clause is: SELECT column_name(s), aggregate_function(column_name) FROM table_name WHERE condition GROUP BY column_name(s) HAVING condition ORDER BY column_name(s). The HAVING condition is often used with the aggregate functions like COUNT, SUM, AVG, MAX, or MIN to filter the results.

It's important to note that the HAVING clause always comes after the GROUP BY clause in the SQL statement. The GROUP BY clause groups the rows that have the same values in specified columns into aggregated data, like sum, average or count. Then the HAVING clause is applied to this grouped result, filtering groups based on the condition specified.

In summary, the HAVING clause in SQL is a powerful tool that allows you to filter the results of your queries based on conditions that apply to the summary of a group of rows. It is particularly useful when working with aggregated data produced by a GROUP BY clause.