Final answer:
The 'HAVING' clause in T-SQL is used to filter aggregated data resulting from the 'GROUP BY' clause. It enables specifying conditions on aggregated data like sums, counts, or averages, which can't be done with the 'WHERE' clause.
Step-by-step explanation:
The 'HAVING' clause in T-SQL (Transact-SQL) is used for filtering data that is grouped by the 'GROUP BY' clause. While the 'WHERE' clause is applied before data is grouped, the 'HAVING' clause is applied after the grouping. This is an important distinction because the 'HAVING' clause allows users to specify conditions on aggregated data, such as sums or averages, that have been created by the 'GROUP BY' clause.
For example, if a database contains sales data, and someone wants to find all departments with a total sale of more than $10,000, they might use a query like:
SELECT DepartmentID, SUM(SalesAmount) AS TotalSales
FROM Sales
GROUP BY DepartmentID
HAVING SUM(SalesAmount) > 10000;
In this query, the 'HAVING' clause filters out any groups that do not meet the condition of having aggregate sales greater than $10,000. Without the 'HAVING' clause, it would be necessary to filter the results after retrieving them from the database, which would be less efficient.