What is the Execution Order of GROUP BY, HAVING, and WHERE Clauses in SQL Server?

Execution Sequence of Group By, Having, and Where Clause in SQL Server

In SQL programming, understanding the execution sequence of various clauses is crucial for efficient query optimization. This article will delve into the specific sequence of GROUP BY, HAVING, and WHERE clauses in SQL Server.

Execution Sequence:

SQL Server executes the following commands in the following order:

1. FROM and JOINs: This step identifies and incorporates data from the specified tables, based on join conditions (if present).
2. WHERE: The WHERE clause filters the data that was selected in step 1, removing any rows that do not meet the specified criteria.
3. GROUP BY: Rows from the previous step are grouped based on the columns specified in the GROUP BY clause.
4. HAVING: The HAVING clause applies filters to the groups created in step 3, removing any groups that do not meet the specified criteria.
5. ORDER BY: The ORDER BY clause arranges the remaining rows or groups in the specified order.
6. LIMIT: The LIMIT clause restricts the results to a specific number of rows or groups.

Example:

Consider the following query:

SELECT SUM(salary) AS total_salary
FROM employees
WHERE department = 'HR'
GROUP BY department
HAVING SUM(salary) > 50000
ORDER BY total_salary DESC

In this query, the data is selected from the "employees" table (step 1). The WHERE clause filters the data to include only employees in the "HR" department (step 2). The GROUP BY clause combines employees from the same department into groups (step 3). The HAVING clause removes groups with a total salary less than 50,000 (step 4). The ORDER BY clause arranges the groups in descending order of total salary (step 5).

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