In what order does SQL Server execute the WHERE, GROUP BY, and HAVING clauses?

Execution Sequence of SQL Server Clauses: WHERE, GROUP BY, and HAVING

Understanding the execution sequence of SQL queries is crucial when incorporating GROUP BY and HAVING with WHERE clauses. The order of these operations directly impacts the result set.

Execution Order

SQL Server executes these clauses in the following order:

1. FROM and JOINs:

• This phase identifies and filters the data using the FROM clause and any specified JOINs.

2. WHERE:

• The WHERE clause filters the rows returned by the FROM clause based on specified conditions.

3. GROUP BY:

• The GROUP BY clause combines rows with identical values in specified columns, creating groups of related data.

4. HAVING:

• The HAVING clause filters groups created by the GROUP BY clause, discarding those that do not meet certain criteria.

5. ORDER BY:

• The ORDER BY clause arranges the remaining rows or groups in ascending or descending order.

6. LIMIT:

• The LIMIT clause restricts the number of returned records, whether rows or groups.

Conclusion

This sequence ensures that data is filtered and grouped appropriately before any aggregate functions or calculations are performed. By following this order, SQL Server maximizes the query's efficiency and accuracy.

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