What's the Execution Order of Clauses in a MySQL Query?

Execution Order of MySQL Clauses

When executing MySQL queries, the order in which clauses are interpreted is predefined. While the actual execution may vary depending on the optimizer, the interpretation order remains consistent. This order is typically as follows:

1. FROM Clause
2. WHERE Clause
3. SELECT Clause
4. GROUP BY Clause
5. HAVING Clause
6. ORDER BY Clause

This order is crucial for understanding query parsing. For instance, column aliases defined in the SELECT clause cannot be used in the WHERE clause because the WHERE clause is interpreted before the SELECT clause. However, they can be used in the ORDER BY clause.

Regarding actual execution, the optimizer optimizes the query for efficient processing. For example, in the following queries:

GROUP BY a, b, c
ORDER BY NULL

and

GROUP BY a, b, c
ORDER BY a, b, c

the ORDER BY clause is not executed, as the GROUP BY clause already performs ordering. In the first query, the GROUP BY ordering is not impacted, and in the second, the ORDER BY is essentially redundant. Understanding this order is essential for optimizing and troubleshooting queries in MySQL.

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