What's the Execution Order of MySQL Query Clauses and How Does it Affect Optimization?

MySQL Query Clause Execution Order

In MySQL, the execution order of query clauses is crucial for understanding query interpretation and performance optimization. While the optimizer may adjust the order for efficiency, the standard defines the interpretation order as follows:

1. FROM Clause: Selects the tables or views from which data will be retrieved.
2. WHERE Clause: Filters the data by matching specified conditions.
3. SELECT Clause: Specifies the columns to be extracted from the queried tables.
4. GROUP BY Clause: Aggregates data based on specified criteria.
5. HAVING Clause: Applies additional filters to the grouped data.
6. ORDER BY Clause: Sorts the results based on specified criteria.

It's important to note that this order is constrained by certain dependencies. For example, column aliases defined in the SELECT clause cannot be used in the WHERE clause because the WHERE clause is parsed before the SELECT clause. However, these aliases can be used in the ORDER BY clause.

The actual execution order of clauses is optimized by the system. For instance, if an ORDER BY clause specifies null values, it may not be executed, as doing so would not affect the results of the GROUP BY clause. This optimization ensures efficient query execution.

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