Logical Order of Clause Processing in MySQL
In MySQL, although queries are written in a specific order, the database engine processes clauses in a logical sequence to produce the result. Understanding this order helps in writing efficient and correct queries.
FROM – Determines the source tables and performs joins if multiple tables are used.
WHERE – Filters rows from the FROM clause based on specified conditions.
GROUP BY – Groups the filtered rows based on one or more columns.
HAVING – Filters the grouped rows based on aggregate conditions.
SELECT – Chooses the columns or expressions to include in the result set, including any aliases.
DISTINCT – Removes duplicate rows from the SELECT output if specified.
ORDER BY – Sorts the final result set according to the specified columns and order.
LIMIT – Restricts the number of rows returned to the specified number.
Even though SQL queries are written starting with SELECT, the execution order starts with FROM and proceeds logically as listed. This logical order explains why certain clauses cannot reference aliases or aggregates that are defined later in the query.