Which clause is executed first in a SQL query?
Explanation
FROM clause is executed first because it forms the set of records all the following clauses (like WHERE and SELECT) will operate over.
Theory
  • SQL Query Order of Operations

    The order in which SQL directives get executed:
    • FROM clause
    • WHERE clause
    • GROUP BY clause
    • HAVING clause
    • SELECT clause
    • ORDER BY clause
    Read more: SQL Query Order of Operations
  • Optimization - FROM Clause

    Since this clause executes first, it is our first opportunity to narrow down possible record set sizes. This is why I put as many of my ON rules (for joins) as possible in this area as opposed to in the WHERE clause:
    FROM
        contact c
    INNER JOIN
        display_status d
    ON
        (
                c.display_status_id = d.id
            AND
                d.is_active = 1
            AND
                d.is_viewable = 1 
        )
    
    This way, by the time we get to the WHERE clause, we will have already excluded rows where is_active and is_viewable do not equal 1.
    Read more: SQL Query Order of Operations

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