Defining MYSQL order of execution

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In MySQL, the order of execution of a SQL query is crucial to understand how the database processes your request. The general order of execution for a MySQL query is as follows:

  1. FROM clause: The database first processes the tables listed in the FROM clause. If your query involves multiple tables, it performs any necessary joins at this stage.

  2. WHERE clause: After the database has determined the result set from the FROM clause (considering any joins and conditions), it filters the rows based on the conditions specified in the WHERE clause. Rows that do not meet the criteria are eliminated from the result set.

  3. GROUP BY clause: If your query includes a GROUP BY clause, the result set is then grouped based on the specified columns. Rows with the same values in the specified columns are grouped together.

  4. HAVING clause: After the grouping is done, the HAVING clause filters the grouped rows. It is similar to the WHERE clause but operates on the grouped rows rather than individual rows. Rows that do not meet the HAVING criteria are eliminated from the result set.

  5. SELECT clause: After the filtering and grouping, the database evaluates the expressions in the SELECT clause to produce the final result set. This can involve computations, column aliases, and other operations specified in the SELECT clause.

  6. ORDER BY clause: If your query has an ORDER BY clause, the final result set is then sorted based on the specified columns and sorting order.

  7. LIMIT and OFFSET clauses: Finally, if your query includes LIMIT and OFFSET clauses, the database limits the number of rows returned by the query and offsets the starting point for the result set, respectively.

It's important to note that not all queries include all these clauses. The order of execution might change if your query doesn't contain certain clauses (for example, a query without a GROUP BY clause won't involve the grouping stage). Understanding this order of execution is essential for optimizing queries and getting the desired results from the database.

Examples

Let's consider a more complex example that involves all the major clauses in a MySQL query: SELECT, FROM, JOIN, WHERE, GROUP BY, HAVING, ORDER BY, LIMIT, and OFFSET.

Example Tables:

Consider two tables, employees and departments:

employees table:

idnamedepartment_idsalary
1Alice150000
2Bob260000
3Charlie155000
4David362000
5Eve258000

departments table:

idname
1HR
2IT
3Sales

Example Query:

Let's say we want to find the average salary of employees in the IT department, ordered by department name, and we want to retrieve only the first two rows.

sql
SELECT d.name as department_name, AVG(e.salary) as average_salary FROM employees e JOIN departments d ON e.department_id = d.id WHERE d.name ='IT'GROUPBY d.name HAVINGAVG(e.salary) >55000ORDERBY d.name LIMIT 2OFFSET0;

Order of Execution:

  1. FROM clause: The database starts by considering the employees and departments tables, then performs an inner join based on the department_id and id columns.

    e.ide.namee.department_ide.salaryd.idd.name
    2Bob2600002IT
    5Eve2580002IT

  2. WHERE clause: Rows where the department name is not 'IT' are eliminated:

    e.ide.namee.department_ide.salaryd.idd.name
    2Bob2600002IT
    5Eve2580002IT

  3. GROUP BY clause: The result set is grouped by the d.name column, which is 'IT' in this case:

    department_nameaverage_salary
    IT59000

  4. HAVING clause: The groups with an average salary less than or equal to 55000 are eliminated (there's only one group in this case, so no rows are eliminated).

  5. SELECT clause: The SELECT clause computes the average salary for the 'IT' department:

    department_nameaverage_salary
    IT59000

  6. ORDER BY clause: The result set is ordered by department_name in ascending order (it's only one row, so no reordering occurs).

  7. LIMIT and OFFSET clauses: Finally, the query limits the result set to 2 rows starting from offset 0, but since there's only one row in the result set, only that row is returned as the final output.

So, the final output of the query would be:

department_nameaverage_salary
IT59000

This example demonstrates how each clause in the MySQL query is executed in sequence to produce the desired result.

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