Using Set Operation Clauses with ORDER BY and LIMIT in MySQL
In MySQL, UNION, INTERSECT (simulated using INNER JOIN or EXISTS), and EXCEPT (simulated using LEFT JOIN ... IS NULL or NOT IN) are set operation clauses that combine results from multiple queries. Their interaction with ORDER BY and LIMIT depends on whether these clauses are applied to individual subqueries or the final combined result.
UNION combines results of two queries and removes duplicates by default. Use UNION ALL to retain duplicates.
INTERSECT can be simulated using INNER JOIN or EXISTS to return rows common to both queries.
EXCEPT can be simulated using LEFT JOIN ... IS NULL or NOT IN to return rows present in the first query but not in the second.
An ORDER BY clause applied after the set operation sorts the final combined result set, not the individual subqueries.
A LIMIT clause applied after the set operation restricts the number of rows returned from the combined result set.
Applying ORDER BY or LIMIT inside individual subqueries may require parentheses to preserve logical grouping, but final sorting and limiting is usually done after the set operation for clarity and correctness.
Essentially, MySQL treats ORDER BY and LIMIT at the end of a set operation as instructions for the overall output, ensuring that the combined results are sorted and limited as intended.