MySQL's ORDER BY and LIMIT Clause Execution: A Clarification
Understanding the interaction between ORDER BY and LIMIT clauses in MySQL queries is crucial for obtaining the expected results. The processing order directly impacts the final output.
Consider this example query:
SELECT article FROM table1 ORDER BY publish_date LIMIT 20;
This query aims to retrieve the 20 most recently published articles.
The Correct Processing Sequence:
MySQL processes these clauses sequentially, not concurrently as might be initially assumed:
-
ORDER BY publish_date: MySQL first sorts all records intable1in ascending order based on thepublish_datecolumn. This creates a completely sorted dataset. -
LIMIT 20: After the sorting is complete, MySQL then applies theLIMITclause, selecting only the first 20 rows from the already sorted dataset.
Therefore, the query's result is the 20 most recent articles, correctly ordered by publication date. The LIMIT clause operates on the output of the ORDER BY clause, not independently.
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