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Comments on Are there best practices for sticking conditions in WHERE clauses vs the JOIN statement?
Parent
Are there best practices for sticking conditions in WHERE clauses vs the JOIN statement?
Lets say I have two tables, A and B and I need to join a subset of them.
Is there best practices of sticking the conditions in the WHERE clause like this,
SELECT *
FROM A
JOIN B on a.fk_b = b.pk
WHERE a.pk <10000
versus sticking the condition in the JOIN like this,
SELECT *
FROM A
JOIN B on a.fk_b = b.pk
AND a.pk <10000
For these, it doesn't make any difference in speed or results, but are there best practices for where to put the conditions?
Post
SQL is a declarative language, and the form of the query does not dictate the form of the query plan that actually retrieves the data. So these two queries might be not only the same speed, but actually map to the exact same query plan to be executed.
So if speed and results don't give an advantage to one form, the best practice is to go with the one that is easier to read. In this case, I'd go with the first one.
In the second query, the JOIN handles both joining and filtering the data. By using a separate WHERE clause, you separate the actions of joining and filtering into different clauses.
If a second filter needs to be added later, it's a little less intuitive to add it to the JOIN. It's possible a later developer might create a WHERE clause, and then you'd have the JOIN filtering and joining, and the WHERE filtering as well.
But if you're already using a WHERE for filtering, it's simple and intuitive to add another condition to the WHERE.
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