![]() So the following is a valid dbo.Table2 t2 ![]() In the FROM clause (as pointed out by in this cross site dupe). Where can I find it in the Microsoft documentation? It's almost like the Reverse Polish Notation (RPN) of HP calculators back in the days of the HP-35 and successors. I think of it as grouped joins or out-of-order joins. ![]() The parenthesis are actually optional, but I prefer to include them to somewhat improve readability (giving the reader a hint that something tricky is going on).īut as to "what is it called?" I don't think it has a name. This allows the order-detail and product rows to be combined and filtered before being fed to the LEFT JOIN. ĪND P.IsBackOrdered = 1 - Not proper design, but it's just an example Where it really comes in useful is when used with outer joins.Īn example use case might be "all orders left join to back-ordered products", but where there is a junction table in between. What it does is logically defines the order that the joins are performed. Personally I prefer your second version most of the time but there are some cases when the first one will be faster - I once had a query where it was not simply possible to limit the rows in the JOINED subquery the right way, but switching to the dependent subquery only few unique combinations were actually read.Not a derived table. If that is available, the difference between both versions will be minimal. That means that in your case all rows in result have u.id = 1 -> to_user_id = 1 and the subquery will be actually executed only once and then the cached results will be used. The dependent subquery can be made faster by using a subquery cache in MariaDB 10 (and IIRC MySQL 5.7, but I did not verify). you can just push the condition in there ( to_user_id = 1 instead of the GROUP BY).īut to make things not so simple there exist some optimizations in newer versions. But in this case the count will be counted even for users which do not really show in the results. If the query is fast (can use index on (to_user_id)), it is good. The one in the JOIN is a derived table - it will be executed only once and materialized into a temporary table, which will then be joined to your other tables. ![]() Even if one execution is fast, potencially thousands of them can make it slow. The (SELECT count(*) FROM review WHERE to_user_id = u.id) as reviewCount is a dependent subquery - it will be executed for each row in your results. Most what I write here should be quite independent but I mostly have experence in MySQL so maybe different systems allow some other optimizations. You do not specify which RDBMS you are working with. Could someone explain me if a solution is better than the other? (Or if there is an other better solution) ? I'm not very an expert with db query performance and tuning. SELECT to_user_id, count(*) reviewCount FROM review GROUP BY to_user_id The other way of doing this is without that subselect and by joining the review table on the right user_id SELECT (SELECT count(*) FROM review WHERE to_user_id = u.id) as reviewCount, The first and easy way I think when doing this query would this: SELECT Let's say I want in one single query retrieve the nickname of a 'given' userId all its pictures url as well as the count of how many people reviewed that user. Often I have to return the count of of some row in a query that fetch other non related rows.įor example A table User a table Review and a Table picture User:
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |