sql tips: i heard you like queries so i put more queries in your query (aka "subqueries")pic.twitter.com/IOvbrEZkIk
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sql tips: i heard you like queries so i put more queries in your query (aka "subqueries")pic.twitter.com/IOvbrEZkIk
I've never heard a CTE called a subquery (usually considered to be an improvement over). CTEs have much better performance and predictability than subqueries (which are usually rewritten by the query optimizer as joins).
That is going to depend on the database engine. MS SQL Server just expands the CTE out in each place it is used, so behaves just as if it were a sub query. All the CTE ends up doing then is improving readability. The query optimiser will still rewrite.
Key is "as if it were". I'm still more comfortable with a stronger distinction to avoid the correlated subquery nonsense entirely. Nothing ever becomes a subquery, but things will become join operations. CTE and subquery are ideological abstractions, joins are actual operations.
A correlated subquery is one that has outer references, i.e. references to table columns from the enclosing query. Often but not always bad, except in the special case of EXISTS which is better thought of as a kind of join.
Once you see how a few of these show up in the execution plan, you just establish a rule to avoid them. Too much rework long term. Just use joins and CTEs.
Used carefully, they can be very effective. (Ditto for lateral joins, which are effectively correlated table subqueries.)
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