问题
This seems so simple but I can't seem to figure it out without doing subqueries (which seem to slow down the queries significantly - takes almost 10 seconds instead of <1).
Let's say I have a table of sent documents, and I want to select the ones that have been updated since they've last been sent, and the ones that have never been sent.
SELECT d.document_id, max(sd.document_sent_date) as last_sent_date
FROM documents d
LEFT JOIN sent_documents sd ON d.document_id=sd.document_id
WHERE last_sent_date is NULL OR last_sent_date<d.last_updated
GROUP BY d.document_id
Is something like this possible? Basically, I want to use the result of max() in my where clause.
回答1:
You want the having clause.
General rule: where operates on date before aggregation, having operates on data after aggregation.
来源：https://stackoverflow.com/questions/14638634/mysql-referencing-aggregate-column-in-where-clause