


How to Correctly Count Rows in PostgreSQL Using WHERE and Aggregate Functions?
Dec 25, 2024 pm 12:06 PMPostgreSQL WHERE count condition
In PostgreSQL, the following query returns an error:
SELECT COUNT(a.log_id) AS overall_count FROM "Log" as a, "License" as b WHERE a.license_id=7 AND a.license_id=b.license_id AND b.limit_call > overall_count GROUP BY a.license_id;
ERROR: column "overall_count" does not exist
The reason for this error is that the WHERE clause references an output column, "overall_count", which is not yet defined in the table structure. To fix this, we must move the count condition to the HAVING clause since it refers to an aggregate function result after WHERE has been applied.
Furthermore, the query should also use a LEFT JOIN instead of a regular JOIN to avoid excluding licenses without any logs and use count(b.license_id) or count(*) for efficiency and clarity.
The correct query should look like this:
SELECT a.license_id, a.limit_call , count(b.license_id) AS overall_count FROM "License" a LEFT JOIN "Log" b USING (license_id) WHERE a.license_id = 7 GROUP BY a.license_id HAVING a.limit_call > count(b.license_id)
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