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SQL Fundamentals · SQL GROUP BY, HAVING and Aggregate Functions Explained with Examples

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Trick to keep in mind+3 XP

COUNT(*) counts rows; COUNT(col) counts values that exist.

Aggregates over a column skip NULLs, because NULL means there is no value to count. Take five rows where a column is NULL in two of them: COUNT(*) gives 5 and COUNT(col) gives 3. The two only agree when the column has no NULLs at all.

Read it once, then a question tests whether the trick stuck.