Loading subject…
Aggregate function
A SQL function that reads the values of a column across many rows and returns one summary value. The five named in the syllabus are AVERAGE (written AVG in most systems), COUNT, MAX, MIN and SUM.
NULL
A marker stored in a column to record that the value is unknown or missing. It is not zero and not an empty string, and every aggregate function except COUNT(*) skips it rather than treating it as a value.