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Databases

kevin nadjarian
kevin nadjarian
1,498 Points
Posted by kevin nadjarian
kevin nadjarian
1,498 Points

COUNT(*) result change

Why the "SELECT TEACHERS.ID, FIRST_NAME, LAST_NAME, COUNT() FROM TEACHERS" Change the result and only display one row, while the same query without the COUNT() Display multiple rows?

Also the actual question on this was "Which teachers teach a class during all 7 periods?" If we just add COUNT(*) from the current results, that just getting the count for every teachers and every period_ID and not only the teacher which teach on each periods ?

I'm lost....

1 Answer

Steven Parker
Steven Parker
231,269 Points
Steven Parker
Steven Parker
231,269 Points

"COUNT" is an aggregate function. This means it operates on the entire result set and returns a single row. The other values are just picked from some single row in the set.

As stated in the SQLite docmentation:

There are two types of simple SELECT statement - aggregate and non-aggregate queries. A simple SELECT statement is an aggregate query if it contains either a GROUP BY clause or one or more aggregate functions in the result-set. Otherwise, if a simple SELECT contains no aggregate functions or a GROUP BY clause, it is a non-aggregate query.

If the SELECT statement is an aggregate query without a GROUP BY clause, then each aggregate expression in the result-set is evaluated once across the entire dataset. Each non-aggregate expression in the result-set is evaluated once for an arbitrarily selected row of the dataset. The same arbitrarily selected row is used for each non-aggregate expression. Or, if the dataset contains zero rows, then each non-aggregate expression is evaluated against a row consisting entirely of NULL values.

An aggregate query without a GROUP BY clause always returns exactly one row of data, even if there are zero rows of input data.