Does `COUNT` discard duplicates?

My professor taught me this SQL statement:

SELECT COUNT(length) FROM product

will return 2 with the following dataset:

product
|id | length | code |
|-------------------|
| 1 |    11  | X00  |
| 2 |    11  | C02  |
| 3 |    40  | A31  |

She justified it by saying that COUNT doesn't count duplicates. I disagreed. After trying a lot of DBMS, I've never found one that has this behaviour. Does such an DBMS exist?

Either your professor made a mistake or you misunderstood what she said. In the context of relational DBMSes, as implemented by various vendors, the aggregate function COUNT() returns the number of non-NULL values of ` in the result set (or a group).

There is a special case of COUNT(*), which returns the number of rows in the result set or group, not the number of values of anything. This is equivalent to COUNT(), such as COUNT(1).

Many databases support COUNT(DISTINCT ), which will return the number of unique values of `.

StackExchange Database Administrators Q#182420, answer score: 46