Question: 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?

Answer: 
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(<expression>)` returns the number of non-NULL values of `<expression>` 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(<constant expression>)`, such as `COUNT(1)`.

Many databases support `COUNT(DISTINCT <expression>)`, which *will* return the number of unique values of `<expression>`.
maybe she mean in conjunction with unique, But Count does COUNT DUPLICATES.
There are some teachers who do not know their stuff, no worries just inform your classmates/friends so that when they go on to higher db and real life they wont forget, better yet send an anonymous message to your teacher asking her that they did not understand some of the sql functions and want a demonstration, have your teacher come up with a way for the class to suggest what to insert include duplicates (do not have the data be large) and when she uses the function count, you got her. Some people will pickup on it, Also when she says other databases, have your friend ask her which ones, then double snare her and say you tried all those databases and they don't work like she said and that count picks up duplicates.