# count

Counts the number of rows or non-`NULL` values.

CoordinatorDB supports:

* `count(expr)` and `count(DISTINCT expr)`.
* `count()` and `count(*)`.

**Details**

`count(*)`, `count()`, and `count(1)` count every admitted row, including rows
containing `NULL`. `count(expr)` evaluates the expression for each row and
counts its non-`NULL` results. `DISTINCT` counts distinct non-`NULL` values using
SQL comparison and the applicable collation.

Use `FILTER (WHERE condition)` to select which rows an aggregate counts, or
[`count_if`](https://coordinatordb.com/docs/reference/functions/aggregate-functions/combinators) to count rows satisfying a condition.

**Syntax**

```text
count()
count(*)
count(expr)
count(DISTINCT expr)
```

**Arguments**

* `expr` — Optional. Any SQL expression. When supplied, only its non-`NULL`
  results contribute to the count.

**Returned value**

A nonnegative signed `INTEGER`. Empty input returns zero.

**Examples**

**Basic row count**

```sql
CREATE TABLE t (num UInt8);
INSERT INTO t VALUES (1), (1), (2), (2), (3);
```

```sql
SELECT count() FROM t FORMAT csv;
```

```text
5
```

**COUNT(DISTINCT) example**

```sql
SELECT count(DISTINCT num) FROM t FORMAT csv;
```

```text
3
```

**Counting missing values**

```sql
INSERT INTO t VALUES (NULL);
```

```sql
SELECT count(*), count(num), count(*) FILTER (WHERE num IS NULL)
FROM t
FORMAT csv;
```

```text
6,5,1
```