COUNTD in Tableau: the SQL equivalent, with a worked example

COUNTD([amount])

id	amount	units	score	other
1	-2.5	2	-0.8	3
2	0	0	0	4
3	1.5	3	0.25	2
4	4	4	0.5	8
5	9	5	0.8	11
6	16	6	1	14
7	null	7	null	9
8	4	8	-0.5	null

SELECT cast(count(DISTINCT amount) AS DOUBLE) AS result FROM input

Runs the example above.

SELECT cast(count(DISTINCT amount) AS DOUBLE) AS result FROM input

Written for this dialect; not run here.

SELECT cast(count(DISTINCT amount) AS FLOAT64) AS result FROM input

Written for this dialect; not run here.

result
6

Same rows from the calculation and from the SQL in DuckDB.

Where a naive translation goes wrong

Counting observations retains duplicates instead of counting distinct values.
Naive SQL
```
SELECT count(amount) AS result FROM input
```
result
7
Matches the calculation
```
SELECT cast(count(DISTINCT amount) AS DOUBLE) AS result FROM input
```
result
6

result
7

COUNTD(expression)