Counts the number of rows or not NULL values.
Syntax
COUNT([ DISTINCT ] <expression>) [FILTER ([WHERE] <condition>)]
Parameters
| Parameter | Description | Supported input types |
|---|
<expression> | The expression to count | Any |
<condition> | An optional boolean expression to filter rows used in aggregation | BOOL |
Valid values for the input expression include column names or functions that return a column name. When DISTINCT is being used, only the unique number of rows with no NULL values are counted. COUNT(*) returns a total count of all rows in the table, while COUNT(<column_name>) returns a count of non-null rows in the specified <column_name>.
By default, COUNT(DISTINCT) returns exact results. If you do not require a precise result and want to have faster performance, consider using the APPROX_COUNT_DISTINCT function. See below for examples and considerations.
Return Type
NUMERIC
Example
For this example, see the following table tournaments:
| name | totalprizedollars |
|---|
| The Drifting Thunderdome | 24768 |
| The Lost Track Showdown | 5336 |
| The Acceleration Championship | 19274 |
| The Winter Wilderness Rally | 21560 |
| The Circuit Championship | 9739 |
| The Singapore Grand Prix | 19274 |
Doing a regular COUNT returns the total number of rows in the column. As the tournaments table contains 6 rows, this will be the returned value.
SELECT
COUNT(name)
FROM
tournaments;
6
A COUNT(DISTINCT) function on the same column returns the number of unique rows. When applied to the totalprizedollars column, the value returned is 5, as there is a repeated number in the column.
SELECT
COUNT(DISTINCT totalprizedollars)
FROM
tournaments;
5
Example of COUNT(DISTINCT) vs. APPROX_COUNT_DISTINCT
To understand the difference between COUNT(DISTINCT pk) with exact precision enabled and using default approximation, consider a table, count_test with 8,388,608 unique pk values. The APPROX_COUNT_DISTINCT function returns the same approximate results as the COUNT(DISTINCT) function with exact precision disabled, so we can see the difference between these methods with the following example.
SELECT
COUNT(DISTINCT pk) as count_distinct,
APPROX_COUNT_DISTINCT(pk) as approx_count
FROM
count_test;
' +----------------+--------------+
' | count_distinct | approx_count |
' +----------------+--------------+
' | 8,388,608 | 8,427,387 |
' +----------------+--------------+
