COUNT
Definition
The COUNT
function returns the number of rows in a SQL expression.
Syntax
COUNT(*) [OVER (...)]
COUNT([DISTINCT] expression) [OVER (...)]
COUNT(*)
counts the number of rows in the input.COUNT(expression)
returns the number of rows in the expression other than NULL
.with my_numbers as (
select 1 as x union all
select 2 as x union all
select 2 as x union all
select 5 as x union all
select null as x
)
select
count(*) as all_rows,
count(x) as non_null,
count(distinct x) as unique_non_null
from
my_numbers
The example above shows three different ways COUNT
can be used.
COUNT(*)
counts the number of rows in the table x.COUNT(x)
counts the number of elements in the table x excluding NULL
values.COUNT(DISTINCT x)
counts the number of unique elements in the table x, excluding NULL
values.COUNT
can also sometimes be used as a window function. The below example runs the COUNT
function for each value of x.
with my_numbers as (
select 1 as x union all
select 2 as x union all
select 2 as x union all
select 5 as x union all
select null as x
)
select
x,
count(*) over (partition by x) AS count_all_rows,
count(x) over (partition by x) AS count_x,
count(distinct x) over (partition by x) AS count_unique_x
from
my_numbers