SELECT knowledge FROM sql_resources WHERE category='sql' AND slug='count'

COUNT

Definition

The COUNT function returns the number of rows in a SQL expression.

  • COUNT(*) counts the number of rows in the input.
  • COUNT(expression) returns the number of rows in the expression other than NULL.
all_rowsnon_nullunique_non_null
543

The example above shows three different ways COUNT can be used.

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  • 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.

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xcount_all_rowscount_xcount_unique_x
null100
1111
2221
2221
5111
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