Oracle COUNT Function

https://www.databasestar.com/oracle-count/
The Oracle COUNT function is used to count the number of rows from the column, or number of values, provided to it.

It’s commonly used to find the number of rows returned by a SELECT query.

The syntax of the COUNT function is:

COUNT ( [ * | [ DISTINCT | ALL ] expression) [ over (analytic_clause) ]

There’s a lot of square brackets here, indicating optional parameters and choices. This just means you can use the COUNT function in a few different ways:

COUNT(*)
COUNT(DISTINCT expression)
COUNT(ALL expression)
COUNT(*) OVER (analytic_clause)
COUNT(DISTINCT expression) OVER (analytic_clause)
COUNT(ALL expression) OVER (analytic_clause)

It can be used as an aggregate or analytic function.

The parameters of the COUNT function are:
- expression: The expression to count the values of. This can be a column or a set of values, for example.
- analytic_clause: This is used to specify the criteria for an analytic query.

COUNT(*) is the most common way to use this function, I’ve found.

As COUNT(*) is an aggregate function, like all aggregate functions the columns mentioned in the SELECT clause need to exist in the GROUP BY clause.

NULL values are ignored by COUNT(expression), but they are counted when using COUNT(*).

COUNT(*) has the exact same calculation and performance as COUNT(1).

For more information about the Oracle COUNT function, including all of the SQL shown in this video and the examples, read the related article here:
https://www.databasestar.com/oracle-count/