COUNT 

COUNT is a set function.

COUNT(*) supplies the total number of values (rows in a result table or group).

COUNT(DISTINCT <expression>) supplies the total number of different values (number of values in the argument of the DISTINCT function).

COUNT(ALL <expression>) supplies the number of values that differ from the NULL value (number of values in the argument of the ALL function)

The result has the data type FIXED(10).

Model table: customer

How many customers are there?

SELECT COUNT (*) number FROM customer

NUMBER

15