COUNT « Aggregate Functions « Oracle PL/SQL Tutorial

12.3.1.COUNT(x) gets the number of rows returned by a query.
12.3.2.Passes ROWID to COUNT() and gets the number of rows
12.3.3.COUNT() with null
12.3.4.Using an aggregate with the GROUP BY clause to count by city
12.3.5.COUNT(1) from a table
12.3.6.Count column with table alias
12.3.7.Count distinct column value
12.3.8.Compare the difference between count(*) and count(distinct course)
12.3.9.Count employees in the same department

12.3.10.JOIN with AND and aggregate function
12.3.11.count(*) - count(onhand)
12.3.12.count(*) vs count(column name)
12.3.13.Count all employees by even/odd employee id
12.3.14.Count date field value, and calculation