Count « Aggregate Functions « SQL Server / T-SQL Tutorial

9.3.1.COUNT returns the total count of records in a column or the count for DISTINCT (unique) values selected by the query.
9.3.2.COUNT(*) returns the total count of all records (rows) in the column, including NULL values of selected records by the query.
9.3.3.Count function with column alias
9.3.4.Counting unique values.
9.3.5.A common mistake with DISTINCT is to code the DISTINCT statement before the COUNT statement.
9.3.6.Counting values including NULL values.
9.3.7.Using ISNULL to count all values including NULLs.
9.3.8.Count table row in dynamic sql