SQL to find the number of distinct values in a column

This will give you BOTH the distinct column values and the count of each value. I usually find that I want to know both pieces of information.

SELECT [columnName], count([columnName]) AS CountOf
FROM [tableName]
GROUP BY [columnName]

You can use the DISTINCT keyword within the COUNT aggregate function:

SELECT COUNT(DISTINCT column_name) AS some_alias FROM table_name

This will count only the distinct values for that column.