SQL to find the number of distinct values in a column

Question

I can select all the distinct values in a column in the following ways:

• SELECT DISTINCT column_name FROM table_name;
• SELECT column_

Answer 1

Be aware that Count() ignores null values, so if you need to allow for null as its own distinct value you can do something tricky like:

select count(distinct my_col)
       + count(distinct Case when my_col is null then 1 else null end)
from my_table
/