The ISNULL() function
The COALESCE() function
The NULLIF() function
Instruction
Good. Finally, we'll refresh your knowledge of NULLs.
NULL indicates that a value is unknown or missing. You might see a NULL in a table with clients if you don't have the client's last name (because you forgot to ask!).
You could have a NULL in the LaunchDate field of a Product table if that product hasn't been launched yet.
A NULL in a product price field doesn't mean that the product is free; it simply means that we don't know the price.
We use the operators IS NULL and IS NOT NULL to check if a value is missing. IS NULL works like this:
SELECT * FROM Product WHERE Category IS NULL;
Exercise
Select all product names where the launch date is not NULL.
Stuck? Here's a hint!
Use a WHERE clause and IS NOT NULL.
