Okay, let's move on. You should also watch out for NULLs when you work with calculations. Most of them will return a NULL if any of the arguments is a NULL. This may not be obvious.
For instance, you could think that the following expression:
firstname || ' ' || lastname
with firstname = 'Anne' and a NULL lastname will return 'Anne ' as a result. That's not true. You will get a NULL if any of the columns is NULL.
Check it out for yourself. For all products, show their category, type and the following text: category and type separated with a colon (:), for example:
kitchen:fork
Name the last column category_type. Note what happens with the row which doesn't have a category.
Use the concatenation symbol || to join the two columns and to separate them by a colon.
