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Review
Comparisons with NULL
Functions with NULL
11. NULL with other functions
COALESCE to the rescue
NULLIF
Review and practice

## Instruction

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.

## Exercise

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.

### Stuck? Here's a hint!

Use the concatenation symbol || to join the two columns and to separate them by a colon.