Text patterns and missing values

When you know part of a text and not all of it, you use LIKE. In the pattern % means "anything, nothing included" and _ exactly one character. 'M%' is those starting with M, '%son' those ending in "son", '%ar%' those with "ar" anywhere inside.

1SELECT title
2FROM books
3WHERE title LIKE 'The %'
4  AND translator IS NULL;

Books that start with "The " and have no translator. A missing value (NULL) means "unknown" and cannot be compared with =; it has a question of its own: IS NULL, and the opposite, IS NOT NULL.

NULL is not zero and not an empty text. If an employee's city is NULL the city is unknown; if it is '' the city is an empty text. They are different rows and are found with different conditions.

Tasks