Conditional update in SQLite

Question

Can I create in SQLite a statement like this?

update books set
(if number_of_pages > 0 then number_of_pages = number_of_pages - 1)
where book         


        

Answer 1

If you just want to conditionally update some rows and leave others intact, this should do:

update books
set number_of_pages = number_of_pages - 1
where number_of_pages > 0 AND book_id = 10

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If you want to update all rows (with book_id = 10) to different values, you can use 2 statements with "opposite" conditions. Assuming 0 is the "other" value, that would look like this:

update books
set number_of_pages = number_of_pages - 1
where number_of_pages > 0 AND book_id = 10

update books
set number_of_pages = 0
where number_of_pages <= 0 AND book_id = 10

Or just use CASE as others have suggested.

Answer 2

Isnt that equal to this statement ?

update books set number_of_pages = number_of_pages - 1 where number_of_pages>0

Edit:

according to new statement :

update books set number_of_pages = number_of_pages - 1 where number_of_pages>0 and book_id = 10

Answer 3

A CASE statement should work with the following syntax:

UPDATE
  books
SET number_of_page = CASE WHEN number_of_pages > 0 THEN (number_of_pages - 1) ELSE 0 END
WHERE whatever_condition