Control several elements based on their values with WHERE

Question

I would like to substract 1 from SEVERAL elements in a table if they are not equal to 0.

Code for one element

UPDATE `database` SET a1=a1-1 WHERE a1!='0' AND username='bob';

I want to do the same thing on the same command for several elements, a2, a3... (username stay the same for all elements)

Thank you

Answer 1

You're probably looking for conditionals within your set expressions. This can be achieved with the keyword CASE:

UPDATE 'database' 
SET
  a1 = CASE WHEN a1 != 0 THEN (a1 - 1) END,
  a2 = CASE WHEN a2 != 0 THEN (a2 - 1) END
WHERE
  username='bob';

Be sure to understand the principles of the sql update syntax which consists basically of a single UPDATE ... SET ... [WHERE] pattern.

Hint: I'm pretty sure your a1,a2,a3,.. fields are of type integer, so - unlike in your code example - the value to check must not put into quotes!