Return Codes coming from a Database. What to check for?

[rudyten]

I always get confused on this part.

I am playing with a mySql Maria DB. in my localhost PC.... ohhh am using PDO

Am I just checking for "0" failure and "1" Success return?
some extra code to see how many rows effected .. 
in DB2 a return code of 100, meant no records were found but it was a successful execution.

What kinda things do i need to check for? do the all return a code?
What is the DB goes down?  will my query return a code? or will it fail in another place?
Besides a down DB  what else must i look for?

Thanks