Selecting data from either of two tables if a key exists

Hi Everyone,

I have a question on how to select data when it exists in either of two tables.

Example:  I have table 'A' that contains the main key (serial number). Table B will have a corresponding record for that key. Table C may have an entry with the matching key.   I would like to using the key from table 'A', extract data from Table 'C' if a matching key exists, otherwise extract data from Table 'B'. NOTE: Table 'B' will always have a matching key/entry, Table 'C' contains only updates for certain keys.

Thanks in advance!