This question already has an answer here:
T1: USER_ID OSX 1 Y 2 Y T2: USER_ID ANDROID 1 Y 3 Y
I want to join the tables as follows but i don't know how
T3: USER_ID ANDROID OSX 1 Y Y 2 null Y 3 Y null
null is null, why should one want a dummy-value 'N/A' on the database-layer?!