從多個表

Question

我有表AA使用列 A B C與價值1,a,a

表BB與A,B,C與價值觀2,b,b

選擇，如果我select B=a and C=a我想O/P爲A柱與價值1

或

if if我想要得到的O/P型柱A與價值2

我想這個

Answer 1

(SELECT a FROM AA WHERE B = 'something' AND C = 'something') 
UNION ALL 
(SELECT a FROM BB WHERE B = 'something' AND C = 'something') 

單查詢