1. 我现在有一个表，如下：
   A1 A2 B1 B2
   111 50 111 20
   121 60 121 100
   122 40
   135 55
   141 15
   NULL NULL 101 40
   NULL NULL 151 21
   想要得到的结果是：
   C1 C2
   111 30
   121 40
   122 40
   135 55
   141 15
   101 40
   151 21
   即 如果A1＝B1时，A1或B1随便取一个，后一个字段的值为 ¦A2－B2 ¦
   A1＝null时，结果取B1, ¦A2－B2 ¦
   B1=null时，结果取A1, ¦A2－B2 ¦
   请问能用SQL语句实现吗？

WITH A AS (SELECT 111 A1,50 A2,111 B1, 20 B2 FROM DUAL
UNION
SELECT 121 A1,60 A2,121 B1, 100 B2 FROM DUAL
UNION
SELECT 122 A1,40 A2,NULL B1, NULL B2 FROM DUAL
UNION
SELECT 135 A1,55 A2,NULL B1, NULL B2 FROM DUAL
UNION
SELECT 141 A1,15 A2,NULL B1, NULL B2 FROM DUAL
UNION
SELECT NULL A1,NULL A2, 101 B1,40 B2 FROM DUAL
UNION
SELECT NULL A1,NULL A2,151 B1, 21 B2 FROM DUAL
)
select NVL(A1,B1),CASE WHEN A1=B1 THEN ABS(A2-B2)
WHEN A1 IS NULL THEN ABS(NVL(A2,0)-NVL(B2,0))
WHEN B1 IS NULL THEN ABS(NVL(A2,0)-NVL(B2,0)) END C2
FROM A

其實仔細看了哈，沒想到還有更簡單

select nvl(A1,B1) C1,abs(NVL(A2,0)-NVL(B2,0)) C2 from A;
一定要加NVL