DECODE在WHERE语句中的妙用

假设我们有两个表，需要用ROW_ID连接，找出值相同或者不相同的数据

WITH TEMP1 AS(
SELECT 1 AS ROW_ID,1 AS COLA FROM DUAL
UNION ALL SELECT 2 ,1  FROM DUAL
UNION ALL SELECT 3 ,NULL FROM DUAL
UNION ALL SELECT 4 ,NULL FROM DUAL
)
SELECT * FROM TEMP1;

WITH TEMP2 AS(
SELECT 1 AS ROW_ID,1 AS COLB FROM DUAL
UNION ALL SELECT 2,NULL FROM DUAL
UNION ALL SELECT 3,1 FROM DUAL
UNION ALL SELECT 4,NULL FROM DUAL
)
SELECT * FROM TEMP2;

两个表的结果集如图:

我们对比较结果的期望如列5，如果：

• 两个比较字段都是NULL，认为是相等
• 两个比较字段其中一个为NULL，另外一个为有效值，认为不等

NULL值的存在有点绕人，

NULL=NULL 结果为NULL

1=NULL结果为NULL

所以必须对当字段可为NULL的时候，比较需要对NULL进行单独的处理

找出COLA和COLB相等的记录

不使用DECODE，必须对NULL进行单独的处理

WITH TEMP1 AS(
SELECT 1 AS ROW_ID,1 AS COLA FROM DUAL
UNION ALL SELECT 2 ,1  FROM DUAL
UNION ALL SELECT 3 ,NULL FROM DUAL
UNION ALL SELECT 4 ,NULL FROM DUAL
)
,TEMP2 AS(
SELECT 1 AS ROW_ID,1 AS COLB FROM DUAL
UNION ALL SELECT 2,NULL FROM DUAL
UNION ALL SELECT 3,1 FROM DUAL
UNION ALL SELECT 4,NULL FROM DUAL
)
SELECT * FROM TEMP1 A
INNER JOIN TEMP2 B
ON A.ROW_ID=B.ROW_ID
WHERE A.COLA=COLB
OR (A.COLA IS NULL AND B.COLB IS NULL) /*必须对NULL进行单独处理*/

使用DECODE替换：

WITH TEMP1 AS(
SELECT 1 AS ROW_ID,1 AS COLA FROM DUAL
UNION ALL SELECT 2 ,1  FROM DUAL
UNION ALL SELECT 3 ,NULL FROM DUAL
UNION ALL SELECT 4 ,NULL FROM DUAL
)
,TEMP2 AS(
SELECT 1 AS ROW_ID,1 AS COLB FROM DUAL
UNION ALL SELECT 2,NULL FROM DUAL
UNION ALL SELECT 3,1 FROM DUAL
UNION ALL SELECT 4,NULL FROM DUAL
)
SELECT * FROM TEMP1 A
INNER JOIN TEMP2 B
ON A.ROW_ID=B.ROW_ID
WHERE DECODE(A.COLA,B.COLB,1,0)=1

结果集如图：

找出COLA和COLB不相等的记录

不使用DECODE，需要对NULL进行一大段处理

WITH TEMP1 AS(
SELECT 1 AS ROW_ID,1 AS COLA FROM DUAL
UNION ALL SELECT 2 ,1  FROM DUAL
UNION ALL SELECT 3 ,NULL FROM DUAL
UNION ALL SELECT 4 ,NULL FROM DUAL
)
,TEMP2 AS(
SELECT 1 AS ROW_ID,1 AS COLB FROM DUAL
UNION ALL SELECT 2,NULL FROM DUAL
UNION ALL SELECT 3,1 FROM DUAL
UNION ALL SELECT 4,NULL FROM DUAL
)
SELECT * FROM TEMP1 A
INNER JOIN TEMP2 B
ON A.ROW_ID=B.ROW_ID
WHERE A.COLA!=COLB
OR (A.COLA IS NULL AND B.COLB IS NOT NULL)
OR (A.COLA IS NOT NULL AND B.COLB IS NULL)  /*必须对NULL进行单独处理*/

使用DECODE

WITH TEMP1 AS(
SELECT 1 AS ROW_ID,1 AS COLA FROM DUAL
UNION ALL SELECT 2 ,1  FROM DUAL
UNION ALL SELECT 3 ,NULL FROM DUAL
UNION ALL SELECT 4 ,NULL FROM DUAL
)
,TEMP2 AS(
SELECT 1 AS ROW_ID,1 AS COLB FROM DUAL
UNION ALL SELECT 2,NULL FROM DUAL
UNION ALL SELECT 3,1 FROM DUAL
UNION ALL SELECT 4,NULL FROM DUAL
)
SELECT * FROM TEMP1 A
INNER JOIN TEMP2 B
ON A.ROW_ID=B.ROW_ID
WHERE DECODE(A.COLA,B.COLB,1,0)=0

结果集如图：

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