DECODE在WHERE語句中的妙用

假設我們有兩個表，需要用row_id連線，找出值相同或者不相同的資料

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;

我們對比較結果的期望如列5，如果：

null值的存在有點繞人，

null=null 結果為null

1=null結果為null

所以必須對當欄位可為null的時候，比較需要對null進行單獨的處理

找出cola和colb相等的記錄

不使用decode，必須對null進行單獨的處理

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) /*必須對null進行單獨處理*/

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 decode(a.cola,b.colb,1,0)=1

找出cola和colb不相等的記錄

不使用decode，需要對null進行一大段處理

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)  /*必須對null進行單獨處理*/

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 decode(a.cola,b.colb,1,0)=0

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