Oracle SQL - 組合兩個表，但是從一個表中複製？

Question

我有這些表：

Table A 
Num Letter 
1 A  
2 B  
3 C  

Table B 
Num Letter 
2 C  
3 D 
4 E 

我想聯盟這兩個表，但我只希望每個號碼出現一次。如果兩個表中出現相同的數字，我希望從表B中取代表A。

Result 
Num Letter 
1 A 
2 C 
3 D 
4 E 

我怎麼能做到這一點？聯合會保持重複，並且相交只會捕捉相同的行 - 無論字母如何，我都認爲行有重複的地方。

Answer 1

效率可能不足，但它會產生正確的答案。

select nums.num, coalesce(b.letter, a.letter) 
from 
(select num from b 
union 
select num from a) nums 
left outer join b 
on (b.num = nums.num) 
left outer join a 
on (a.num = nums.num); 

Answer 2

試試這個：http://www.sqlfiddle.com/#!4/0b796/1

with a as 
(
    select Num, 'A' as src, Letter 
    from tblA 
    union 
    select Num, 'B' as src, Letter 
    from tblB 
) 
select 

    Num 

    ,case when count(*) > 1 then 
     min(case when src = 'B' then Letter end) 
    else 
     min(Letter) 
    end as Letter 

from a 
group by Num 
order by Num; 

輸出：

| NUM | LETTER | 
---------------- 
| 1 |  A | 
| 2 |  C | 
| 3 |  D | 
| 4 |  E | 

Answer 3

或者你可以使用Oracle特定的技術，使代碼更短：http://www.sqlfiddle.com/#!4/0b796/11

with a as 
(
    select Num, 'A' as src, Letter 
    from tblA 
    union 
    select Num, 'B' as src, Letter 
    from tblB 
) 
select Num, min(Letter) keep(dense_rank first order by src desc) as Letter 
from a 
group by Num 
order by Num; 

輸出：

| NUM | LETTER | 
---------------- 
| 1 |  A | 
| 2 |  C | 
| 3 |  D | 
| 4 |  E | 

代碼工作不管min(letter)或max(letter)，它具有相同的輸出，它給出了相同的輸出。重要的是你使用keep dense_rank。另一個重要的事情是，訂單問題，我們使用order by src desc來保持行時優先源表B。

---

而真正使其更短，使用keep dense_rank last，並省略desc訂單，asc是默認反正http://www.sqlfiddle.com/#!4/0b796/12

with a as 
(
    select Num, 'A' as src, Letter 
    from tblA 
    union 
    select Num, 'B' as src, Letter 
    from tblB 
) 
select Num, min(Letter) keep(dense_rank last order by src) as Letter 
from a 
group by Num 
order by Num; 

同樣，使用min或max上信不要緊，只要您保持dense_rank獲得優先級/首選行

Answer 4

另一種選擇是按如下方式組合UNION和MINUS命令：

SELECT 
    NUM, LETTER 
FROM 
    TABLE B 
UNION 
(SELECT 
     NUM, LETTER 
    FROM 
     TABLE A 
    WHERE 
     NUM IN (SELECT 
        NUM 
       FROM 
        TABLE A 
       MINUS 
       SELECT 
        NUM 
       FROM 
        TABLE B)) 

Answer 5

而另一個問題：

SELECT COALESCE(b.num, a.num) num, COALESCE(b.letter, a.letter) letter 
    FROM a FULL JOIN b ON a.num = b.num 
ORDER BY 1; 

與您的數據：

WITH a AS 
(SELECT 1 num, 'A' letter FROM dual 
UNION ALL SELECT 2, 'B' FROM dual 
UNION ALL SELECT 3, 'C' FROM dual), 
b AS 
(SELECT 2 num, 'C' letter FROM dual 
UNION ALL SELECT 3, 'D' FROM dual 
UNION ALL SELECT 4, 'E' FROM dual) 
SELECT COALESCE(b.num, a.num) num, COALESCE(b.letter, a.letter) letter 
    FROM a FULL JOIN b ON a.num = b.num 
ORDER BY 1; 

     NUM L 
---------- - 
     1 A 
     2 C 
     3 D 
     4 E 

Answer 6

SELECT A.* 
FROM A 
WHERE A.NUM NOT IN 
    (SELECT A.NUM 
    FROM B 
    WHERE A.NUM=B.NUM 
    AND B.NUM IS NOT NULL 
    AND A.NUM IS NOT NULL 
) 
UNION 
SELECT * FROM B;