MySQL：如何将两个表与一列合并？

Question

每个人。

我有2张桌子。

tbl_a

id    Col_A   Col_B
1     1       2
3     3       4

tbl_b

id    Col_C   Col_D
1     100     101
2     102     103
4     104     105

我想制作一张表格，例如：

id    Col_A   Col_B   Col_C   Col_D
1     1       2       100     101
2     0       0       102     103
3     3       4       0       0
4     0       0       104     105

我从How to do a FULL OUTER JOIN in MySQL? 中尝试了 SQL ？ ，但这不是我想要的正确结果。

查询语句：

SELECT * FROM tbl_a
LEFT  JOIN tbl_b ON tbl_a.id = tbl_b.id
UNION
SELECT * FROM tbl_a
RIGHT JOIN tbl_b ON tbl_a.id = tbl_b.id

结果：

id     Col A  Col B   id(1)   Col C     Col D
1      1      2       1       100       101
3      3      4       (NULL)  (NULL)    (NULL)
(NULL) (NULL) (NULL)  2       102       103
(NULL) (NULL) (NULL)  4       104       105

我怎样才能得到正确的结果？ 任何帮助将不胜感激。

谢谢你。

Answer 1

这将给出所需的结果：

SELECT * FROM
(
    SELECT tbl_a.id, 
    tbl_a.Col_A, 
    tbl_a.Col_B, 
    COALESCE(tbl_b.Col_C, 0) AS Col_C, 
    COALESCE(tbl_b.Col_D, 0) AS Col_D
    FROM tbl_a
    LEFT  JOIN tbl_b ON tbl_a.id = tbl_b.id
    UNION
    SELECT tbl_b.id, 
    COALESCE(tbl_a.Col_A, 0) AS Col_A, 
    COALESCE(tbl_a.Col_B, 0) AS Col_B, 
    tbl_b.Col_C, 
    tbl_b.Col_D
    FROM tbl_a
    RIGHT JOIN tbl_b ON tbl_a.id = tbl_b.id
)
AS a 
ORDER BY id;

Answer 2

像这样尝试

SELECT 
    IFNULL(tbl_a.id, tbl_b.id) AS id,
    IFNULL(tbl_a.Col_A, 0) as Col_A,
    IFNULL(tbl_a.Col_B, 0) as Col_B,
    IFNULL(tbl_b.Col_C, 0) as Col_C,
    IFNULL(tbl_b.Col_D, 0) as Col_D
FROM
    tbl_a
       LEFT JOIN
           tbl_b ON tbl_a.id = tbl_b.id 
UNION 
SELECT 
     IFNULL(tbl_a.id, tbl_b.id) AS id,
     IFNULL(tbl_a.Col_A, 0) as Col_A,
     IFNULL(tbl_a.Col_B, 0) as Col_B,
     IFNULL(tbl_b.Col_C, 0) as Col_C,
     IFNULL(tbl_b.Col_D, 0) as Col_D
 FROM
     tbl_a
       RIGHT JOIN
         tbl_b ON tbl_a.id = tbl_b.id