Question

我有两张桌子，我想彼此结合。在这一点上，我正在使用UNION来组合它们，这非常适合我的所有需求，但有一点。由于UNION用T2代替T1中的所有内容，因此T3.votes将为0.但我不希望它为0 - 我希望它保持T1.votes的状态（不要介意T2只有四行长）因为我已经设法将这些结合在一起，但我只是没有得到T3.votes来显示T1.votes内容。）

我如何做到这一点？澄清：除T2.votes外，一切都应该合并。

T1     
+-----------+-------+---------------+
| CANDIDATE | VOTES | RANKED_CHOICE |
+-----------+-------+---------------+
|         1 |     8 |             0 |
|         2 |     1 |             0 |
|         3 |     2 |             0 |
|         4 |     4 |             0 |
|         5 |     2 |             0 |
|         6 |     3 |             0 |
+-----------+-------+---------------+

T2
+-----------+-------+---------------+
| CANDIDATE | VOTES | RANKED_CHOICE |
+-----------+-------+---------------+
|         2 |     0 |             4 |
|         3 |     0 |             1 |
|         4 |     0 |             2 |
|         5 |     0 |             1 |
+-----------+-------+---------------+

COMBINED TABLE AS I EXPECT IT TO BE (T3)
+-----------+-------+---------------+
| CANDIDATE | VOTES | RANKED_CHOICE |
+-----------+-------+---------------+
|         1 |     8 |             0 |
|         2 |     1 |             4 |
|         3 |     2 |             1 |
|         4 |     4 |             2 |
|         5 |     2 |             1 |
|         6 |     3 |             0 |
+-----------+-------+---------------+

Answer 1

如果T1的所有候选人都在T2，你可以进行左连接。

select t1.candidate, max(t1.votes,ifnull(t2.votes,0)), max(t1.ranked_choice,ifnull(t2.ranked_choice,0))
from t1 left join t2
on t1.candidate = t2.candidate;

否则，您可以将T1和T2的并集作为子查询，并从此子查询中获取总票数或最大值。

select candidate, sum(votes),max(ranked_choice) from
(
select candidate,votes,ranked choice from t1
union
select candidate,votes,ranked choice from t2
) sub
group by candidate;

编辑：我的查询中有两个小错误 - 使用最大（）而不是max（）和＃34;排名选项之间的下划线＆＃34;：

select t1.candidate, greatest(t1.votes,ifnull(t2.votes,0)) as votes, greatest(t1.ranked_choice,ifnull(t2.ranked_choice,0)) as rank
from t1 left join t2
on t1.candidate = t2.candidate;

select candidate, sum(votes),max(ranked_choice) from
(
select candidate,votes,ranked_choice from t1
union
select candidate,votes,ranked_choice from t2
) sub
group by candidate;

两个查询都会产生所需的结果。

结合表格内容，同时排除＆＃39;给出MySQL中的列

1 个答案: