多列计数

Question

我在编写对 2 列中所有值的出现次数求和的查询时遇到问题。我有一个具有以下结构的 table：

+-----------+------------+
| player1ID | player2ID  |
+-----------+------------+
| 1         | 2          |
| 1         | 3          |
| 2         | 3          |
| 1         | 4          |
+-----------+------------+

在运行查询之后，我想要这样的结果 table：

+-----------+------------+
| playerID  | count      |
+-----------+------------+
| 1         | 3          |
| 2         | 2          |
| 3         | 2          |
| 4         | 1          |
+-----------+------------+

我试过以下查询

select g1.player1ID, g1.count1 + g2.count2 
from 
    (select player1ID, count(*) from table group by player1ID) as g1, 
    (select player2ID, count(*) from table group by player2ID) as g2 
where player1ID = player2ID

但这只会在玩家出现在两列（player1ID 和 player2ID）中时给出计数，但如果它只出现在其中一列中则不会出现。

Answer 1

使用union all合并两列。然后做聚合：

select playerID, count(*)
from ((select player1ID as playerID from table) union all
      (select player2ID as playerID from table)
     ) t
group by playerID;

Answer 2

您可以像这样在派生的 table 中使用 union：

select player, count(*) as count
from (
  select player1id player from table1
  union all
  select player2id player from table1
) sub
group by player;

Sample SQL Fiddle

多列计数

Count of multiple columns

mysql

sql

sum

count