Question

比方说，我有一个看起来像这样的表

ID  NAME            YREF    UREF
1   ADAM BLAISE     0001    0007
2   JAMES HARDY     0002    0005
3   PHILIP HENRY    0003    0002
4   PETER SCHWARTZ  0004    0001
5   WILL MADISON    0005    0002
6   JEREMY PINOT    0006    0002
7   JOHN WILLIARD   0007    0007
8   MARK WILLIARD   0008    0005
9   JOHN VAUGHN     0009    0002
10  DAVID JAMES     0010    0002

我希望能够为每一行计算每个用户将YREF发送给另一个用户的次数，以便所述用户在提交自己的名称时引用它，以便表格看起来像这可以得到：

ID  NAME            YREF    COUNT (i.e. how many times their YREF appeared in the UREF column of other users)
1   ADAM BLAISE     0001    1
2   JAMES HARDY     0002    5
3   PHILIP HENRY    0003    0
4   PETER SCHWARTZ  0004    0
5   WILL MADISON    0005    2
6   JEREMY PINOT    0006    0
7   JOHN WILLIARD   0007    2
8   MARK WILLIARD   0008    0
9   JOHN VAUGHN     0009    0
10  DAVID JAMES     0010    0

Answer 1

select t1.id, t1.name, t1.yref, coalesce(t2.cnt, 0) as count
from your_table t1
left join 
(
  select uref, count(*) as cnt
  from your_table
  group by uref
) t2 on t1.yref = t2.uref

Answer 2

生成一组带有计数的数据并将其连接回基本集。其他数据库引擎将允许使用OVER语法的窗口集，但mySQL不支持这些。

SELECT A.ID, A.Name, A.YREF, coalesce(B.MyCount,0)
FROM tableName A
LEFT JOIN (SELECT count(*) myCount, UREF 
            FROM tableName 
            GROUP BY UREF) B
 on A.YREF = B.UREF

从同一个表中选择然后按计数列排序，获取项目数

2 个答案: