Question

我只想问你一个关于SQL的问题。

让我们考虑一下这个EMPLOYEE表：

Employee     Department
A               10
A               10
A               11
A               12
B               13
B               13

我要显示的是每个雇员，所有不同部门（无重复）以及这些不同部门的总数。因此，如下所示：

Employee     Department      total_dept     
A               10             3
A               11             3
A               12             3
B               13             1

如果可能的话，我甚至更喜欢这样的东西：

Employee     Department      total_dept     
A               10             3
A               11             null
A               12             null
B               13             1

我有一个很大的表（有很多列和许多数据），所以我认为这可以是一种“优化”，不是吗？我的意思是，无需在所有行中存储total_dept。只要足够就放它。没问题，如果之后我将该列留空。但是我不知道是否有可能在SQL中做这样的事情。

那么，我该如何解决？我尝试过，但似乎无法将count（column）与同一列组合在一起...

提前谢谢

Answer 1

这可能就是您要寻找的

SELECT
  emp,
  dept,
  (select count(distinct dept) from TB as tbi where tb.emp = tbi.emp ) x
FROM TB
group by emp, dept;

Answer 2

MySQL 8.0支持窗口COUNT：

SELECT *,COUNT(*) OVER (PARTITION BY Employee) AS total_dept
FROM (SELECT DISTINCT * FROM Employees) e

db<>fiddle demo

您甚至可能会有第二个结果（我建议将演示文稿留给应用层）：

SELECT *, CASE WHEN ROW_NUMBER() OVER(PARTITION BY Employee ORDER BY Department) = 1
               THEN COUNT(*) OVER (PARTITION BY Employee) END AS total_dept
FROM (SELECT DISTINCT * FROM Employees) e
ORDER BY Employee, Department;

db<>fiddle demo

Answer 3

对于第二版：

SELECT 
  DISTINCT e.Employee, e.Department, 
  CASE 
    WHEN e.Department = 
      (SELECT MIN(Department) FROM Employees WHERE Employees.Employee = e.Employee) 
      THEN
        (SELECT COUNT(DISTINCT Department) FROM Employees WHERE Employees.Employee = e.Employee) 
    END AS total_dept  
FROM Employees e
ORDER BY e.Employee, e.Department;

请参见demo

在SQL上显示列值及其计数

3 个答案: