Question

我想在一行中检索并显示员工的销售数量，然后是销售总数。

SELECT COUNT(SalesID) AS SalesForEmployee, COUNT(SalesID) AS TotalSales
FROM Sales
WHERE EmployeeID = 123

如何使where子句仅适用于select？

中的第一列

Answer 1

您不能拥有仅适用于一列的where。

为了在只扫描一次表时获得两个计数，您可以这样做：

select 
    sum(case when EmployeeID=123 then 1 else 0 end) as SalesForEmployee, 
    Count(SalesID) as TotalSales
from Sales

没有where条款，因为Count(SalesID)需要计算每一行以计算总数。

由于您必须查看每一行，case when EmployeeID=123 then 1 else 0 end为每个属于目标员工的行提供1，为每行不提供0 。因此，总结该表达式只会为该员工提供计数。

SalesID     EmployeeID (case when ... )
1           123         1
2           311         0
3           333         0
4           123         1
5           300         0
count = 5               sum = 2

你也可以这样做：

select
    (select count(SalesID) from Sales where EmployeeID=123) as SalesForEmployee,
    (select count(SalesID) from Sales) as TotalSales

但是现在你要扫描Sales表两次，这会慢一些。

Answer 2

SELECT 
  sum(case when EmployeeID = 123 then 1 else 0 end) AS SalesForEmployee
 ,COUNT(SalesID) AS TotalSales
FROM Sales

Answer 3

select count(SalesEmp.SalesID) AS SalesForEmployee count(Sales.salesID) As TotalSales 
from Sales left outer join Sales as SalesEmp
on Sales.salesID=SalesEmp.SalesID
and SalesEmp.EmployeeID = 123

Answer 4

这是一个嵌套的子查询方法，将两个select语句合并为一个。

SELECT 
    (SELECT COUNT(SalesID) FROM Sales WHERE EmployeeID = 123) AS SalesForEmployee, 
    (SELECT COUNT(SalesID) FROM Sales) AS TotalSales

另一种写它的方式就是这样。

SELECT COUNT(SalesID) AS SalesForEmployee, 
    (SELECT COUNT(SalesID) FROM Sales) AS TotalSales
FROM Sales WHERE EmployeeID = 123

使用相关子查询，您可以将外部查询与内部查询链接。假设您希望获得的总销售额不包括EmployeeID 123的销售额

SELECT COUNT(SalesEmployee.SalesID) AS SalesForEmployee, 
    (SELECT COUNT(SalesID) FROM Sales WHERE Sales.EmplyeeID <> SalesEmployee.EmployeeID) AS TotalSales
FROM Sales As SalesEmployee WHERE SalesEmployee.EmployeeID = 123

这里内部查询引用WHERE子句中的outeeeries EmployeeeID来过滤它们。

具有不同条件的多个计数

4 个答案: