Question

如何将这两个计数函数合并到一个查询中？

SELECT 
    COUNT(MaritalStatus) as 'Married'
FROM 
    Person.Person PP 
INNER JOIN 
    HumanResources.Employee HRE ON PP.BusinessEntityID = HRE.BusinessEntityID
WHERE 
    MaritalStatus = 'M';

SELECT 
    COUNT(MaritalStatus) as 'Single'
FROM 
    Person.Person PP 
INNER JOIN 
    HumanResources.Employee HRE ON PP.BusinessEntityID = HRE.BusinessEntityID
WHERE 
    MaritalStatus = 'S';

Answer 1

您可以尝试在以下情况下使用总和案例：

SELECT SUM(CASE WHEN MaritalStatus = 'M' THEN 1 ELSE 0 END), 
SUM(CASE WHEN MaritalStatus = 'S' THEN 1 ELSE 0 END) 
FROM Person.Person PP INNER JOIN HumanResources.Employee HRE
ON PP.BusinessEntityID = HRE.BusinessEntityID

Answer 2

对于这种请求，您需要使用聚合函数。

SELECT MaritalStatus, COUNT(MaritalStatus)
FROM Person.Person PP
    INNER JOIN HumanResources.Employee HRE
        ON PP.BusinessEntityID = HRE.BusinessEntityID
GROUP BY MaritalStatus;

这将输出如下内容：

MaritalStatus  | COUNT(MaritalStatus)
               |
M              | 50
S              | 20

两排。你可以加 ORDER BY MaritalStatus以确保您按正确顺序接收数据，因为您收到数据的顺序不确定。

使用此优于SUM（）的优点是，如果您添加其他婚姻状态，您的请求将不会更改。

Answer 3

这也很有效，并且很容易扩展到其他MaritalStatuses。

SELECT married.*, single.* from
(   SELECT COUNT(MaritalStatus) as 'Married'
    FROM Person.Person PP INNER JOIN HumanResources.Employee HRE
    ON PP.BusinessEntityID = HRE.BusinessEntityID
    WHERE MaritalStatus = 'M'
    ) married
, (
    SELECT COUNT(MaritalStatus) as 'Single'
    FROM Person.Person PP INNER JOIN HumanResources.Employee HRE
    ON PP.BusinessEntityID = HRE.BusinessEntityID
    WHERE MaritalStatus = 'S'
    ) single

Answer 4

由于from和join子句相同，只需将where条件移动到case语句中即可。

SELECT COUNT(case when MaritalStatus = 'M' then 1 end) as 'Married',
       COUNT(case when MaritalStatus = 'S' then 1 end) as 'Single'
FROM Person.Person PP 
JOIN HumanResources.Employee HRE
  ON PP.BusinessEntityID = HRE.BusinessEntityID

Answer 5

您可以使用group并同时拥有：

SELECT COUNT(MaritalStatus) as 'Status'
    FROM Person.Person PP INNER JOIN HumanResources.Employee HRE
    ON PP.BusinessEntityID = HRE.BusinessEntityID
  group by MaritalStatus;

或者您可以使用decode

Answer 6

使用MySQL，这应该有效：

SELECT SUM(MaritalStatus = 'M') as 'Married',
       SUM(MaritalStatus = 'S') as 'Single'
FROM Person.Person PP INNER JOIN HumanResources.Employee HRE
ON PP.BusinessEntityID = HRE.BusinessEntityID
WHERE MaritalStatus IN ('M', 'S');

（“MaritalStatus ='X'）是一个布尔值，计算结果为0（假）或1（真）。在MySQL以外的其他服务器上，可能需要强制转换。

SQL：单个查询中的两个计数函数？

6 个答案: