查询数据库中特定项目的数量

Question

我只是想知道如何计算特定设备的数量..

SELECT 
    EQUIPMENTS.DESCRIPTION AS [EQUIPMENT TYPE], 
    Count(EQUIPMENTS.EQNAME) AS QUANTITY,
    (SELECT Count(EQUIPMENTS.CONDITION) FROM EQUIPMENTS WHERE EQUIPMENTS.CONDITION = 'Functional') AS WORKING,
    (SELECT Count(EQUIPMENTS.CONDITION) FROM EQUIPMENTS WHERE EQUIPMENTS.CONDITION = 'Non-Functional') AS [NON-WORKING]
FROM EQUIPMENTS
GROUP BY EQUIPMENTS.DESCRIPTION;

此查询返回以下内容：

EQUIPMENT NAME : PROJECTOR
QUANTITY : 3
WORKING : 2
NON-WORKING :1

现在，如果我添加另一个具有不同类型的设备，例如计算器，它将具有相同的 WORKING 和 NON-WORKING 计数，仅适用于 PROJECTOR。我该如何让它也计算计算器的数量以及工作和非工作本身的数量？我的意思是每当我添加另一个具有特定描述的设备时，查询也会独立计算它？

我使用的是 VB.NET，这个查询是在 MS ACCESS 2007 中进行的。

Answer 1

使用IIf()表达式，当条件满足时返回1，不满足时返回0。然后Sum 那些值。

SELECT 
    e.DESCRIPTION AS [EQUIPMENT TYPE], 
    Count(e.EQNAME) AS QUANTITY,
    Sum(IIf(e.CONDITION = 'Functional', 1, 0)) AS WORKING,
    Sum(IIf(e.CONDITION = 'Non-Functional', 1, 0)) AS [NON-WORKING]
FROM EQUIPMENTS AS e
GROUP BY e.DESCRIPTION;