Question

我正在尝试为Microsoft Access编写以下查询，并且在这里遇到了最后三个字符的问题，有人可以告诉我出于访问目的的语法有什么问题：

SELECT DISTINCT Searches.Search，IIF（Chemical_Names.Category IS NOT NULL，Chemical_Names.Category，IIF（CAS_Numbers.Category IS NOT NULL，CAS_Numbers.Category，IIF（Brand_Name.Category IS NOT NULL，Brand_Name.Category， IIF（PART.Category IS NOT NULL，PART.Category，CAT.Category））））AS类别来自搜索 LEFT JOIN （SELECT * FROM Cat1 UNION SELECT * FROM Cat2 UNION SELECT * FROM Cat3 UNION 选择*来自Cat4）作为CAT 上 Searches.Search = CAT.Search LEFT JOIN （SELECT * FROM Part1 UNION SELECT * FROM Part2 UNION SELECT * FROM Part3 UNION
SELECT * FROM Part4）作为PART 上 Searches.Search = PART.Search LEFT JOIN 供应商名称上 Searches.Search = Supplier_Name.Search LEFT JOIN Chemical_Names 上 Searches.Search = Chemical_Names.Search LEFT JOIN Chemical_Names 上 Searches.Search = CAS_Number.Search LEFT JOIN 品牌上 Searches.Search = Brand_Name.Search;

Answer 1

我认为您可以尝试使用Nz()函数有条件地在您的首选项链中显示第一个非null类别：

SELECT DISTINCT
    Nz(Chemical_Names.Category,
        Nz(CAS_Number.Category,
            Nz(Brand_Name.Category,
                Nz(PART.Category, CAT.Category)))) AS Category
FROM Searches
...

Nz()函数是大多数其他数据库中COALESCE()的“Access”版本。

您还可以处理此使用IIF()，这是CASE表达式的Access'版本：

SELECT DISTINCT
    IIF(Chemical_Names.Category IS NOT NULL, Chemical_Names.Category,
        IIF(CAS_Number.Category IS NOT NULL, CAS_Number.Category,
            IIF(Brand_Name.Category IS NOT NULL, Brand_Name.Category,
                IIF(PART.Category IS NOT NULL, PART.Category, CAT.Category)))) AS Category
FROM Searches
...

Answer 2

如果我理解您想要的结果，应按如下方式应用Switch：Switch(Chemical_Names.Category IS NOT NULL, Chemical_Names.Category, CAS_Namber.Category IS NOT NULL, CAS_Number.Category, Brand_Name.Category IS NOT NULL, Brand_Name.Category, PART.Category IS NOT NULL, PART.Category, 1=1, CAT.Category) AS Category

关于Case的语法错误问题

2 个答案: