Question

如何执行类似于SQL IN查询的查询表达式？

我正试图沿着这些方向做点什么：

let customerNumbers = set ["12345"; "23456"; "3456"]
let customerQuery = query {
    for c in dataContext.Customers do
    where(customerNumbers.Contains(c.CustomerNumber))
    select c
}

但是我收到了一个错误：

System.NotSupportedException: Method 'Boolean Contains(System.String)' has no supported translation to SQL.

查看http://msdn.microsoft.com/en-us/library/hh225374.aspx处查询表达式的文档我应该对contains部分使用另一个查询，但是这段代码不起作用，示例已经破了：

// Select students where studentID is one of a given list.
let idQuery = query { for id in [1; 2; 5; 10] do select id }
query { 
    for student in db.Student do
    where (idQuery.Contains(student.StudentID))
    select student
}

idQuery确实不包含任何“包含”方法。

我也尝试过：

let customerNumbers = set ["12345"; "23456"; "3456"]
let customerQuery = query { 
    for c in dataContext.Customers do
    where (query { for x in customerNumbers do exists (c.CustomerNumber=x)})
    select r
}

但这会出现此错误消息：

System.NotSupportedException: Local sequence cannot be used in LINQ to SQL implementations of query operators except the Contains operator

我注意到经过一些测试后，除了Gene的建议之外，以下内容也可以正常使用：

let customerNumbers = set ["12345"; "23456"; "3456"]
query {
    for customer in dataContext.Customer do
    where (query { for x in customerNumbers do contains customer.CustomerNumber})
    select customer
}

Answer 1

我认为问题来自F＃Set实现方法Contains的方式。它属于ICollection接口，这个事实以某种方式扰乱了LINQ-to-SQL查询构建器。

如果您明确强制Contains进入IEnumerable领域的扩展方法，一切都会正常：

let customerNumbers = set ["12345"; "23456"; "3456"]
let customerQuery = query {
    for c in dataContext.Customers do
    where((customerNumbers :> IEnumerable<string>).Contains(c.CustomerNumber))
    select c
}

或者，等效地，您可以添加非LINQ-to-SQL查询

let idQuery = query { for id in customerNumbers do select id }

在枚举set产生seq<string>然后将其用于Contains

时没有问题

....
where (idQuery.Contains(c.CustomerNumber))
....

或者，首先，您可以将customerNumbers作为seq：

let customerNumbers = set ["12345"; "23456"; "3456"] |> Set.toSeq

并将其用作直觉提示：

....
where(customerNumbers.Contains(c.CustomerNumber))
....

如何使用f＃查询表达式执行“包含”查询？

1 个答案: