Question

我的数据库中有这样的表格，例如Customer，Member，Instructor，Employee等。每个用户都有自己的电子邮件地址。我需要检查是否已经有给定电子邮件的用户。我在考虑：

检查每个表格，如下所示：

    public bool IsEmailAddressExists(string email)
    {
        if (!Context.Customers.Any(c => string.Equals(c.Email, email, StringComparison.InvariantCultureIgnoreCase)))
            if (!Context.Members.Any(m => string.Equals(m.Email, email, StringComparison.InvariantCultureIgnoreCase)))
              ...
    }

选择所有电子邮件并查看：

    public bool IsEmailAddressExists(string email)
    {
        var emails = Context.Customers.Select(c => c.Email).Union(Context.Members.Select(m => m.Email))...; //other unions
        return emails.Any(e => string.Equals(e, email, StringComparison.InvariantCultureIgnoreCase));
    }

有更多的表和许多用户，所以我想知道实现这种检查的最有效方法是什么。

谢谢。

Answer 1

听起来你有几个选择。

创建视图。您可以在数据库中创建一个仅显示电子邮件地址的视图。假设您正在使用MSSQL Server，例如：

CREATE VIEW EmailView AS
SELECT Email from Customers
UNION ALL
SELECT Email from Instructors
....

...然后使用绑定到该视图的实体，以便您可以检查电子邮件列表以查看该电子邮件是否已存在。有关详细信息，请查看documentation。

规范化您的数据库。这些表中的每一个都是否共享除电子邮件之外的公共信息，例如，名字和/或姓氏？您可能值得花时间重新组织数据模型以将该信息放在“人员”表中，然后将其他表外键键入其中。（如果您的用户是两个不同的东西，比如客户和讲师，这也会有所帮助。）

Answer 2

在纯SQL中，这将是您最有效的，因为它会在匹配时立即停止搜索：

...作为存储过程：

CREATE PROCEDURE EmailExists 
    @email varchar(254) = NULL
AS
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON;

    DECLARE @emailExists bit

    SET @emailExists = 0

    SELECT @emailExists = 1 WHERE EXISTS(SELECT 1 FROM Customer WHERE email = @email)
    IF @emailExists = 0
    BEGIN
        SELECT @emailExists = 1 WHERE EXISTS(SELECT 1 FROM Member WHERE email = @email)
        IF @emailExists = 0
        BEGIN
            SELECT @emailExists = 1 WHERE EXISTS(SELECT 1 FROM Instructor WHERE email = @email)
            IF @emailExists = 0
            BEGIN
                SELECT @emailExists = 1 WHERE EXISTS(SELECT 1 FROM Employee WHERE email = @email)
            END
        END
    END

    SELECT @emailExists
END

......作为一个标量值函数：

CREATE FUNCTION EmailExists 
(
    @email varchar(254)
)
RETURNS bit
AS
BEGIN
    DECLARE @emailExists bit

    SET @emailExists = 0

    SELECT @emailExists = 1 WHERE EXISTS(SELECT 1 FROM Customer WHERE email = @email)
    IF @emailExists = 0
    BEGIN
        SELECT @emailExists = 1 WHERE EXISTS(SELECT 1 FROM Member WHERE email = @email)
        IF @emailExists = 0
        BEGIN
            SELECT @emailExists = 1 WHERE EXISTS(SELECT 1 FROM Instructor WHERE email = @email)
            IF @emailExists = 0
            BEGIN
                SELECT @emailExists = 1 WHERE EXISTS(SELECT 1 FROM Employee WHERE email = @email)
            END
        END
    END

    -- Return the result of the function
    RETURN @emailExists
END

在使用Linq的C＃中，您可以使用Any扩展名和||运营商。由于Any通常在SQL中被翻译为EXISTS并且对||进行了评估C＃中的运算符是惰性的，一旦达到电子邮件的第一次发生，评估就会停止。

bool emailExists = customerEmails.Any(e => string.Equals(e, email, StringComparison.InvariantCultureIgnoreCase))
                   || memberEmails.Any(e => string.Equals(e, email, StringComparison.InvariantCultureIgnoreCase))
                   || instructorEmails.Any(e => string.Equals(e, email, StringComparison.InvariantCultureIgnoreCase))
                   || employeeEmails.Any(e => string.Equals(e, email, StringComparison.InvariantCultureIgnoreCase));

检查多个表中是否存在值

2 个答案: