Question

I have such tables in my database like Customer, Member, Instructor, Employee etc. Each of these users has his email adrress. I need to check if there is already a user with given email. I was thinking about:

• Check each table, something like this:

    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)))
              ...
    }

• Select all emails and check:

    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));
    }

There are more tables and many users, so I would like to know what would be the most efficient way to implement such kind of checking.

Thank you.

Answer 1

In pure SQL this would be your most efficient because it stops searching as soon as it hits a match:

… As a stored procedure:

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

… As a scalar-valued function:

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

In C# with Linq, you can use the Any extension and the || operator. Since Any usually gets translated to EXISTS in SQL and evalutation of the || operator in C# is lazy, evaluation will stop as soon as the first ocurrence of an email is reached.

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));