Validate that a new username doesn’t match a pre-existing username

I wanted to implement a validation in my SQL stored procedure that’s designed to create new users. I wanted to validate that a new user can’t use the same username as another user. This is my current stored procedure:

ALTER PROCEDURE NewUserCreation
    @Login NVARCHAR(50), 
    @Password NVARCHAR(50), 
    @FirstName NVARCHAR(40), 
    @LastName NVARCHAR(40),
    @DOB DATE(10),
    @Email NVARCHAR(100),
    @Phone varchar(10),
    @Address NVARCHAR(250),
AS
BEGIN
    SET NOCOUNT ON

    INSERT INTO dbo.[USERS] (USERNAME, PASSWORDHASH, USER_F_NAME, USER_L_NAME, USER_DOB, USER_EMAIL)
        VALUES(@Login, HASHBYTES('SHA2_512', @Password), @FirstName, @LastName, @Email)
END

I assume I need to do something using an if statement that takes the value entered from the user in @login, and cross checks it with the USERS table’s username column. Any guidance on the best approach?

Answer

This is what I would do

    INSERT INTO dbo.[USERS] (USERNAME, PASSWORDHASH, USER_F_NAME, USER_L_NAME, USER_DOB, USER_EMAIL)
        SELECT @Login, HASHBYTES('SHA2_512', @Password), @FirstName, @LastName, @Email
        WHERE NOT EXISTS (SELECT 1 FROM dbo.[USERS] WHERE USER_EMAIL = @Email);

    IF @@ROWCOUNT = 0 BEGIN
        THROW 51000, 'Duplicate email address not allowed', 1;
    END;