跨栏唯一值

Question

I have a case in which I have two columns, but the value should be unique across both. So you can consider the below table

CREATE TABLE user_details(
  id int(11) AUTO_INCREMENT PRIMARY KEY,
  email varchar(200),
  secondary_email varchar(200),
  unique key(email),
  unique key(secondary_email)
);

The feature I want is, email cannot repeat across email and secondary_email columns. So a given email can be linked only with one user. Can we achieve it in MySQL without triggers? Currently I am managing it through application logic, and it sometimes very hard to handle as different people across modules from different teams insert/update data in the table.

Also to add, my user table is in order of millions of records and there are option for secondary mobile also. Creating an second (or third) table of 2x size and joining always the two(or three) tables will worsen my case.

Thanks in advance for all your inputs..

Answer 1

If you normalize your design, you can have an e-mail table that references your user details:

user_details_email
------------------
user_details_id (FK)
email_address

With a primary key across user_details_id, email_address . You can optionally add a boolean column to flag it as a primary.