DB integrity, Users and SSO

In an existing DB, we have several tables associated with a central USERS table. Tables such as list of old password or User's preferences are associated based on USER_ID column in the main USERS table.

A new requirment is to allow login and preferences handling using 开发者_开发知识库SSO, i.e. case where USERS are not existing in the USERS table. On the face of it, it may require moving away from handling the database integirty using keys because the core USER_ID entry in the USERS table will not existing and will break following entries in the chain.

Any thoughts on how to handle such case AND keeping keys in place?

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You can create an entry in the USERS table when ever a user logs in that you don't know yet.