How to indicate primary customer information for accounts

need to design database tables to manage account and customer.
1 Account can have 1 Primary Holder but zero or more Joint holders.
Accounts and Customers can be linked using Link table.

Question - where can Primary Account Holder information be kept for above requirements ?

I have put few thoughts below but I am not if they are in right direction.

Is there another solution I can use ?

Solution 1: Keep IsPrimary column in link table to show which customer is PrimaryHolder.

Issue - NO way to control this to 1 row. By mistake 2 customers could be marked primary for any account.

create table Accounts (
    AccountId INT
)

create table Customers (
    CustomerId INT
)

create table AccCustLink (
    AccCustLinkId INT,
    AccountId INT,
    CustomerId INT,
    IsPrimary BOOLEAN // this shows if customer is primary

)

Solution 2: Keep Primary Holder in main table. Use link table for only Joint Account Holders.

Issue - "Account - Client" link is split across muliple tables and 2 tables have information around which clients are associated with an account.

create table Accounts (
    AccountId INT,
    PrimaryCustomerId INT, // this has primary holder id and will be restricted to one data point.
    PRIMARY(AccountId)
)

create table Customers (
    CustomerId INT
)

create table AccJointHolders (
    AccCustLinkId INT,
    AccountId INT,
    CustomerId INT // only joint account holders will be present in this table.
)

CodePudding user response：

Solution 2 in my opinion as long as there is a unique constraint on account I’d in accounts table . So no 2 people can become primary account holder accidentally