SQL: Right way to model multible relations in UML/ ERD -

how do uml example:

order(id, user_id, invoice_user_id) user_id , invoice_user_id foreign keys , user_id != null 

both users stored in same entity. how cardinalities or uml priciple? guessed way:

[order]0..n--1..2[user]

so order can have 2 users. option (i hope understand textpicture...) should mean order can have 2 users. or way right:

[order]n -- 1[user] |n |1 [(invoice)user]

one user can have many orders and/or can invoice user of many orders...

you should naming unnamed association-end properties , using 2 separate associations. way have it, there no way invoice user separately other user. , these users anyway? need more analysis understand roles involved. here's guess:

• seller creates number of invoices
• buyer receives number of invoices
• invoice received by 1 buyer
• invoice created by 1 seller

that gives model this:

  ------ |seller|  ------  1 | creatingseller    |  * | createdinvoice  -------  *                              1  ----- |invoice|----------------------------------|buyer|  -------  receivedinvoice   receivingbuyer  ----- 

now can navigate 2 "users".

btw, both buyer , seller roles played agent, person or organization. i'm not going model whole thing you.

just aware, there other ways realize model in rdbms, using "reference data" / "enumeration literals" differentiate 2 associations , 2 roles of agent. original question indicates want more physical model, think you're asking trouble skipping analysis model, can realized in rdbms.