Question

What does a plus symbol mean when it appears next to a key in a table?

 ----------------------------
 TABLES
 ----------------------------
 Order        | OrderLine
 -------      | -----------
 orderid      |  orderid+
 orderdate    |  total price
              |
 ----------------------------

The order table and orderline table are linked through the orderid key, but I’m
not sure what the plus sign next to the orderid key in the orderline table means.

Thanks for the answer in advance.

Answer 1

Looking at the tables and the attribute names, one can hazard a guess that this stands for a foreign key.

That is, the orderid attribute of OrderLine is a foreign key to the orderid of the Order table.

Note that this is a guess based on context of the table and attribute names, in conjunction with knowledge of how Order and OrderLine entities are normally modelled.