Referential join integrity

Hi HANA experts,

I have a question regarding the referential join integrity.I have seen two statements as following, which one is correct?

1 Each entry in the left table must have a correspondingentry in the right table.

2 For each row in one table, there is at least one join partner in the other table, and that holds true in both directions at all times.

Obviously statement 2 is more restricted. However if statement 1 alone is sufficient  to do a valid referential join?

Thanks,