I suspect that this is because some tables are defined as having an additional key column.
So for instance if you are looking at an "events" table you can't join to a "labs" table because the "labs" table has multiple rows per participant visit.
In general to join columns from two table in the UI (using customize view) you have to start with the table with more rows (well actually more columns in the key, e.g. participant,visit,labbcode) and then select column from the dataset with fewer rows (fewer columns in the key: participant visit).
Yes, this is confusing. We have a proposal alternate UI for selecting columns. |