Combine two tables by rows using key variables, in a restricted form.
This is not a "real" relational join operation. It has the restrictions that: 1) The key values in B must be unique. 2) Every key value in A must map to a key value in B. These are restrictions inherited from the Matlab definition of table.join.
You probably don’t want to use this method. You probably want to use innerjoin or outerjoin instead.
See also: table.innerjoin, table.outerjoin