I would like to know why, in the dataset screen editor, it is not possible to use a calculate field as field in a join (ex: one source A contains a column with a postal code and the two first characters of this field could be used as a joining-field to enrich the dataset thanks to a source B which gets a column with the short code (2 characters))
I’ve seen that this is possible by creating first a dataset with the source A and a calculate field and then, recreating a second dataset to set up the join.
You are right in that it is not possible - Not sure what the reason behind this limitation is, but could be related to having some calculated fields which are impossible to use in joins.
I hit this limitation in the past and my workaround was to switch to custom sql and add the calculated column to the query (then it allows me to use that as a join column). In actual fact, if you are not joining across datasources, you might also do the join in the custom sql query itself.