There are several rules, most of them listed in online help. Check for
"updatable query" or whatever (I don't have Access here to check).
In general, for a query to be updatable, Access must be able to look at any
field in the query output, and answer the following question with absolute
certainty: "Exactly which one field from which one record in which one
table, did that value come from?". If it can answer that question for every
field in the query, it will probably be updatable. If it can not answer that
question for one or more fields, it will probably not be fully updatable.
For example, this query will be updatable:
SELECT MyField FROM MyTable WHERE PrimaryKey = 123
This query *will not* be updatable:
SELECT SUM (MyField) FROM MyTable
HTH,
TC