The unit comes first

One row might represent a request, an update to that request, an individual item, or a daily snapshot. All four can be valid records, but they cannot be counted interchangeably. Write down the unit of each table before considering a relationship.

For example, a request can have several status events. Joining the request table directly to every event can multiply the apparent number of requests. A source map should show this one-to-many relationship and explain when the events must be summarized before joining.

A compact source inventory

For each source, record its purpose, owner, extraction method, update pattern, and expected fields. Note whether a file replaces its previous version or adds new records. Identify fields that may be blank, delayed, reused, or edited after an event.

An identifier needs more than a convenient column name. Check whether it stays stable over time, whether another system uses a different identifier, and whether it uniquely identifies the intended unit. If a matching rule is approximate, make the uncertainty visible.

Map transformations as well as systems

A calculation, a lookup, or a manual correction can change the meaning of a record. Include those steps in the relationship map. A reader should be able to trace a displayed measure back through the transformation to its source fields.

Use a small reconciliation sample to examine the route. Compare counts before and after joins, list unmatched identifiers, and inspect duplicated keys. A total that happens to match is useful evidence, but it does not replace checking individual relationships.

Define an output that can be reviewed

A bounded source-mapping scope can include a source register, a relationship diagram, join rules, a list of unresolved questions, and a reconciliation note. Each unknown should have an owner or an explicit decision to defer it.

The work is ready for its next stage when a reviewer can explain what one row means, how records connect, and which gaps could change an interpretation. The record-grain guide provides a worked example for that review.