sources_to_institutions table contains dead source IDs

[ahankinson]

The sources_to_institutions table contains entries for source IDs that have been deleted or are no longer in Muscat.

e.g., 402003050, 402004875, 402005008, etc.

In theory, these records are the ones where marc_tag is NULL.

SELECT count(*) from sources_to_institutions WHERE marc_tag is NULL;

Gives me 1,457 records. I haven't checked all of them, but a spot check suggests that they are no longer in Muscat.