Which key

accounts.cu_number is a charter, not an institution. When a credit union converts its charter NCUA issues a new number, and its series under the old one stops. No query fails, so a large institution simply appears to have closed. NORTHERN did it between 2026Q1 and 2026Q2:

-- NORTHERN converted its charter between 2026Q1 and 2026Q2.
SELECT max(quarter) FILTER (WHERE cu_number = 68696) AS last_quarter_on_charter_68696,
       max(quarter) FILTER (WHERE cuid = 5163)       AS last_quarter_on_cuid_5163
FROM read_parquet([
  'https://data.quartercall.com/curated/accounts/quarter=2026Q1/data.parquet',
  'https://data.quartercall.com/curated/accounts/quarter=2026Q2/data.parquet'])
WHERE account_code = 'A010';

examples/concepts/which-key/charter-or-institution.sql

On cu_number the series ends at 2026Q1. On accounts.cuid, NCUA's entity id, it continues.

key	on	use it for	where it fails
`cu_number`	every table	one charter; joining to NCUA's own files, which are keyed on it	a conversion mints a new one
`cuid`	accounts, profiles, the index	one institution over time	NULL for a filing with no Profile block (gotcha 3); NCUA's own id, seen to change once
`rssd_number`	profiles	joining to Federal Reserve data	a string, on profiles only, with its own lifecycle

None is a perfect entity key. For a series over time, filter on cuid; the recipe does it without reading every row of every quarter.