SELECT table_a.*
FROM table_a
INNER JOIN (
SELECT invoice_id
FROM table_a
GROUP BY invoice_id
HAVING COUNT(invoice_id) = 1
) AS TempTable ON TempTable.invoice_id=table_a.invoice_id
WHERE 1=1
AND category='211'
https://dbfiddle.uk/?rdbms=sqlserver_2017&fiddle=d6060a66f0078cb5f3161339f40645f9
SELECT invoice_id, item, category, name, count(invoice_id)
FROM table_a
GROUP BY invoice_id
HAVING count(invoice_id) = 1 AND category = 211