Jeff's nailed it:
-- If *this* query returns any rows
SELECT 1
from Interface a
INNER JOIN AnotherDb.dbo.Trans b
ON (convert(varchar, a.TICKET_NUM) = b.TICKET_NUM and a.TXN_DT = b.TXN_DT)
where a.P_STATUS <> 'R'
-- and b.TICKET_NUM is not null
-- and b.TXN_DT is not null
OPTION (maxdop 0)
-- Then *this* unrestricted update will run, affecting every row
Update dbo.INT_AS_VENTE_TITRE
set P_STATUS = 'R', P_ERROR_CODE = 'E000026'
- which is probably not the intended behaviour.
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden