You could use and EXISTS in you case statement rather than an =

Not sure if I've got you logic right, but something like the following

-- Some Test Data

;with Fact as (

SELECT *

FROM (VALUES (1), (2), (3), (4), (5)) AS Fact(VisitID)

)

, Error as (

SELECT *

FROM (VALUES (1,'a'), (3,'w also has another'), (3,'a'), (4, 'w just this one'), (5,'a')) AS Error(VisitID, [Message])

)

-- The Query

SELECT f.VisitID, CASE

WHEN EXISTS(SELECT 1 FROM Error e WHERE e.visitID = f.visitID and [Message] not like 'w%') THEN 'false'

ELSE 'true'

END

FROM Fact f