You write the following subquery to define custom age groups for your patients:

SELECT

CASE

WHEN pats.BIRTH$\_$DATE $<=$DATEADD$($year$,-18,$CURRENT$\_$TIMESTAMP $)$

THEN 'Adult'

WHEN pats.BIRTH$\_$DATE $<=$DATEADD$($year$,-1,$CURRENT$\_$TIMESTAMP $)$

THEN 'Child"

WHEN pats.BIRTH$\_$DATE IS NOT NULL

THEN 'Infant' ELSE 'Missing Birthdate"

END PATIENT$\_$AGE$\_$GROUP

FROM

PATIENT pats

However, when you try to join to this subquery, shown below, you get an error.

SELECT

PAT$\_$NAME

$,$PATIENT$\_$AGE$\_$GROUP

FROM PATIENT pat

INNER JOIN $(/*$subquery $*/)$pat$\_$age ON pat.PAT$\_$ID $=$pat$\_$age.PAT$\_$ID

$$What is causing the error?