stored procedures - Using MIN(left(@prefix, 3)) in SQL -

i'm trying use min(left) in sql find prefix of id (for example want ujh ujh5433)

@prefix varchar(25) select  [description] prefix prefix = min(left(@prefix, 3))

when try execute

    alter procedure [dbo].[prefixinfo2]

statement, sql doesn't seem min(left(@prefix, 3)) format. error message get:

an aggregate may not appear in clause unless in subquery contained in having clause or select list, , column being aggregated outer reference.

is there way min work?

all need prefix of id left(@id, 3). don't need min.

OSX