select t2.*,
       (case
           when AIV between minAIV and (minAIV * 1.2) then 'Низкий'
           when AIV between (maxAIV * 0.8) and maxAIV then 'Высокий'
           else 'Средний'
           end) over (partition by CategoryLevel4) as PriceLevel
    from
(
    select ItemID,
           CategoryLevel4,
           ID1,
           Name1,
           Value1,
           AIV,
           min(AIV) over (partition by CategoryLevel4) as minAIV,
           max(AIV) over (partition by CategoryLevel4) as maxAIV
    from (select t.RezonItemID                  as ItemID,
                 t.CategoryLevel4               as CategoryLevel4,
                 t.AttributeID1                 as ID1,
                 t.AttributeName1               as Name1,
                 t.AttributeValue1              as Value1,
                 t.AIV as AIV
          from table t) t1
    group by ItemID, CategoryLevel4, ID1, Name1, Value1, AIV) t2