Mysql Case用法

CREATE view BIWareHouse.Stock_Status as
select
    `T_k3onhandqty`.`FStockName` AS `FStockName`,
    `T_k3onhandqty`.`FMaterialModel` AS `FMaterialModel`,
    `T_k3onhandqty`.`Taste` AS `Taste`,
    `T_k3onhandqty`.`FCUUQty` AS `FCUUQty`,
    `T_k3onhandqty`.`FCUUnitName` AS `FCUUnitName`,
    (to_days(`T_k3onhandqty`.`FMaturityDate`) - to_days(now())) AS `dateDIFF(FMaturityDate, now())`,
    case 
    when (to_days(`T_k3onhandqty`.`FMaturityDate`) - to_days(now())) <= 30 then "新鲜度<30日"
    when (to_days(`T_k3onhandqty`.`FMaturityDate`) - to_days(now())) <=90 and  (to_days(`T_k3onhandqty`.`FMaturityDate`) - to_days(now())) >30 then "30日<新鲜度<=90日" 
    when (to_days(`T_k3onhandqty`.`FMaturityDate`) - to_days(now())) <180 and  (to_days(`T_k3onhandqty`.`FMaturityDate`) - to_days(now())) >=90 then "90日<新鲜度<=180日"
    end as "Freshness"
    from
    BIWareHouse.T_k3onhandqty;