Flink SQL string literal does not support double quotation?

[徐涛]

Hi Experts,
        When I am running the following SQL in FLink 1.6.2, I got 
org.apache.calcite.sql.parser.impl.ParseException
        
        select 
                 BUYER_ID, 
                 AMOUNT, 
                 concat( 
                         from_unixtime(unix_timestamp(CREATE_TIME, 'EEE MMM dd 
HH:mm:ss zzz yyyy'),'yyyyMMdd'), 
                         case when 1>= 10 and 1<= 21 
                         then "02" else "01” 
                         end 
                 ) as date12 
         from 
                 vip_order

        but when I use single quotes , such as  ’02’ , it does not complain 
anything, it is a bit wired, because normally in SQL user can use double 
quotation or single quotation equally.
        I want to know it is a Flink limitation or a bug? If it is a 
limitation, why?
        Thank you.


Best 
Henry