在SAS中使用proc SQL返回最近的值

我希望根据字母类型为P29的日期返回字母来源为CCMA的最新结果

基本上，我只需要知道最后一封CCMA信函的日期，即有P29信函的账户。CCMA信函可以是任何日期，只要是最近的，但P29必须在特定的时间段内

到目前为止，我的代码是：

proc sql;
   select distinct
      account_no        as account_no
     ,letter_date   as letter_date
     ,letter_type   as letter_type
     ,source        as letter_source
   from uulster.perm_aura_letters
   where letter_type="P29"
     and letter_date >= '01jul2015'd and letter_date <= '09jul2015'd
   order by letter_date, letter_type;
quit;

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请您帮忙好吗？

我想您可以使用两个烫发字母表来完成这项工作。这并不漂亮，但我现在还没有想出一个更有效的方法

select distinct
    account_no      as account_no
    ,letter_date    as letter_date
    ,letter_type    as letter_type
    ,source         as letter_source
from uulster.perm_aura_letters p29
join uulster.perm_aura_letters ccma on p29.account_no = ccma.account_no
where p29.letter_type="P29"
and ccma.letter_type = "CCMA"
and p29.letter_date >= '01jul2015'd and p29.letter_date <= '09jul2015'd
and ccma.letterdate = (subselect max date for account and ccma)
order by p29.letter_date, p.29.letter_type

)

proc sql;
select * from (select distinct
        account_no      as account_no
        ,letter_date    as letter_date
        ,letter_type    as letter_type
        ,source         as letter_source
        from uulster.perm_aura_letters
        where letter_type="P29"
        and letter_source contains "CCMA"
        and letter_date >= '01jul2015'd and letter_date <= '09jul2015'd)
having letter_date=max(letter_date)
order by letter_date, letter_type
;
quit;