参考楼上的回复，将程序整理了一下，以供参考。有更简便的方法希望大家能提出来，多多交流。
/*make preparation for dataset*/
%macro repeat;
%do i=1 to 100;
data a&i.;
        set a&i;
        source=&i.;
run;
%end;
%mend repeat;
data all;
        set a1-a100;
run;

/*Create a dummy dataset for analyze */
data all;
        input custid txndate txnamt mcc $ source;
datalines;
001 1 500 A 1
002 1 569 A 1
001 5 895 B 5
001 3 898 C 3
003 2 589 B 2
003 1 589 D 1
001 2 589 A 2
004 2 410 C 2
005 1 482 A 1
005 3 785 B 3
;
run;
/*Q1:count the number of customer who has pos transaction in the past 100 months*/
proc sql;
                 select COUNT(distinct custid) into: count_custid
                from all;
quit;
%PUT the number of customer who has pos transaction in the past 100 months is &count_custid;

/*Q2: count the max, min transaction amount in the past ten months */
proc sql;
        select min(txnamt), max(txnamt) into: min_txnamt, : max_txnamt
        from all
        where source le 10;
quit;
%put in the past ten months, the max transaction amount is: &max_txnamt.. the min is: &min_txnamt..;

/*Q3: count the number of customer who has more than three pos transaction records in the past half year*/
proc sql;
        select count(distinct custid) into: count_3half_custid
        from (select *, count(txndate) as count_txndate_half
                from all
                where txndate<6
                group by custid)
        where count_txndate_half>3;
quit;
%put the number of customer who has more than three pos transaction records in the past half year is: &count_3half_custid..;

/*Q4: count the number of customer who has transaction record in consecutive two months in the past three months*/
proc sort data=all nodupkey;
        by custid source;
run;  *in case of a cutomer who has multiple transaction records in a month;
data all1;
        set all;
        by custid source;
        if first.custid then seq=1;
        else seq+1;
run;
/*customers have transaction records at least in both past two months*/
data all_1;
        set all1;
        where source=2 and seq=2;
run;
/*customers have transaction record at the 2rd and 3rd month*/
data all_21;
        set all1;
        where source=2 and seq=1;
run;
data all_22;
        set all1;
        where source=3 and seq=2;
run;
proc sort data=all_21;
        by custid;
run;
proc sort data=all_22;
        by custid;
run;
data all2;
        merge all_21(in=a) all_22(in=b);
        by custid;
        if a and b;
run;
data all3;
        set all_1 all2;
run;
proc sql;
        select count(distinct custid) into: custid_2month
        from all3;
quit;
%put the number of customers who has at least two consecutive two-month transaction records in the past three month is :&custid_2month..;

/*Q5: count the transaction amount group by MCC*/
proc sql;
        create table mcc as
        select mcc, source, sum(txnamt) as sum
        from all
        where source<=20
        group by mcc, source;
quit;

因为没有具体的数据，但可以参考以下思路：
(1) 用Proc SQL，count distinct CUSTID;
(2) 用Proc Univariate去找;
(3) 先用时间找出“过去半年”的transaction记录，还是用SQL count 每个CUSTID的transaction的次数，然后条件设置成"where transaction次数>3";
(4) 先用时间找出“过去3个月”的transaction记录，在data step里根据CUSTID、trsanction时间排序，然后用first.obs, n+1作为indicator，找出来；
(5) 先用时间找出“过去20个月”的transaction记录，用proc tabulate做出一张表格，Row是MCC项目，Column是sum of 交易金额，然后就一目了然啦！

希望能帮到你，↖(^ω^)↗加油！

感谢回复。我会加油！

十分感谢

大家都好强啊！