不知道以下代码是什么意思：

求指点：

proc sql;

create tablesasuser.qiang_pom3 as

select a.*, b.xbeta

fromsasuser.qiang_pom3 a left join temp3 b

on a.hotel_id =b.hotel_id and a.customer_id=b.customer_id and a.review_date = b.review_date;

run;

/* convert toinverse mill ratio */

datasasuser.qiang_pom3;

setsasuser.qiang_pom3;

adjustment =pdf('normal', xbeta)/(1-cdf('normal', xbeta));

run;

datasasuser.qiang_pom3;

set  sasuser.qiang_pom3;

by hotel_idcustomer_id;

lag_adjustment= lag(adjustment);

iffirst.customer_id then lag_adjustment = 0;

run;