【sas proc sql】子查询

proc sql feedback;
    select * from merge_a
    where flight>(select max(flight) from merge_b);
quit;

proc sql feedback;
    select * from merge_a
    where flight in (select flight from merge_b);
quit;

-----correlated subquery

proc sql feedback;
    select * from merge_a a
    where flight=(select flight from merge_b where mergeb.flight=a.flight);
quit;

correlated subquery会把每一次匹配的外部值传回子查询中，子查询匹配后返回select值。

 If subquery returnes a single result then "=" is acceptable ,otherwise you have to use in union any or all operator. 

proc sql feedback;
    select * from merge_a a
    where exists (select flight from merge_b where merge_b.flight=a.flight);
quit;

exists和correlated subquery 起类似的作用。

子查询可以重复嵌套。