直接上题目求解释这个sql语句

Question

            sql = "select m.id, m.mobile, m.destid, m.content from cs_user u, CS_MO_SMGP m where u.extend = substr(m.destid, 9,4) and u.id = " + 
                    userid + " and m.ifflag=0 and rownum<=20"

Answer 1

这还要解释呀，几个关键的信息解释如下：
substr :http://www.cnblogs.com/nicholas_f/articles/1526063.html
rownum:http://www.blogjava.net/freeman1984/archive/2011/01/20/343253.html
where:http://www.w3school.com.cn/sql/sql_where.asp