高人帮我看看这段代码什么意思

String pmid="";


formParams = "mid=" + toURL(getParam( request, "mid")) + "&";

pmid = getParam( request, "mid");
if ( ! isNumber (pmid)) {
pmid = "";
}

if (pmid != null && ! pmid.equals("")) {

hasParam = true;
sWhere += "m.message_parent_id=" + pmid;
}

if (hasParam) { sWhere = " WHERE (" + sWhere + ")"; }
// Build ORDER statement
sOrder = " order by m.date_entered Desc";

// Build full SQL statement

sSQL = "select m.author as m_author, " +
"m.date_entered as m_date_entered, " +
"m.message as m_message, " +
"m.message_parent_id as m_message_parent_id, " +
"m.topic as m_topic " +
" from messages m ";

sSQL = sSQL + sWhere + sOrder;


帮我详细一点解释一下这段代码的意思，谢啦