First, it's a bad idea to use one letter meaningless table aliases. It makes code harder to read and maintain. Second, if you assign allias to a table you have to use it everywhere in a query. Third, it's better to use JOIN

SELECT site, security ;
	FROM cur_sec_2_3 cs23 ;
	JOIN cur_t_oblgr_site ctos ;
		ON cs23.site = ctos.site_id

>SELECT site, security FROM cur_sec_2_3, cur_t_oblgr_site where cur_sec_2_3.site = cur_t_oblgr_site.site_id
>
>This does not work
>SELECT site, security FROM cur_sec_2_3 a, cur_t_oblgr_site b where cur_sec_2_3.site = cur_t_oblgr_site.site_id
>
>The only difference in the a, and b, for the table references in the FROM.
>