第一种:
select a.a1,a.a2,a.a3,b.b2,c.c2,d.d2 from a,b,c,d where a.a1=b.b1 and b.b1=c.c1 and c.c1=d.d1
第二种:
select a.a1,a.a2,a.a3,b.b2,c.c2,d.d2 from a inner join b on a.a1=b.b1 inner join c on b.b1=c.c1 inner join d on c.c1=d.d1
附上一个链接:
http://www.5idev.com/p-php_mysql_join_multi_table.shtml