方法1：
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
方法2：
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
方法1：
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
方法2：
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