拼接多个相同结构的数据表,并剔除重复
a表(a,b,c)
b表(a,b,c)
c表(a,b,c)

SELECT DISTINCT a, b, c
FROM (SELECT A.A,a.b,a.c FROM a LEFT JOIN b ON (a.a=b.a))
UNION ALL (SELECT B.A,b.b,b.c FROM a right JOIN b ON (a.a=b.a))
UNION ALL (SELECT C.A,c.b,c.c FROM A RIGHT JOIN C ON (A.A=C.A));