db2 表关联查询

今天在MapReduce的练习中看到了一个题目：

file:

CHILD      PARENT
---------- ----------
tom        lucy
tom        jack
jone       lucy
jone       jack
lucy       mary
lucy       ben
jack       alice
jack       jesse
terry      alice
terry      jesse
philip     terry
philip     alma
mark       terry
mark       alma

输出结果要求：

GRANDCHILD GRANDPARENT
---------- -----------
jone       mary
jone       ben
jone       alice
jone       jesse
mark       alice
mark       jesse
philip     alice
philip     jesse
tom        mary
tom        ben
tom        alice
tom        jesse  

我在思考，这个如果是DB2的一个表，应该能通过表连接来实现这个要求。于是生成表parent：

[db2inst1@win ~]$ db2 "select * from parent"

CHILD      PARENT
---------- ----------
tom        lucy
tom        jack
jone       lucy
jone       jack
lucy       mary
lucy       ben
jack       alice
jack       jesse
terry      alice
terry      jesse
philip     terry
philip     alma
mark       terry
mark       alma      

  14 record(s) selected.

要达到这样的结果，一定要用到表的hash join。下面是我的SQL实现：

[db2inst1@win ~]$ db2 "select u.child GRANDCHILD, b.parent GRANDPARENT from (select * from parent where parent in (select child from parent)) as u ,(select * from parent where child in (select parent from parent)) as b where u.parent=b.child order by u.child"

DB2的优化器重写成这样：

Optimized Statement:
-------------------
SELECT
  DISTINCT Q1.CHILD AS "GRANDCHILD",
  Q3.PARENT AS "GRANDPARENT",
  Q3.CHILD,
  Q1.PARENT
FROM
  DB2INST1.PARENT AS Q1,
  DB2INST1.PARENT AS Q2,
  DB2INST1.PARENT AS Q3,
  DB2INST1.PARENT AS Q4
WHERE
  (Q1.PARENT = Q2.CHILD) AND
  (Q2.CHILD = Q4.PARENT) AND
  (Q2.CHILD = Q3.CHILD)
ORDER BY
  Q1.CHILD

关于SQL要怎么优化这一方面还有很多不足。。。