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2.4.2 [/b]Partition Outer Joins [/b]
Partition outer
joins 是Oracle10g中令人感兴趣的的特性。它可以将数据分成组，句体我也说不清，看了下面的例子你就明白了：
(表employee_expanse)
SELECT * FROM employee_expense;
EMP_ID
YEAR MONTH EXPENSE_CLAIM
APPROVED_AMT PAID_DATE
---------- ---------- ----------
------------- ------------ ---------

7369
2002 2 3072.43 3072.43 03-MAR-02
7369
2002 4 30 30 01-JUN-02
7369
2002 5 235.03 35.03 01-JUN-02
7369
2002 9 5095.98 5095.08 31-OCT-02
7369
2002 12 1001.01 1001.01 01-FEB-03
7782
2002 1 111.09 111.09 01-FEB-02
7782
2002 3 9.85 9.85 01-APR-02
7782
2002 7 3987.32 3987.32 01-AUG-02
7782
2002 9 1200 1200 01-OCT-02

（表months）
SELECT * FROM months WHERE year = 2002;

YEAR MONTH

---------- ----------

2002 1

2002 2

2002 3

2002 4

2002 5

2002 6

2002 7

2002 8

2002 9

2002 10

2002 11 2002
12

你可以用month表中的反回的12行，做一个left outer join 把某个员工(employee)每月的expense显示出来：

SELECT NVL(ee.emp_id, 7782), m.year,
m.month, NVL(ee.expense_claim,0)

FROM (SELECT * FROM months WHERE year =
2002) m

LEFT OUTER JOIN (SELECT *

FROM employee_expense

WHERE emp_id = 7782) ee

ON m.year = ee.year AND m.month = ee.month

ORDER BY m.month;
（NVL（value,0）该函数的意思是当value的值为空时，以0代替）

NVL(EE.EMP_ID,7782) YEAR
MONTH NVL(EE.EXPENSE_CLAIM,0)

------------------- ---------- ----------
-----------------------

7782 2002 1 111.09

7782
2002 2 0

7782 2002 3 9.85

7782 2002 4 0

7782 2002 5 0

7782 2002 6 0

7782 2002 7 3987.32

7782 2002 8 0

7782 2002 9 1200

7782 2002 10 0

7782 2002 11 0

我们可以看到emp_id 为 7782的员工在2002今天各个月份的EXPENSE_CLAIM 被显示出来了，那么如果我人要显示每个员工的在2002今天各个月份的EXPENSE_CLAIM么怎么办，那么Partition Outer Joins 就起作用了：

SELECT ee.emp_id, m.year, m.month,
NVL(ee.expense_claim,0)

FROM (SELECT * FROM months WHERE year =
2002) m

LEFT OUTER JOIN employee_expense ee

PARTITION BY (ee.emp_id)

ON m.year = ee.year AND m.month = ee.month

ORDER BY ee.emp_id, m.month;

EMP_ID YEAR MONTH NVL(EE.EXPENSE_CLAIM,0)

---------- ---------- ----------
-----------------------

7369 2002 1 0

7369 2002 2 3072.43

7369 2002 3 0

7369 2002 4 30

7369 2002 5 235.03

7369 2002 6 0

7369 2002
7 0

7369 2002 8 0

7369 2002 9 5095.98

7369 2002 10 0

7369 2002 11 0

7369 2002 12 1001.01

7782 2002 1 111.09

7782 2002 2 0

7782 2002 3 9.85

7782
2002 4 0

7782 2002 5 0

7782 2002 6 0

7782 2002 7 3987.32

7782 2002 8 0

7782 2002 9 1200

7782 2002 10 0

7782 2002 11 0

7782 2002 12 0

上面那种写法我们也可以写成，下面这样（把分别每个员工在2002年各月份EXPENSE_CLAIM取出，再连接起来）：

SELECT NVL(ee.emp_id, 7369), m.year,
m.month, NVL(ee.expense_claim,0)

FROM (SELECT * FROM months WHERE year =
2002) m

LEFT OUTER JOIN (SELECT *

FROM employee_expense

WHERE emp_id = 7369) ee

ON m.year = ee.year AND m.month = ee.month

ORDER BY m.month

UNION ALL

SELECT NVL(ee.emp_id, 7782), m.year,
m.month, NVL(ee.expense_claim,0)

FROM (SELECT * FROM months WHERE year =
2002) m

LEFT
OUTER JOIN (SELECT *

FROM employee_expense

WHERE emp_id = 7782) ee

ON m.year = ee.year AND m.month = ee.month

ORDER BY m.month;本文出自 “感性V理性” 博客，请务必保留此出处http://longsoft.blog.51cto.com/378540/82040