oracle分析函数执行顺序

靠山王有些人对oracle分析函数中select over(partition by col1 order by col2) from test order by ...关于partition by 和 组内order by以及最后的order by的执行顺序产生疑惑。

http://www.itpub.net/showthread.php?s=&threadid=732957&perpage=10&pagenumber=1

 

over 中的partition为分组， order by是视窗内排序, 先执行 partition 然后order by 如 partition by col_a order by col_b 的执行排序效果类似于order by col_a, col_b 这样的排序效果，如果再在最后加order by，是在前边分组排序的结果基础上进行排序。

SQL> create table test(id varchar2(20));

Table created

SQL> insert into test values('1');

1 row inserted

SQL> insert into test values('1');

1 row inserted

SQL> insert into test values('8');

1 row inserted

SQL> insert into test values('5');

1 row inserted

SQL> insert into test values('5');

1 row inserted

SQL> commit;

Commit complete

 

SQL> select * from test;

ID

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

1

1

8

5

5

1.按照id排序:

SQL> select row_number() over(order by id),id,rownum from test;

ROW_NUMBER()OVER(ORDERBYID) ID ROWNUM

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

1 1 1

2 1 2

3 5 5

4 5 4

5 8 3

2.组内(没有分组就是所有数据1组)按照id排序,最后order by在组内排序基础上按照rownum排序:

SQL> select row_number() over(order by id),id,rownum from test order by rownum;

ROW_NUMBER()OVER(ORDERBYID) ID ROWNUM

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

1 1 1

2 1 2

5 8 3

4 5 4

3 5 5

3.按照rownum排序:

SQL> select row_number() over(order by rownum),id,rownum from test;

ROW_NUMBER()OVER(ORDERBYROWNUM ID ROWNUM

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

1 1 1

2 1 2

3 8 3

4 5 4

5 5 5

4.按照id分组,组内按照id排序

SQL> select row_number() over(partition by id order by id),id,rownum from test;

ROW_NUMBER()OVER(PARTITIONBYID ID ROWNUM

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

1 1 1

2 1 2

1 5 5

2 5 4

1 8 3

5.按照id分组,组内按照rownum(这个是早已经出来的结构)排序:

SQL> select row_number() over(partition by id order by rownum),id,rownum from test;

ROW_NUMBER()OVER(PARTITIONBYID ID ROWNUM

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

1 1 1

2 1 2

1 5 4

2 5 5

1 8 3

oracle在提取数据库的时候是按over(partition by ... order by ...)这个里边的order by后边的字段的一个个distinct值取出数据的。

SQL> select * from t;

A B C D

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

1 111 G 87

1 111 G 87

1 222 G 85

1 222 G 86

2 111 G 80

2 111 G 80

2 222 G 81

2 222 G 80

8 rows selected

只有partition by a，distinct a有2个值1和2：分2次提取数据

为1的提取一次，4条a值相同，4条平均86.25

为2的提取一次，4条a值相同，4条平均80.25

SQL> select a,b,c,avg(d) over(partition by a ),d from t;

A B C AVG(D)OVER(PARTITIONBYA) D

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

1 111 G 86.25 87

1 111 G 86.25 87

1 222 G 86.25 85

1 222 G 86.25 86

2 111 G 80.25 80

2 111 G 80.25 80

2 222 G 80.25 81

2 222 G 80.25 80

8 rows selected

partition by a，order by b，distinct a，b有4个值：

1---111

1---222

2---111

2---222

分四次提取数据：

1---111：取出2条，a=1的2条取平均87

1---222：取出2条，a=1的4条取平均86.25

2---111：取出2条，a=2的2条取平均80

2---222：取出2条，a=2的4条取平均80.25

SQL> select a,b,c,avg(d) over(partition by a order by b ),d from t;

A B C AVG(D)OVER(PARTITIONBYAORDERBY D

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

1 111 G 87 87

1 111 G 87 87

1 222 G 86.25 85

1 222 G 86.25 86

2 111 G 80 80

2 111 G 80 80

2 222 G 80.25 81

2 222 G 80.25 80

8 rows selected

SQL>