南大通用GBase 8a OLAP分析函数NTH_VALUE的使用样例

GBase 8a的NTH_VALUE返回当前窗口的指定n行的值，对比first_value则取的是第一个值。本文介绍NTH_VALUE的语法和使用样例。

语法

NTH_VALUE(field,num) OVER([PARTITION BY col_name1,col_name2,…] ORDER BY col_name1 [asc/desc], col_name2 [asc/desc],…)

说明

field为指定的列，与first_value一样

num是截至窗口当前行，指定第几行。如果当前行小于指定num行，则返回null. 该参数必须大于0。

样例

数据

gbase> create table td(id int, type int,val decimal(20,3));
Query OK, 0 rows affected (Elapsed: 00:00:00.17)

gbase> insert into td values(1,1,111),(1,1,222),(2,1,333),(2,2,444),(3,2,555),(3,2,666),(4,2,777);
Query OK, 7 rows affected (Elapsed: 00:00:00.12)
Records: 7  Duplicates: 0  Warnings: 0

gbase> insert into td values(5,1,null),(50,2,null);
Query OK, 2 rows affected (Elapsed: 00:00:00.09)
Records: 2  Duplicates: 0  Warnings: 0

gbase> select * from td;
+------+------+---------+
gbase> select * from td order by type;
+------+------+---------+
| id   | type | val     |
+------+------+---------+
|    1 |    1 | 222.000 |
|    2 |    1 | 333.000 |
|    5 |    1 |    NULL |
|    1 |    1 | 111.000 |
|    3 |    2 | 666.000 |
|    4 |    2 | 777.000 |
|    3 |    2 | 555.000 |
|    2 |    2 | 444.000 |
|   50 |    2 |    NULL |
+------+------+---------+
9 rows in set (Elapsed: 00:00:00.04)

正常NTH_VALUE样例

num为1时，与first_value完全相同，返回第一行。

num为2，则第一行数据找不到第二行，所以返回null

num为3时，则第一行和第二行都找不到第三行，都返回null

gbase> select *,nth_value(val,1)over(partition by type order by id) maxOfType from td;
+------+------+---------+-----------+
| id   | type | val     | maxOfType |
+------+------+---------+-----------+
|    1 |    1 | 111.000 |   111.000 |
|    1 |    1 | 222.000 |   111.000 |
|    2 |    1 | 333.000 |   111.000 |
|    5 |    1 |    NULL |   111.000 |
|    2 |    2 | 444.000 |   444.000 |
|    3 |    2 | 555.000 |   444.000 |
|    3 |    2 | 666.000 |   444.000 |
|    4 |    2 | 777.000 |   444.000 |
|   50 |    2 |    NULL |   444.000 |
+------+------+---------+-----------+
9 rows in set (Elapsed: 00:00:00.12)

gbase> select *,nth_value(val,2)over(partition by type order by id) maxOfType from td;
+------+------+---------+-----------+
| id   | type | val     | maxOfType |
+------+------+---------+-----------+
|    1 |    1 | 111.000 |      NULL |
|    1 |    1 | 222.000 |   222.000 |
|    2 |    1 | 333.000 |   222.000 |
|    5 |    1 |    NULL |   222.000 |
|    2 |    2 | 444.000 |      NULL |
|    3 |    2 | 555.000 |   555.000 |
|    3 |    2 | 666.000 |   555.000 |
|    4 |    2 | 777.000 |   555.000 |
|   50 |    2 |    NULL |   555.000 |
+------+------+---------+-----------+
9 rows in set (Elapsed: 00:00:00.07)

gbase> select *,nth_value(val,3)over(partition by type order by id) maxOfType from td;
+------+------+---------+-----------+
| id   | type | val     | maxOfType |
+------+------+---------+-----------+
|    1 |    1 | 111.000 |      NULL |
|    1 |    1 | 222.000 |      NULL |
|    2 |    1 | 333.000 |   333.000 |
|    5 |    1 |    NULL |   333.000 |
|    2 |    2 | 444.000 |      NULL |
|    3 |    2 | 555.000 |      NULL |
|    3 |    2 | 666.000 |   666.000 |
|    4 |    2 | 777.000 |   666.000 |
|   50 |    2 |    NULL |   666.000 |
+------+------+---------+-----------+
9 rows in set (Elapsed: 00:00:00.12)

其它例子

gbase> select *,nth_value(val,3)over(partition by type) maxOfType from td;
+------+------+---------+-----------+
| id   | type | val     | maxOfType |
+------+------+---------+-----------+
|    1 |    1 | 222.000 |      NULL |
|    2 |    1 | 333.000 |      NULL |
|    5 |    1 |    NULL |      NULL |
|    1 |    1 | 111.000 |      NULL |
|    3 |    2 | 666.000 |   555.000 |
|    4 |    2 | 777.000 |   555.000 |
|    3 |    2 | 555.000 |   555.000 |
|    2 |    2 | 444.000 |   555.000 |
|   50 |    2 |    NULL |   555.000 |
+------+------+---------+-----------+
9 rows in set (Elapsed: 00:00:00.06)

gbase> select *,nth_value(val,3)over(order by id) maxOfType from td;
+------+------+---------+-----------+
| id   | type | val     | maxOfType |
+------+------+---------+-----------+
|    1 |    1 | 111.000 |      NULL |
|    1 |    1 | 222.000 |      NULL |
|    2 |    1 | 333.000 |   333.000 |
|    2 |    2 | 444.000 |   333.000 |
|    3 |    2 | 555.000 |   333.000 |
|    3 |    2 | 666.000 |   333.000 |
|    4 |    2 | 777.000 |   333.000 |
|    5 |    1 |    NULL |   333.000 |
|   50 |    2 |    NULL |   333.000 |
+------+------+---------+-----------+
9 rows in set (Elapsed: 00:00:00.33)