Question

有桌子即。 table1，其中包含Time，Col1和Col2字段。我尝试做的是创建一个新表table2，其中包含基于Col3＆amp;的每行的计算Col1值。 SQL Col2中当前行和上一行的table1值。 table2所需的格式如下所示。

table2

   Time             Col1    Col2   Col3
   09:37.43.755       1      0      0.0    #=> col3 row1 start value = 0
   09:37.44.255       5      2      0.3333 #=> (row2.col2-row1.col2)/((row2.col2-row1.col2)+(row2.col1-row1.col1)) => (2-0)/((2-0) + (5-1)) => 2/6 = 0.3333
   09:37.44.755       10     2      0.0    #=> (row3.col2-row2.col2)/((row3.col2-row2.col2)+(row3.col1-row2.col1)) => (2-2)/((2-2) + (10-5)) => 0/5 = 0.0
   09:37.45.255       10     2      0.0    # similar to row2,3 but a div by 0 case => (2-2)/( (2-2) + (10-10)) = 0/0 => this should be mapped to 0 if the numerator is 0.

Answer 1

这里为您提供最佳服务的工具可能是LAG窗口聚合函数。不过，可能有几种不同的方法可以做到这一点。这是一个例子，使用LAG和子查询。窗口化的聚合函数非常有用，所以我要说了解它们！

select * from table1 order by time, col1;

     TIME     | COL1 | COL2
--------------+------+------
 09:37:43.755 |    1 |    0
 09:37:44.255 |    5 |    2
 09:37:44.755 |   10 |    2
 09:37:45.255 |   10 |    2
(4 rows)


select time, col1, col2,
   coalesce ( case
      when (
            col2-lag_col2
         )
         + (col1-lag_col1) = 0 then 0
      else ((col2-lag_col2)*1.0/((col2-lag_col2)+(col1-lag_col1)))::NUMERIC(5,4)
   END , 0) col3
from (
      select time, col1, col2,
         lag(col1) over (order by time, col1) lag_col1,
         lag(col2) over (order by time, col1) lag_col2
      from table1
   )
   foo;

     TIME     | COL1 | COL2 |  COL3
--------------+------+------+--------
 09:37:43.755 |    1 |    0 | 0.0000
 09:37:44.255 |    5 |    2 | 0.3333
 09:37:44.755 |   10 |    2 | 0.0000
 09:37:45.255 |   10 |    2 | 0.0000
(4 rows)

根据当前行和上一行中的值计算当前行col值

1 个答案: