更新值而不是 NULL

Question

我正在使用 Oracle SQL。

我有以下 table:

Timestamp             | A   | B
13-11-14 06.49.54.004 | 50  | 70
13-11-14 06.49.54.005 | NULL| 80
13-11-14 06.49.54.006 | NULL| NULL
13-11-14 06.49.54.007 | 40  | 70
13-11-14 06.49.54.008 | 20  | 90
13-11-14 06.49.54.009 | 30  | NULL

如何用每列的最后一个值替换 NULL 值？这是预期的输出 table：

Timestamp             | A   | B
13-11-14 06.49.54.004 | 50  | 70
13-11-14 06.49.54.005 | 50  | 80
13-11-14 06.49.54.006 | 50  | 80
13-11-14 06.49.54.007 | 40  | 70
13-11-14 06.49.54.008 | 20  | 90
13-11-14 06.49.54.009 | 30  | 90

请指教

Answer 1

使用合并仅更新 NULL，使用子选择查找最新值。

update tablename t1 set a = coalesce(a,(select max(a) from tablename
                                        where Timestamp < t1.Timestamp)),
                        b = coalesce(b,(select max(b) from tablename
                                        where Timestamp < t1.Timestamp))
 where a is null or b is null

现已编辑！（Max 可能不是最近的...）

update tablename t1 set a = coalesce(a,(select a from tablename
                                        where Timestamp < t1.Timestamp
                                        order by Timestamp desc
                                        fetch first 1 row only)),
                        b = coalesce(b,(select b from tablename
                                        where Timestamp < t1.Timestamp
                                        order by Timestamp desc
                                        fetch first 1 row only))
 where a is null or b is null

FETCH FIRST 需要较新的 Oracle 版本。

Answer 2

我建议两种方法。首先，您可以很容易地使用 PL/SQL 块来完成此操作，例如：

DECLARE
  v_prev_timestamp TIMESTAMP;
  v_prev_A INT;
  v_prev_B INT;
BEGIN
  FOR x IN (SELECT * FROM table_name) LOOP
    IF (x.A IS NULL) THEN
      UPDATE table_name SET A = v_prev_A where timestamp = v_prev_timestamp;
    END IF;
    ... same for B ...
  END LOOP;
END;

或者，您可以使用 Oracle 的 LAG 函数创建一个视图，用前几行的值覆盖空值。

CREATE VIEW table_name_no_nulls AS
  SELECT timestamp, 
      nvl(A, lag(A) OVER (ORDER BY timestamp)) AS A,
      nvl(B, lag(B) OVER (ORDER BY timestamp)) AS B
    FROM table_name;

在第二个版本中，NVL 在第一个参数中查找空值，如果为空值，则用第二个参数的输入结果（即同一列的滞后值）替换该值。您必须告诉 Oracle table 是如何排序的，以便它知道要使用哪个先前的值。

我建议使用第二个版本，因为您不需要运行任何一次性批处理来更新数据。只需查询一个转换空值的新视图。

Answer 3

有 LEAD() 和 LAG() 分析函数。在您的情况下，您需要 LAG()。它非常易于使用，有很多示例 - 查看文档并搜索 Whosebug...

Answer 4

您可以将 the first_value() analytic function 与 windowing 子句一起使用，使用时间戳列进行排序并忽略空值：

select timestamp, a, b,
  first_value(a) ignore nulls over (order by timestamp desc
    rows between current row and unbounded following) as new_a,
  first_value(b) ignore nulls over (order by timestamp desc
    rows between current row and unbounded following) as new_b
from table_name
order by timestamp;

TIMESTAMP                             A          B      NEW_A      NEW_B
---------------------------- ---------- ---------- ---------- ----------
13-NOV-14 06.49.54.004000000         50         70         50         70 
13-NOV-14 06.49.54.005000000                    80         50         80 
13-NOV-14 06.49.54.006000000                               50         80 
13-NOV-14 06.49.54.007000000         40         70         40         70 
13-NOV-14 06.49.54.008000000         20         90         20         90 
13-NOV-14 06.49.54.009000000         30                    30         90

或者用 last_value() 代替：

select timestamp, a, b,
  last_value(a) ignore nulls over (order by timestamp
    rows between unbounded preceding and current row) as new_a,
  last_value(b) ignore nulls over (order by timestamp
    rows between unbounded preceding and current row) as new_b
from table_name
order by timestamp;

window 包含当前行，因此如果该值不为空，则将使用它；否则它将使用 first/last 非空值（因为 ignore null 子句），因为它以指定的顺序遍历 window。

更新值而不是 NULL

Updating values instead of NULLs

sql

oracle

null