用于将列值设置为与下一行的列值相同的 SQL [英] SQL for setting column value same as next row's column value

问题描述

我有一张这样的桌子

--------------------------
id    some_column
--------------------------
1      null
2      abc
3      null
4      null
5      def
6      ghi

在我的场景中，表中的一些初始行在 some_column 中为空.所有最新的行都正确设置了 some_column 值.

In my scenario, some initial rows in the table have null in some_column. All the latest rows have some_column value properly set.

我想运行一个查询，它可以通过从下一个非空值复制值来替换 some_column 的所有空值.

I want to run a query which can replace all the null values of some_column by copying value from next not null value.

比如运行查询后，表会变成

For example, after running the query, table will become

--------------------------
id    some_column
--------------------------
1      abc
2      abc
3      def
4      def
5      def
6      ghi

我正在尝试这样的事情

update table as t1 set some_column = (select some_column from table as t2 where t2.id > t1.id and t2.some_column is not null limit 1) where some_column is null

但出现错误

You can't specify target table 't1' for update in FROM clause

推荐答案

你可以用 JOIN 来做到:

UPDATE t AS t0
JOIN (
    SELECT t1.id, MIN(t2.id) new_id
    FROM t AS t1
    JOIN t AS t2 ON t1.id < t2.id
    WHERE t1.some_column IS NULL
    AND t2.some_column IS NOT NULL
    GROUP BY t1.id) AS tnew ON t0.id = tnew.id
JOIN t AS t3 ON t3.id = tnew.new_id
SET t0.some_column = t3.some_column

tnew 子查询将每个具有空值的 ID 与具有非空值的次高 ID 进行匹配.然后我将它与原始表连接两次，以配对要更新的行具有替换值的行中的值.

The tnew subquery matches up each ID with a null value with the next highest ID with a non-null value. I then join this twice with the original table, to pair up the row to be updated with the value from the row with the replacement value.

演示

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