sql - 更新空值

Question

这个问题反映了我的问题。如何在 SQLite 中做到这一点？

我尝试过UPDATE使用自联接，在子查询、触发器和类似的东西中隔离自联接。这是一个例子：

UPDATE stage 
SET title = 
(
  SELECT 
    prior.title
  FROM 
    stage prior,
    stage now
  WHERE 
    prior.rownum+1 = now.rownum
)
WHERE
title is null

Answer 1

SQLite 中的每个表都有一个名为rowid的伪列（可以在几个不同的名称下访问它：rowid, oid, _rowid_，除非这些名称分配给其他真实的列）。rowid 列本质上是一个唯一的行标识符，您可以将其用作排序标准和/或条件。

以下查询演示了如何在rowid的帮助下解决您的问题：

UPDATE stage
SET title = (
  SELECT title
  FROM stage AS prev
  WHERE title IS NOT NULL AND prev.rowid < stage.rowid
  ORDER BY prev.rowid DESC
  LIMIT 1
)
WHERE title IS NULL

这是关于 SQL Fiddle 的演示。

您可以在此 SQLite 手册中阅读有关rowid的更多信息。

Answer 2

我针对您提到的问题提出了一个解决方案（我在 SQL2008、SQLite3 和 Oracle11g 上测试成功）。我在下面复制了该解决方案：

CREATE TABLE test(mysequence INT, mynumber INT);
INSERT INTO test VALUES(1, 3);
INSERT INTO test VALUES(2, NULL);
INSERT INTO test VALUES(3, 5);
INSERT INTO test VALUES(4, NULL);
INSERT INTO test VALUES(5, NULL);
INSERT INTO test VALUES(6, 2);
SELECT t1.mysequence, t1.mynumber AS ORIGINAL
, (
    SELECT t2.mynumber
    FROM test t2
    WHERE t2.mysequence = (
        SELECT MAX(t3.mysequence)
        FROM test t3
        WHERE t3.mysequence <= t1.mysequence
        AND mynumber IS NOT NULL
       )
) AS CALCULATED
FROM test t1;

-- below here it was only tested in SQLite3, but I believe it should
-- work on other DBMS since it uses standard/non-proprietary SQL
UPDATE test
SET mynumber = (
    SELECT t2.mynumber
    FROM test t2
    WHERE t2.mysequence = (
        SELECT MAX(t3.mysequence)
        FROM test t3
        WHERE t3.mysequence <= test.mysequence
        AND mynumber IS NOT NULL
       )
);