如何在 SQLite 上解决基于游标的 SQL 任务？

Question

我最近参加了 SQL 任务的工作面试，这需要使用游标，但代码必须是为 SQLite doesn't have 游标编写的。那么，我该如何解决呢？任务如下：

Exists a table which has two columns: "b" - the beginning and "e" - the end. 
Each row of this table represents 1-dimentional line - column "b"
contains coordinate of the beginning, column "e" - coordinate of
the end. e.g.:
 b | e
-------
 1 | 5
 2 | 4
 3 | 7
-------
Column "e" always greater than "b" in each row. It is required to calculate 
length of covered surface on coordinate axis. For this example answer 
is 6 = ((5 - 1) + (7 - 5)).

这是我的游标解决方案：

DECLARE @temp_table TABLE(b int, e int);
DECLARE [cursor] CURSOR FOR SELECT b, e FROM [table]
DECLARE @b INT
DECLARE @e INT

OPEN [cursor]
FETCH NEXT FROM [cursor] INTO @b, @e
WHILE @@FETCH_STATUS = 0
BEGIN
    IF (EXISTS(SELECT * FROM @temp_table))
        FETCH NEXT FROM [cursor] INTO @b, @e
    IF (EXISTS(SELECT * FROM @temp_table WHERE b <= @b AND e >= @e)) 
        CONTINUE;
    IF (EXISTS(SELECT * FROM @temp_table WHERE e < @b OR b > @e)
        OR NOT EXISTS(SELECT * FROM @temp_table)) 
    BEGIN
        INSERT INTO @temp_table VALUES(@b, @e)
        CONTINUE
    END
    IF (EXISTS(SELECT * FROM @temp_table WHERE b <= @b AND e <= @e)) 
    BEGIN
        UPDATE @temp_table SET e = @e WHERE b <= @b AND e <= @e 
        CONTINUE;
    END
    IF (EXISTS(SELECT * FROM @temp_table WHERE b >= @b AND e >= @e)) 
    BEGIN
        UPDATE @temp_table SET b = @b WHERE b >= @b AND e >= @e     
        CONTINUE;
    END
END

CLOSE [cursor]
DEALLOCATE [cursor]

SELECT SUM(e - b) FROM @temp_table

如何为 SQLite 编写相同的代码？我问过我的朋友，没人知道。我多次尝试解决这个任务，但都没有成功。想到这个悬而未决的问题让我心烦意乱。请帮帮我！感谢任何帮助。

Answer 1

SQLite 是一个嵌入式 数据库，也就是说，它设计用于某些应用程序内部。您将在代码中处理光标。

总之，你真的不需要迭代算法。

您需要忽略所有被另一条线覆盖的 start/end 坐标。假设没有两条线 start/end 在同一个坐标，这可以用一个简单的 EXISTS 来完成：

SELECT (SELECT SUM(e)
        FROM MyTable
        WHERE NOT EXISTS (SELECT 1
                          FROM MyTable AS T2
                          WHERE T2.e >= MyTable.e
                            AND T2.b <= MyTable.e
                            AND T2.rowid <> MyTable.rowid)
       ) -
       (SELECT SUM(b)
        FROM MyTable
        WHERE NOT EXISTS (SELECT 1
                          FROM MyTable AS T2
                          WHERE T2.e >= MyTable.b
                            AND T2.b <= MyTable.b
                            AND T2.rowid <> MyTable.rowid)
       )

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