使用 sqlite db 创建处理原始表的视图答案

【问题标题】：Create a view processing the original table using sqlite db使用 sqlite db 创建处理原始表的视图
【发布时间】：2014-11-18 07:46:03
【问题描述】：

我正在开发一个 C# Winforms 应用程序并尝试使用以下表结构在 sqlite 中创建一个视图：

这就是视图的样子：

这是 sql 命令，但不会产生所希望的视图：

SELECT tbl1.Time AS C1,
(SELECT MIN(Value) FROM tbl1 AS T WHERE Time = T.Time AND T.VarId = 2) AS C2, 
(SELECT MIN(Value) FROM tbl1 AS T WHERE Time = T.Time AND T.VarId = 3) AS C3,
(SELECT MIN(Value) FROM tbl1 AS T WHERE Time = T.Time AND T.VarId = 4) AS C4, 
(SELECT MIN(Value) FROM tbl1 AS T WHERE Time = T.Time AND T.VarId = 5) AS C5,
(SELECT MIN(Value) FROM tbl1 AS T WHERE Time = T.Time AND T.VarId = 6) AS C6, 
(SELECT MIN(Value) FROM tbl1 AS T WHERE Time = T.Time AND T.VarId = 7) AS C7
FROM tbl1
WHERE tbl1.VarId IN (1, 2, 3, 4, 5, 6, 7)
GROUP BY tbl1.Time
ORDER BY tbl1.Time

非常感谢任何帮助。

【问题讨论】：

标签： c# sql sqlite

【解决方案1】：

在子查询中，没有表名的列名（如Time）指的是最内层表，在本例中为T。这意味着Time = T.Time 被解释为T.Time = T.Time。

对于关联子查询，必须比较内表的一列 (T.Time) 和外表的一列 (tbl1.Time)：

SELECT tbl1.Time AS Time,
(SELECT MIN(Value) FROM tbl1 AS T WHERE T.Time = tbl1.Time AND T.VarId = 1) AS C1,
(SELECT MIN(Value) FROM tbl1 AS T WHERE T.Time = tbl1.Time AND T.VarId = 2) AS C2,
(SELECT MIN(Value) FROM tbl1 AS T WHERE T.Time = tbl1.Time AND T.VarId = 3) AS C3,
(SELECT MIN(Value) FROM tbl1 AS T WHERE T.Time = tbl1.Time AND T.VarId = 4) AS C4,
(SELECT MIN(Value) FROM tbl1 AS T WHERE T.Time = tbl1.Time AND T.VarId = 5) AS C5,
(SELECT MIN(Value) FROM tbl1 AS T WHERE T.Time = tbl1.Time AND T.VarId = 6) AS C6,
(SELECT MIN(Value) FROM tbl1 AS T WHERE T.Time = tbl1.Time AND T.VarId = 7) AS C7,
(SELECT MIN(Value) FROM tbl1 AS T WHERE T.Time = tbl1.Time AND T.VarId = 8) AS C8,
(SELECT MIN(Value) FROM tbl1 AS T WHERE T.Time = tbl1.Time AND T.VarId = 9) AS C9,
tbl1.Method AS Method    -- ???
FROM tbl1
GROUP BY tbl1.Time
ORDER BY tbl1.Time

【讨论】：

但是，当我在原始表中使用它而不是在上面的演示表中时，查询需要一段时间才能执行。
优化查询是一个不同的问题。（但您可能只需要 Time 列上的索引。）
嗨 CL 我必须修改视图，新要求是从另一个表中获取 varId 和 C1、C2...。有什么提示吗？
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