Question

如果以下SELECT返回0或没有行，我只想插入或更新一行。

SELECT (value = ? AND status = ? AND connected = ?)
FROM channels, data
WHERE data.channel_id = channels.channel_id AND channels.channel_name = ? AND sample_time < ?
ORDER BY sample_time DESC
LIMIT 1

基本上，它是一个只写更改的数据存档程序。也就是说，如果数据与前一个sample_time所写的数据不同，它只会为给定的sample_time写入数据。此SELECT获取前一个sample_time的给定通道的数据，并将其与当前sample_time的数据进行比较。因此，如果返回0，即数据不同，则应该继续写入。如果还没有为此通道写入数据，那么它应该不返回任何行，并且应该写入新数据。以下是我编写数据的查询：

INSERT INTO data (acquire_time, sample_time, channel_id, value, status, connected)
SELECT ?, ?, channels.channel_id, ?, ?, ?
FROM channels
WHERE channel_name = ?
ON DUPLICATE KEY UPDATE acquire_time = ?, value = ?, status = ?, connected = ?

当前sample_time的新数据可能会使用ON DUPLICATE KEY UPDATE覆盖当前sample_time的先前数据，而不是与前一个sample_time存储的数据相同。

重复键是channel_id和sample_time的组合。 channel_id和acquire_time上还有一个唯一索引。

感谢您的时间。

Answer 1

在您做出以下澄清之后，我相信这会做您想做的事情：

INSERT INTO data (acquire_time, sample_time, channel_id, value, status, connected)
SELECT ?, ?, channels.channel_id, ?, ?, ?
FROM channels
WHERE channel_name = ?
AND NOT EXISTS (
    SELECT 1
    FROM (
        SELECT value, status, connected
        FROM channels, data
        WHERE data.channel_id = channels.channel_id AND channels.channel_name = ? 
        AND sample_time < ?
        ORDER BY sample_time DESC 
        LIMIT 1 
    ) a
    WHERE a.value = ? and a.status = ? and a.connected = ? 
) 
ON DUPLICATE KEY UPDATE acquire_time = ?, value = ?, status = ?, connected = ?;

mysql如果选择然后插入

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