MySQL 离开加入子查询分组依据

Question

我有一个要求，我需要将数据分组为相等数量的 ob 行。由于 mysql 没有 rownum() 我正在模拟这种行为:

SET @row:=6; 
SELECT MAX(agg.timestamp) AS timestamp, MAX(agg.value) AS value, COUNT(agg.value) AS count 
FROM 
    (
        SELECT timestamp, value, @row:=@row+1 AS row 
        FROM data 
        WHERE channel_id=52 AND timestamp >= 0 ORDER BY timestamp
    ) AS agg 
GROUP BY row div 8
ORDER BY timestamp ASC;

注:根据Can grouped expressions be used with variable assignments?此查询可能不是 100% 正确，但它确实有效。

另一个要求是计算分组集之间的行差。我一直在寻找将同一个表与子查询连接起来的解决方案:

SET @row:=6; 
SELECT MAX(agg.timestamp) AS timestamp, MAX(agg.value) AS value, COUNT(agg.value) AS count 
FROM 
    (
        SELECT timestamp, value, @row:=@row+1 AS row 
        FROM data 
        WHERE channel_id=52 AND timestamp >= 0 ORDER BY timestamp
    ) AS agg 
LEFT JOIN data AS prev
    ON prev.channel_id = agg.channel_id
    AND prev.timestamp = (
        SELECT MAX(timestamp) 
        FROM data
        WHERE data.channel_id = agg.channel_id
            AND data.timestamp < MIN(agg.timestamp)
    )
GROUP BY row div 8
ORDER BY timestamp ASC;

不幸的是错误:

Error Code: 1054. Unknown column 'agg.channel_id' in 'on clause'

知道如何编写这个查询吗？

Answer 1

您从未从 sbuquery 中选择 channel_id，因此它不会返回到父查询，因此是不可见的。尝试

SELECT MAX(agg.timestamp) AS timestamp, MAX(agg.value) AS value, COUNT(agg.value) AS count 
FROM 
    (
        SELECT timestamp, value, @row:=@row+1 AS row, channel_id
                                                    ^^^^^^^^^^^^-- need this
        FROM data 

由于 MySQL 只看到并使用您从该子查询显式返回的字段，并且不会“深入”查询底层的表，因此您需要选择/返回您将使用父查询的所有字段.