SQL Query giving an error of Unknown Column

Question

I am creating an application in PHP of Power Meter Analysis. I have following table structure:

table: 'feeds'

| feed_id | device_no | current1 | voltage1 | power_factor_1 | vc1 | ic1 | date_added
-------------------------------------------------------------------------------------
| 36752   | 2         | 36.048   | 196.01   | 0.9            | 1   | 1   | 2014-06-23 14:14:44
| 36753   | 2         | 35.963   | 195.59   | 0.9            | 1   | 1   | 2014-06-23 14:15:34

and so on.

table: 'machine'

| machine_id | machine_phone | machine_name | company_id |
----------------------------------------------------------
| 1          | 2             | ABC Machine  | 1          |
| 2          | 093           | DEF Machine  | 1          |

I need records on hourly basis and I have written the following query for this purpose:

$sql =  "
            SELECT
                SUM(t.power1) AS 'power1'
                , HOUR(t.date) AS 'pulse_hour' 

            FROM    (
                SELECT
                    IF(@diff = 0, 0, (((f.voltage1*f.vc1)*(f.current1*f.ic1)*(f.power_factor_1))/1000) * (@diff/3600)) AS 'power1'
                    , IF(@diff = 0,0, TIME_TO_SEC(f.date_added) - @diff) AS 'deltaT'
                    , @diff := TIME_TO_SEC(f.date_added)
                    , f.date_added AS 'date'
                FROM
                    feeds f,
                    (SELECT @diff := 0) AS X
                left join
                    machine m
                on
                    f.device_no = m.machine_phone
                left join
                    company c
                on
                    c.company_id = m.company_id
            ";

    $sql .= $params['machine_id'] ? " where f.device_no = '".$params['machine_id']."'" : " where f.device_no > 0";
    $sql .= $params['machine_pulse_datetime_from'] ? " and f.date_added >= '".$params['machine_pulse_datetime_from']."'" : "";
    $sql .= $params['machine_pulse_datetime_to'] ? " and f.date_added <= '".$params['machine_pulse_datetime_to']."'" : "";
    $sql .= $params['company_id'] ? " and c.company_id = '".$params['company_id']."'" : "";

    $sql .= "
                ORDER BY
                    f.date_added ASC
                ) t
            GROUP BY HOUR(t.date)
            ORDER BY HOUR(t.date) ASC

            ";

The query is running OK if I remove the following part from the query:

left join
                    machine m
                on
                    f.device_no = m.machine_phone
                left join
                    company c
                on
                    c.company_id = m.company_id

But with this part It is giving me following error:

Error Code : 1054
Unknown column 'f.device_no' in 'on clause'

can you please help me to sort this out... I have spent an hour with this query :(

Answer 1

This is your join :

         FROM feeds f,
              (SELECT @diff := 0) AS X left join
              machine m
              on f.device_no = m.machine_phone left join
              company c
              on c.company_id = m.company_id

The problem is that you are mixing explicit and implicit joins. You can fix this by replacing the comma with cross join :

        FROM feeds f cross join
              (SELECT @diff := 0) AS X left join
              machine m
              on f.device_no = m.machine_phone left join
              company c
              on c.company_id = m.company_id

The issue, which is buried deep in the documentation for select , is that , is a lot like a cross join with the exception of scoping rules -- that is, when the table aliases are recognized. With a comma, the table aliases are not recognized as you expect. They are with a cross join .

Here is the reference:

INNER [CROSS] JOIN and , (comma) are semantically equivalent in the absence of a join condition: both produce a Cartesian product between the specified tables (that is, each and every row in the first table is joined to each and every row in the second table).

However, the precedence of the comma operator is less than of INNER JOIN , CROSS JOIN , LEFT JOIN , and so on. If you mix comma joins with the other join types when there is a join condition, an error of the form Unknown column 'col_name' in 'on clause' may occur. Information about dealing with this problem is given later in this section.