DOUBLE gets returned as BLOB

Question

I am having some confusion with my SELECT:

SELECT  
    mergedData.rawValue,
    mergedData.validatedValue,
    IFNULL(mergedData.validatedValue, mergedData.rawValue) as mergedValue
FROM
    [...]

While mergedData.rawValue and mergedData.validatedValue return as DOUBLE, the IFNULL statement returns a BLOB. Why is that happening? And how can I make it return a DOUBLE instead?

EDIT: I am merging two tables that have exactly the same structure in my Select by emulating a FULL JOIN. The data is stored as a DOUBLE in both tables:

TableRawData (NOX gets loaded into mergedData.rawValue):

timed (BIGINT(20)) --- NOX (DOUBLE)
1380583800000          8.972167

TableValidatedData (NOX gets loaded into mergedData.validatedValue):

timed (BIGINT(20)) --- NOX (DOUBLE)
1380583800000          9.506166

EDIT2: My full SELECT:

SET @StartTime  = 1356998400000;
SET @EndTime    = 1386546000000;

SELECT  
    mergedData.rawValue,
    mergedData.validatedValue,
    CAST(IFNULL(mergedData.validatedValue, mergedData.rawValue) AS DECIMAL(10, 5)) as mergedValue
FROM
 (SELECT 
        Month(FROM_UNIXTIME(timed / 1000)) as months,
            Year(FROM_UNIXTIME(timed / 1000)) as years,
            DAYOFMONTH(FROM_UNIXTIME(timed / 1000)) as days,
            HOUR(FROM_UNIXTIME(timed / 1000)) as hours,
            MINUTE(FROM_UNIXTIME(timed / 1000)) as minutes,
            IF(true, avg(NOX), null) as rawValue,
            FROM_UNIXTIME(timed / 1000) as timeValue,
            IF(true, 1, 0) as IsValid,
            NULL as validatedValue
    FROM
        nabelnrt_bas
    WHERE
        timed > @StartTime
            AND timed < @EndTime
    GROUP BY concat(years, months, days) UNION (
 SELECT 
        Month(FROM_UNIXTIME(timed / 1000)) as months,
            Year(FROM_UNIXTIME(timed / 1000)) as years,
            DAYOFMONTH(FROM_UNIXTIME(timed / 1000)) as days,
            HOUR(FROM_UNIXTIME(timed / 1000)) as hours,
            MINUTE(FROM_UNIXTIME(timed / 1000)) as minutes,
            IF(true, avg(NOX), null) as validatedValue,
            FROM_UNIXTIME(timed / 1000) as timeValue,
            IF(true, 1, 0) as IsValid,
            NULL as rawValue
    FROM
        nabelvalidated_bas
    WHERE
        timed > @StartTime
            AND timed < @EndTime
    GROUP BY concat(years, months, days))) as mergedData

Answer 1

Just cast it

SELECT  
    mergedData.rawValue,
    mergedData.validatedValue,
    CAST(IFNULL(mergedData.validatedValue, mergedData.rawValue) AS DECIMAL[(M[,D])]) as mergedValue
FROM

Please note the DECIMAL[(M[,D])] you should fill it as your table is defined.

Edit, COALESCE might also be interesting. Example usage;

SELECT  
    mergedData.rawValue,
    mergedData.validatedValue,
    COALESCE(mergedData.validatedValue, mergedData.rawValue) as mergedValue
FROM

Perheps you would not need to cast it then