Mysql – SELECTing multiple columns through a subquery

MySQLperformanceselectsubquery

I am trying to SELECT 2 columns from the subquery in the following query, but unable to do so. Tried creating alias table, but still couldn't get them.

SELECT
  DISTINCT petid,
  userid,
  (SELECT MAX(comDate) FROM comments WHERE petid=pet.id) AS lastComDate,
  (SELECT userid FROM comments WHERE petid=pet.id ORDER BY id DESC LIMIT 1) AS lastPosterID
FROM 
  pet LEFT JOIN comments ON pet.id = comments.petid
WHERE 
  userid='ABC'      AND 
  deviceID!='ABC'   AND 
  comDate>=DATE_SUB(CURRENT_TIMESTAMP, INTERVAL 2 MONTH);

Basically, I am trying to get the lastComDate & lastPosterID from the same row – the row which is the latest one in comments for the specific pet. Please suggest how can I get them in an efficient way.

The above query works, but seems overkill as same row is fetched twice. Moreover, the ORDER BY clause is significantly slower than the aggregate function – as I found while profiling query. So, a solution avoiding sorting would be appreciated.

Best Answer

SELECT DISTINCT petid, userid, lastComDate, lastPosterId
FROM 
    pet 
    LEFT JOIN comments ON pet.id = comments.petid 
    LEFT JOIN (
        SELECT MAX(comDate), userid, petid FROM comments GROUP BY userid
    ) a ON a.petid = pet.id
WHERE 
    userid='ABC' 
    AND deviceID!='ABC' 
    AND comDate>=DATE_SUB(CURRENT_TIMESTAMP, INTERVAL 2 MONTH)
;

You can also pull your subquery out into a temp table if performance becomes impacted somewhere down the road.

Related Solutions

Sql-server – SELECT multiple sensor values in one query

First things first, I notice that your 'what I do now' query:

SELECT TOP (1)
    ca.SensorValue,
    ca.Date
FROM sys.partitions AS p
CROSS APPLY
(
    SELECT TOP (1)
        v.Date, 
        v.SensorValue
    FROM SensorValues AS v
    WHERE 
        $PARTITION.SensorValues_Date_PF(v.Date) = p.[partition_number]
        AND v.DeviceId = @fDeviceId
        AND v.SensorId = @fSensorId
        AND v.Date <= @fDate
    ORDER BY 
        v.Date DESC
) AS ca
WHERE 
    p.[partition_number] <= $PARTITION.SensorValues_Date_PF(@fDate)
    AND p.[object_id] = OBJECT_ID(N'dbo.SensorValues', N'U')
    AND p.index_id = 1
ORDER BY
    p.[partition_number] DESC, 
    ca.Date DESC;

...produces an execution plan like this:

Original Plan

This execution plan has an estimated total cost of 0.02 units. Over 50% of this estimated cost is the final Sort, running in Top-N mode. Now estimates are just that, but sorts can be expensive in general, so let's remove it without changing the semantics:

SELECT TOP (1)
    ca.SensorId,
    ca.SensorValue,
    ca.Date
FROM
(
    -- Partition numbers
    SELECT DISTINCT
        partition_number = prv.boundary_id
    FROM
        sys.partition_functions AS pf
    JOIN sys.partition_range_values AS prv ON
        prv.function_id = pf.function_id
    WHERE
        pf.name = N'SensorValues_Date_PF'
        AND prv.boundary_id <= $PARTITION.SensorValues_Date_PF(@fDate)
) AS p
CROSS APPLY
    (
    SELECT TOP (1)
        v.Date,
        v.SensorValue,
        v.SensorId
    FROM dbo.SensorValues AS v
    WHERE
        $PARTITION.SensorValues_Date_PF(v.Date) = p.partition_number
        AND v.DeviceId = @fDeviceId
        AND v.SensorId = @fSensorId
        AND v.Date <= @fDate
    ORDER BY
        v.Date DESC
  ) AS ca
ORDER BY
    p.partition_number DESC,
    ca.Date DESC

Now the execution plan has no blocking operators, and no sorts in particular. The estimated cost of the new query plan below is 0.01 units and the total cost is distributed evenly over the data access methods:

Improved Query Plan

With the improvement in place, all we need to produce a result for each Sensor ID is to make a list of Sensor IDs and APPLY the previous code to each one:

SELECT
    PerSensor.SensorId,
    PerSensor.SensorValue,
    PerSensor.Date
FROM 
(
    -- Sensor ID list
    VALUES 
        (@fSensorId1),
        (@FSensorId2),
        (@FSensorId3)
) AS Sensor (Id)
CROSS APPLY
(
    -- Optimized code applied to each sensor
    SELECT TOP (1)
        ca.SensorId,
        ca.SensorValue,
        ca.Date
    FROM
    (
        -- Partition numbers
        SELECT DISTINCT
            partition_number = prv.boundary_id
        FROM
            sys.partition_functions AS pf
        JOIN sys.partition_range_values AS prv ON
            prv.function_id = pf.function_id
        WHERE
            pf.name = N'SensorValues_Date_PF'
            AND prv.boundary_id <= $PARTITION.SensorValues_Date_PF(@fDate)
    ) AS p
    CROSS APPLY
        (
        SELECT TOP (1)
            v.Date,
            v.SensorValue,
            v.SensorId
        FROM dbo.SensorValues AS v
        WHERE
            $PARTITION.SensorValues_Date_PF(v.Date) = p.partition_number
            AND v.DeviceId = @fDeviceId
            AND v.SensorId = Sensor.Id--@fSensorId1
            AND v.Date <= @fDate
        ORDER BY
            v.Date DESC
      ) AS ca
    ORDER BY
        p.partition_number DESC,
        ca.Date DESC
) AS PerSensor;

The query plan is:

Final Query Plan

Estimated query plan cost for three Sensor IDs is 0.011 - half that of the original single-sensor plan.

Mysql – Selecting minimum value using a subquery

Modifying slightly your second query, will give you both the merchant id and the lowest price (over all products that pass the conditions - I guess that's what you want):

SELECT p.p_m_id, MIN(p_price) AS min_p_price 
FROM tgmp_affiliates ga 
JOIN tgmp_prices p 
    ON ga.a_code = p.p_gtin 
        AND ga.a_code > '' 
JOIN tgmp_merchants m 
    ON m.m_id = p.p_m_id 
WHERE ga.site_id = '34' 
    AND p.site_id = '34' 
    AND ga.a_parent = '25573' 
    AND p.p_type = 'games' 
    AND m.m_hide = 0 
GROUP BY p.p_m_id ;

Then you can join this - as a derived table - to all the tables that you need data from in the results:

SELECT
    m.*, p.*, ga.*                     -- whatever columns you want  
FROM tgmp_affiliates ga 
JOIN tgmp_prices p 
    ON ga.a_code = p.p_gtin 
        AND ga.a_code > '' 
JOIN tgmp_merchants m 
    ON m.m_id = p.p_m_id 
JOIN
      ( SELECT p.p_m_id, MIN(p_price) AS p_price 
        FROM tgmp_affiliates ga 
        JOIN tgmp_prices p 
            ON ga.a_code = p.p_gtin 
                AND ga.a_code > '' 
        JOIN tgmp_merchants m 
            ON m.m_id = p.p_m_id 
        WHERE ga.site_id = '34' 
            AND p.site_id = '34' 
            AND ga.a_parent = '25573' 
            AND p.p_type = 'games' 
            AND m.m_hide = 0 
        GROUP BY p.p_m_id 
      ) AS tmp
    ON  tmp.p_m_id = p.p_m_id 
    AND tmp.p_price = p.p_price
WHERE ga.site_id = '34' 
    AND p.site_id = '34' 
    AND ga.a_parent = '25573' 
    AND p.p_type = 'games' 
ORDER BY p.p_price ;

Best Answer

Related Solutions

Sql-server – SELECT multiple sensor values in one query

Mysql – Selecting minimum value using a subquery

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