SELECT only first and last results - mysql

Question

A client can have more than one equipment (SerialNo). Each equipment has a cost and every month there is data recorded for each equipment. I'm trying to select only the first and last result for each equipment based on the queried period.
"
SELECT i.SerialNo
, p.Name
, c.Cost
, ci.DataDate
, ci.Data
,
FROM install i
JOIN product p USING (ProductId)
JOIN counter c USING (InstallId)
JOIN counter_item ci USING (CounterId)
WHERE i.ClientId LIKE $clientId
AND MONTH(ci.DataDate) BETWEEN $mStart AND $mEnd
";
This select works but it retrieves all records between the starting date and finishing date.
I tried, to get the top results and figured I would use A UNION ALL to combine with the bottom results (ci.DataDate ASC), but it's not working. I only get the first record encounter.
GROUP BY i.SerialNo
ORDER BY ci.DataDate DESC
It's like ORDER BY has no effect at all.

Answer 1

In counter_item you find the first and last DataDate per CounterId for the time range. So find these first by aggregation and use this information in order to join the desired records:
SELECT i.SerialNo,
p.Name,
c.Cost,
ci.DataDate,
ci.Data
FROM install i
JOIN product p ON p.ProductId = i.ProductId
JOIN counter c ON c.InstallId = i.InstallId
JOIN
(
SELECT CounterId, MIN(DataDate) AS MinDate, MAX(DataDate) AS MaxDate
FROM counter_item
WHERE MONTH(DataDate) BETWEEN $mStart AND $mEnd
GROUP BY CounterId
) minmax ON minmax.CounterId = c.CounterId
JOIN counter_item ci ON ci.CounterId = minmax.CounterId
AND ci.DataDate IN (minmax.MinDate, minmax.MaxDate)
WHERE i.ClientId LIKE $clientId
ORDER BY i.SerialNo, ci.DataDate

Answer 2

You could do it in next way, here is just general idea of how that could be done:
select * from table
where
([row] = (select max([row]) from table ) or
[Date] = (select min([row]) from table ))

Answer 3

You may also be able to use a cross apply. Something like this, untested rough sample:
SELECT i.SerialNo,
p.Name,
c.Cost,
MIN(ci.DataDate) as MinDate,
b.MaxDate,
ci.Data,
FROM install i
CROSS APPLY (SELECT
MAX(ci.DataDate) as MaxDate
FROM install
JOIN counter_item ci USING (CounterId)
WHERE i.ClientId LIKE $clientId
AND MONTH(ci.DataDate) BETWEEN $mStart AND $mEnd) b
WHERE i.ClientId LIKE $clientId
AND MONTH(ci.DataDate) BETWEEN $mStart AND $mEnd
GROUP BY i.SerialNo
ORDER BY ci.DataDate DESC