SQL Query to get oldest date - sql

Question

I'm new to SQL and have a large database that contains IDs and Service Dates and I need to write a query to give me the first date each ID had a service.
I tried:
SELECT dbo.table.ID, dbo.otherTable.ServiceDate AS EasliestDate
FROM dbo.table INNER JOIN dbo.table.ID = dbo.otherTable.ID
But the output is every service for every ID, which has too many results to sort through. I want the output to only show the ID and the oldest service date. Any advice is appreciated.
EDIT: To be more precise, the output I am looking for is the ID and service date if the oldest service date is during the year that I specify. I.E. if ID = 1 has a service in 2015 and 2016 and I am searching for IDs in 2016 then ID = 1 should not appear in the results because there was an earlier service in 2015.
EDIT: Thanks everyone who helped with this! The answer I accepted did exactly what I asked. Major kudos to Patty though who who elaborated on how to further filter the outcome by year.

Answer 1

Use GROUP BY and MIN to get the first date for each ID:
SELECT dbo.table.ID,
MIN(dbo.otherTable.ServiceDate) AS EasliestDate
FROM dbo.table
INNER JOIN otherTable
ON dbo.table.ID = dbo.otherTable.ID
GROUP BY dbo.table.ID;
ADDENDUM
In reference to a question in the comments:
how would I also restrict it to show only those who had a service in a specific year?
It would depend on your exact requirements, consider the following set:
ID ServiceDate
--------------------
1 2014-05-01
1 2015-08-01
1 2016-07-07
2 2015-08-19
You would only want to include ID = 1 if the year you specified was 2016, but assuming you still wanted to return the first date of 2014-05-01 then you would need to add a having clause with a case statement to get this.
DECLARE #Year INT = 2016;
DECLARE #YearStart DATE = DATEADD(YEAR, #Year - 1900, '19000101'),
#YearEnd DATE = DATEADD(YEAR, #Year - 1900 + 1, '19000101');
SELECT #YearStart, #YearEnd
SELECT t.ID,
MIN(o.ServiceDate) AS EasliestDate
FROM dbo.table AS t
INNER JOIN otherTable AS o
ON o.ID = r.ID
GROUP BY t.ID
HAVING COUNT(CASE WHEN o.ServiceDate >= #YearStart
AND o.ServiceDate < #YearEnd THEN 1 END) > 0;
If you only want the earliest date in 2016 the a where clause would suffice
DECLARE #Year INT = 2016;
DECLARE #YearStart DATE = DATEADD(YEAR, #Year - 1900, '19000101'),
#YearEnd DATE = DATEADD(YEAR, #Year - 1900 + 1, '19000101');
SELECT #YearStart, #YearEnd
SELECT t.ID,
MIN(o.ServiceDate) AS EasliestDate
FROM dbo.table AS t
INNER JOIN otherTable AS o
ON o.ID = r.ID
WHERE o.ServiceDate >= #YearStart
AND o.ServiceDate < #YearEnd
GROUP BY t.ID;
It is worth noting there is a very good reason I have chosen to calculate the start of the year, and the start of the next year and used
WHERE o.ServiceDate >= #YearStart
AND o.ServiceDate < #YearEnd
Instead of just
WHERE DATEPART(YEAR, o.ServiceDate) = 2016;
In the former, an index on ServiceDate can be used whereas in the latter, the DATEPART calculation must be done on every record and this can cause significant performace issues.
ADDENDUM 2
To do the following:
The exact thing I want then would be IDs who's earliest service is in the year I specify.
Then you would need a having clause, just a different one to the one I posted before:
DECLARE #Year INT = 2016;
DECLARE #YearStart DATE = DATEADD(YEAR, #Year - 1900, '19000101'),
#YearEnd DATE = DATEADD(YEAR, #Year - 1900 + 1, '19000101');
SELECT #YearStart, #YearEnd
SELECT t.ID,
MIN(o.ServiceDate) AS EasliestDate
FROM dbo.table AS t
INNER JOIN otherTable AS o
ON o.ID = r.ID
GROUP BY t.ID
HAVING MIN(o.ServiceDate) >= #YearStart
AND MIN(o.ServiceDate) < #YearEnd;
ADDENDUM 3
CREATE VIEW dbo.YourView
AS
SELECT dbo.table.ID,
MIN(dbo.otherTable.ServiceDate) AS EasliestDate
FROM dbo.table
INNER JOIN otherTable
ON dbo.table.ID = dbo.otherTable.ID
GROUP BY dbo.table.ID;
Then you can apply your criteria to the view:
SELECT *
FROM dbo.YourView
WHERE EasliestDate >= '2015-01-01'
AND EasliestDate < '2016-01-01';

Answer 2

You have to include a WHERE in your current query:
SELECT dbo.table.ID, dbo.otherTable.ServiceDate AS EasliestDate
FROM dbo.table INNER JOIN dbo.table.ID = dbo.otherTable.ID
WHERE Month(dbo.otherTable.ServiceDate) = 1
Or you can search with Year(dbo.otherTable.ServiceDate) = 2016
Or you can use Day(dbo.otherTable.ServiceDate) = 1
Or an specific date.

Answer 3

use group by and min to get records. Else you can refer http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/ for better understanding.

Answer 4

You need to use a "Group by" statement. Try this:
SELECT dbo.table.ID, Max(dbo.otherTable.ServiceDate) AS LatestDate, Min(dbo.otherTable.ServiceDate as EarliestDate)
FROM dbo.table INNER JOIN dbo.table.ID = dbo.otherTable.ID
group by dbo.table.ID

Answer 5

Use nested statement to get the min date , and then just match based on ID.
select t1.ID from table1 t1 INNER JOIN
(
SELECT ID, MIN(servicedate) MinServiceDate
FROM table2
GROUP BY ID
) t2 ON t1.ID = t2.ID