I have an MS Access table with employee information and contract periods. Each contract period has a unique ID,...
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but may of course have the same employee number with different contract start date and contract end date. How can I use this table to find the last contract period for each employee? You can assume that one contract doesn't start until the previous one ends.
Assuming your table looks like this --
cID cStart cEnd cEmpl 122 2002-01-01 2002-03-31 john 123 2002-01-01 2002-02-28 fred 124 2002-01-01 2002-09-30 bill 125 2002-04-01 2002-04-30 fred 126 2002-05-01 2002-05-31 fred 127 2002-04-01 2002-06-30 john 128 2002-11-01 2002-12-31 bill
The query must find the latest contract for each employee, and since one contract doesn't start until the previous one ends, we can use MAX(cStart). Using the start date rather than end date is preferable, since this allows ongoing contracts to have a null end date, although this might not apply in your situation.
However, that's not enough to give the entire contract period. You need both the start date and the end date. But how can you get the end date when selecting MAX(cStart)? The answer is to use a correlated subquery:
select cID, cStart, cEnd, cEmpl from yourtable XX where cStart = ( select max(cStart) from yourtable where cEmpl = XX.cEmpl )
Here, the correlation is on the employee column. The outer query's WHERE clause accepts only those rows where cStart is the highest for that employee.
There's another way to do this, using a derived table in a join; see Rows having maximum group value in MySQL. Note that in MySQl, you have to do it in two steps using a temp table.
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