c# print barcode labels Orders with the Maximum OrderDate for Each Employee in .NET

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Table 4-4. Orders with the Maximum OrderDate for Each Employee
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OrderID CustomerID EmployeeID OrderDate RequiredDate
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The output contains one example of multiple orders for an employee, in the case of employee 2. If
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you want to return only one row for each employee, you have to introduce a tiebreaker. For example, out of the multiple rows with the maximum OrderDate , return the one with the maximum OrderID . This can be achieved by adding another subquery that keeps the order only if OrderID is equal to the maximum among the orders with the same EmployeeID and OrderDate as in the outer row: SELECT OrderID, CustomerID, EmployeeID, OrderDate, RequiredDate FROM dbo.Orders AS O1 WHERE OrderDate = (SELECT MAX(OrderDate) FROM dbo.Orders AS O2 WHERE O2.EmployeeID = O1.EmployeeID) AND OrderID = (SELECT MAX(OrderID) FROM dbo.Orders AS O2 WHERE O2.EmployeeID = O1.EmployeeID AND O2.OrderDate = O1.OrderDate);
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Notice in the output shown in Table 4-5 , that of the two orders for employee 2 in the previous query's output, only the one with the maximum OrderID remains.
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11077 RATTC 1 1998-05-06 1998-06-03 11073 PERIC 2 1998-05-05 1998-06-02 11063 HUNGO 3 1998-04-30 1998-05-28 11076 BONAP 4
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1998-05-06 1998-06-03 11043 SPECD 5 1998-04-22 1998-05-20 11045 BOTTM 6 1998-04-23 1998-05-21 11074 SIMOB 7 1998-05-06 1998-06-03 11075 RICSU 8 1998-05-06 1998-06-03 11058 BLAUS 9 1998-04-29 1998-05-27
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Table 4-5. Most Recent Order for Each Employee; Tiebreaker: MAX(OrderID)
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Instead of using two separate subqueries for the sort column (OrderDate ) and the tiebreaker (OrderID ), you can use nested subqueries:
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OrderID
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CustomerID
EmployeeID
OrderDate
RequiredDate
Instead of using two separate subqueries for the sort column (OrderDate ) and the tiebreaker (OrderID ), you can use nested subqueries: SELECT OrderID, CustomerID, EmployeeID, OrderDate, RequiredDate FROM dbo.Orders AS O1 WHERE OrderID = (SELECT MAX(OrderID) FROM dbo.Orders AS O2 WHERE O2.EmployeeID = O1.EmployeeID AND O2.OrderDate = (SELECT MAX(OrderDate) FROM dbo.Orders AS O3 WHERE O3.EmployeeID = O1.EmployeeID));
I compared the performance of the two and found it very similar, with a slight advantage to the latter. I find the nested approach more complex, so as long as there's no compelling performance benefit, I'd rather stick to the simpler approach. Simpler is easier to understand and maintain, and therefore less prone to errors. Going back to the simpler approach, for each tiebreaker attribute you have, you need to add a subquery. Each such subquery must be correlated by the group column, sort column, and all preceding tiebreaker attributes. So, to use MAX(RequiredDate), MAX(OrderID) as the tiebreaker, you would write the following query: SELECT OrderID, CustomerID, EmployeeID, OrderDate, RequiredDate FROM dbo.Orders AS O1 WHERE OrderDate = (SELECT MAX(OrderDate) FROM dbo.Orders AS O2 WHERE O2.EmployeeID = O1.EmployeeID) AND RequiredDate = (SELECT MAX(RequiredDate) FROM dbo.Orders AS O2 WHERE O2.EmployeeID = O1.EmployeeID AND O2.OrderDate = O1.OrderDate) AND OrderID = (SELECT MAX(OrderID) FROM dbo.Orders AS O2 WHERE O2.EmployeeID = O1.EmployeeID AND O2.OrderDate = O1.OrderDate AND O2.RequiredDate = O1.RequiredDate);
The indexing guideline for the tiebreaker queries above is to create an index on (group_cols, sort_cols, tiebreaker_cols ). For example, when the tiebreaker is MAX(OrderID) , you want an index on (EmployeeID, OrderDate, OrderID ). When the tiebreaker is MAX(RequiredDate), MAX(OrderID) , you want an index on (EmployeeID, OrderDate, RequiredDate, OrderID ). Such an index would allow retrieving the relevant sort value or tiebreaker value for an employee using a seek operation within the index. When you're done testing the tiebreaker solutions, run the following code to drop the indexes that were created just for these examples:
DROP INDEX dbo.Orders.idx_eid_od_oid; DROP INDEX dbo.Orders.idx_eid_od_rd_oid;
I presented here only one approach to solving tiebreaker problems using ANSI correlated subqueries. This approach is neither the most efficient nor the simplest. You will find other solutions to tiebreaker problems in 6 in the "Tiebreakers " section, and in 7 in the "TOP n for Each Group " section.
EXISTS
EXISTS is a powerful predicate that allows you to efficiently check whether or not any rows result from a given query. The input to EXISTS is a subquery, which is typically but not necessarily correlated, and the predicate returns TRUE or FALSE, depending on whether the subquery returns at least one row or none. Unlike other predicates and logical expressions, EXISTS cannot return UNKNOWN. Either the input subquery returns rows or it doesn't. If the subquery's filter returns UNKNOWN for a certain row, the row is not returned. Remember that in a filter, UNKNOWN is treated like FALSE. In other words, when the input subquery has a filter, EXISTS will return TRUE only if the filter is TRUE for at least one row. The reason I'm stressing this subtle point will become apparent shortly. First, let's look at an example that will demonstrate the use of EXISTS. The following query returns all customers from Spain that made orders, generating the output shown in Table 4-6 : SELECT CustomerID, CompanyName FROM dbo.Customers AS C WHERE Country = N'Spain' AND EXISTS (SELECT * FROM Orders AS O WHERE O.CustomerID = C.CustomerID);
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