asp.net c# qr code generator Inside Microsoft SQL Server 2008: T-SQL Querying in C#

Make QR Code in C# Inside Microsoft SQL Server 2008: T-SQL Querying

Inside Microsoft SQL Server 2008: T-SQL Querying
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the order only if orderid is equal to the maximum among the orders with the same empid and orderdate as in the outer row:
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SELECT orderid, custid, empid, orderdate, requireddate FROM Sales.Orders AS O1 WHERE orderdate = (SELECT MAX(orderdate) FROM Sales.Orders AS O2 WHERE O2.empid = O1.empid) AND orderid = (SELECT MAX(orderid) FROM Sales.Orders AS O2 WHERE O2.empid = O1.empid AND O2.orderdate = O1.orderdate);
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Of the two orders for employee 2, only the one with the maximum orderid remains, as the following output shows:
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orderid ----------11077 11073 11063 11076 11043 11045 11074 11075 11058 custid ----------65 58 37 9 74 10 73 68 6 empid ----------1 2 3 4 5 6 7 8 9 orderdate ----------------------2008-05-06 00:00:00.000 2008-05-05 00:00:00.000 2008-04-30 00:00:00.000 2008-05-06 00:00:00.000 2008-04-22 00:00:00.000 2008-04-23 00:00:00.000 2008-05-06 00:00:00.000 2008-05-06 00:00:00.000 2008-04-29 00:00:00.000 requireddate ----------------------2008-06-03 00:00:00.000 2008-06-02 00:00:00.000 2008-05-28 00:00:00.000 2008-06-03 00:00:00.000 2008-05-20 00:00:00.000 2008-05-21 00:00:00.000 2008-06-03 00:00:00.000 2008-06-03 00:00:00.000 2008-05-27 00:00:00.000
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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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SELECT orderid, custid, empid, orderdate, requireddate FROM Sales.Orders AS O1 WHERE orderid = (SELECT MAX(orderid) FROM Sales.Orders AS O2 WHERE O2.empid = O1.empid AND O2.orderdate = (SELECT MAX(orderdate) FROM Sales.Orders AS O3 WHERE O3.empid = O1.empid));
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I compared the performance of the two and found it very similar. I nd the nested approach more complex, so as long as there s no compelling performance bene t, 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,
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Subqueries, Table Expressions, and Ranking Functions
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and all preceding tiebreaker attributes. So, to use MAX(requireddate), MAX(orderid) as the tiebreaker, you would write the following query:
SELECT orderid, custid, empid, orderdate, requireddate FROM Sales.Orders AS O1 WHERE orderdate = (SELECT MAX(orderdate) FROM Sales.Orders AS O2 WHERE O2.empid = O1.empid) AND requireddate = (SELECT MAX(requireddate) FROM Sales.Orders AS O2 WHERE O2.empid = O1.empid AND O2.orderdate = O1.orderdate) AND orderid = (SELECT MAX(orderid) FROM Sales.Orders AS O2 WHERE O2.empid = O1.empid AND O2.orderdate = O1.orderdate AND O2.requireddate = O1.requireddate);
The indexing guideline for the preceding tiebreaker queries 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 (empid, orderdate, orderid). When the tiebreaker is MAX(requireddate), MAX(orderid), you want an index on (empid, 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 Sales.Orders.idx_eid_od_oid; DROP INDEX Sales.Orders.idx_eid_od_rd_oid;
I presented here only one approach using ANSI-correlated subqueries to solving the problem of isolating one row per group using a tiebreaker. This approach is neither the most ef cient nor the simplest. You will nd other solutions to tiebreaker problems in 8, Aggregating and Pivoting Data, in the Tiebreakers section, and in 9, TOP and APPLY, in the TOP n for Each Group section.
EXISTS
EXISTS is a powerful predicate that allows you to ef ciently check whether 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 lter returns UNKNOWN for a certain row, the row is not returned. Remember that in a lter, UNKNOWN is treated like FALSE.
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