c# print barcode zebra Natural Join x1 x2 x3 y1 y2 y3 y1 y2 y3 y4 z1 z2 z3 z4 x1 x2 x3 y1 y2 y3 z1 z2 z3 in C#

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3 Natural Join x1 x2 x3 y1 y2 y3 y1 y2 y3 y4 z1 z2 z3 z4 x1 x2 x3 y1 y2 y3 z1 z2 z3
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The Relational Model
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FIGURE 3-4 The Natural Join operator
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Probably the most poorly understood relational operator is the Divide operator. A divisor relation is used to partition a dividend relation and produce a quotient relation. The quotient relation is made up of those values of one column from the dividend table for which the second column contains all of the values in the divisor. Although this is a theoretical chapter, I am going to use code to explain the Divide operator and a problem you can meet if you divide with an empty set, a zero divide problem. I ll use an example that you saw in 2 and that you ll see again in 6. The problem, which refers to the InsideTSQL2008 database, asks you to return all customers for whom every employee from the USA has handled at least one order. In this case, you divide the set of all orders by the set of all employees from the USA, and you expect the set of matching customers back. T-SQL has no Divide operator. To show the problem, I ll rephrase the problem as it appears in 6:
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Return customers for whom you cannot find any employee from the USA for whom you cannot find any order placed for the subject customer and by the subject employee
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The query for this problem is quite intuitive:
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USE InsideTSQL2008; SELECT custid FROM Sales.Customers AS C WHERE NOT EXISTS (SELECT * FROM HR.Employees AS E WHERE country = N'USA' AND NOT EXISTS (SELECT * FROM Sales.Orders AS O WHERE O.custid = C.custid AND O.empid = E.empid));
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This query returns 23 rows, which means there are 23 customers for whom every employee from the USA has handled at least one order. Let s ask the same question with a different
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Inside Microsoft SQL Server 2008: T-SQL Querying
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country: How many customers are there for whom every employee from Israel has handled at least one order Here is the same query with one changed parameter:
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SELECT custid FROM Sales.Customers AS C WHERE NOT EXISTS (SELECT * FROM HR.Employees AS E WHERE country = N'IL' AND NOT EXISTS (SELECT * FROM Sales.Orders AS O WHERE O.custid = C.custid AND O.empid = E.empid));
This query returns 91 rows, representing all customers. This might not be the result you expected, given that there are no employees from Israel in the HR.Employees table. This is the way the Divide operator was de ned originally. Because the HR.Employees table has no employee from Israel, the condition that a customer was served by all employees from Israel is true for every customer (it is vacuously true). In other words, every customer was served by every employee from Israel. However, something else is also true: every customer was served by no employees from Israel. Note that there is no preferred truth here; the one you take depends on the problem you are solving. Do we have something like Russell s Paradox here (which you remember from 2) Not really. The problem is that we did not think through the possibility of having no employees from Israel. If the original question s customers . . . for whom . . . at least one order was intended to mean there were in fact some orders, we can answer the question by simply adding a condition to the predicate requiring to return customers served by all employees from Israel if there is at least one employee from Israel:
Return customers for whom you cannot find any employee from Israel for whom you cannot find any order placed for the subject customer and by the subject employee if there is at least one employee from Israel
The query now looks like this:
SELECT custid FROM Sales.Customers AS C WHERE NOT EXISTS (SELECT * FROM HR.Employees AS E WHERE country = N'IL' AND NOT EXISTS (SELECT * FROM Sales.Orders AS O WHERE O.custid = C.custid AND O.empid = E.empid)) AND EXISTS (SELECT * FROM HR.Employees AS E WHERE country = N'IL');
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