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LEFT ON LEFT ON OUTER JOIN Production.Products AS P P.productid = OD.productid OUTER JOIN Production.Suppliers AS S S.supplierid = P.supplierid;
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Joins and Set Operations
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The left outer joins keep the outer customers in the intermediate virtual tables. This query correctly produces 1,238 rows, including the two customers that made no orders. However, if you had orders with no related order details, order details with no related products, or products with no related suppliers, this query would have produced incorrect results. That is, you would have received result rows that were unmatched by several join conditions when you wanted only the unmatched rows from the rst join condition. Also, remember that the optimizer cannot apply join ordering optimization with outer joins those have to be processed in speci ed order, so this technique might hurt optimization. Another option is to make sure the join with the Customers table is logically last. This can be achieved by using inner joins between all other tables and nally a right outer join with Customers:
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SELECT DISTINCT C.companyname AS customer, S.companyname AS supplier FROM Sales.Orders AS O JOIN Sales.OrderDetails AS OD ON OD.orderid = O.orderid JOIN Production.Products AS P ON P.productid = OD.productid JOIN Production.Suppliers AS S ON S.supplierid = P.supplierid RIGHT OUTER JOIN Sales.Customers AS C ON O.custid = C.custid;
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This scenario was fairly simple, but in cases where you mix different types of joins not to mention other table operators (APPLY, PIVOT, UNPIVOT) it might not be that simple. Furthermore, using left outer joins all along the way is very arti cial. It s more intuitive to think of the query as a single left outer join, where the left table is the Customers table and the right table is the result of inner joins between all the other tables. Both ANSI SQL and T-SQL allow you to control the logical order of join processing:
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SELECT DISTINCT C.companyname AS customer, S.companyname AS supplier FROM Sales.Customers AS C LEFT OUTER JOIN ( Sales.Orders AS O JOIN Sales.OrderDetails AS OD ON OD.orderid = O.orderid JOIN Production.Products AS P ON P.productid = OD.productid JOIN Production.Suppliers AS S ON S.supplierid = P.supplierid) ON O.custid = C.custid;
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Technically, the parentheses are ignored here, but I recommend you use them because they will help you write the query correctly. Using parentheses caused you to change another
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Inside Microsoft SQL Server 2008: T-SQL Querying
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aspect of the query, which is the one that the language really uses to determine the logical order of processing. If you haven t guessed yet, it s the ON clause order. Specifying the ON clause ON O.custid = C.custid last causes the other joins to be logically processed rst; the left outer join occurs logically between Customers and the inner join of the rest of the tables. You could write the query without parentheses, and it would mean the same thing:
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SELECT DISTINCT C.companyname AS customer, S.companyname AS supplier FROM Sales.Customers AS C LEFT OUTER JOIN Sales.Orders AS O JOIN Sales.OrderDetails AS OD ON OD.orderid = O.orderid JOIN Production.Products AS P ON P.productid = OD.productid JOIN Production.Suppliers AS S ON S.supplierid = P.supplierid ON O.custid = C.custid;
Other variations that specify the ON clause that refers to C.custid last include the following two:
SELECT DISTINCT C.companyname AS customer, S.companyname AS supplier FROM Sales.Customers AS C LEFT OUTER JOIN Sales.Orders AS O JOIN Production.Products AS P JOIN Sales.OrderDetails AS OD ON P.productid = OD.productid ON OD.orderid = O.orderid JOIN Production.Suppliers AS S ON S.supplierid = P.supplierid ON O.custid = C.custid; SELECT DISTINCT C.companyname AS customer, S.companyname AS supplier FROM Sales.Customers AS C LEFT OUTER JOIN Sales.Orders AS O JOIN Sales.OrderDetails AS OD JOIN Production.Products AS P JOIN Production.Suppliers AS S ON S.supplierid = P.supplierid ON P.productid = OD.productid ON OD.orderid = O.orderid ON O.custid = C.custid;
The obvious disadvantage to not using parentheses is a decrease in the readability and clarity of code. Without parentheses, the queries are far from intuitive. But we have another issue, too. It s important to note that you cannot play with the ON clause s order any way you d like. There s a certain relationship that must be maintained between the order of the speci ed tables and the order of the speci ed ON clauses for the query to be valid. The relationship is called a chiastic relationship. A chiastic relationship is neither unique to SQL nor unique to computer science; rather, it appears in many elds, including poetry, linguistics, mathematics, and others. In an ordered series of items, this relationship correlates the rst item with the last, the second with the next to last, and so on. For example, palindromes such as never odd or
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