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Table 4-13. Comparing Current Year to Previous Year's Number of Customers
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OrderYear CurNumCusts PrvNumCusts Growth 1996 1997 1998 67 86 81 NULL 67 86 NULL 19 5
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Common Table Expressions (CTE)
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A common table expression (CTE) is a new type of table expression introduced in SQL Server 2005. As for the standard, CTEs were introduced in the ANSI SQL:1999 specification. In many aspects, you will find CTEs very similar to derived tables. However, CTEs have several important advantages, which I'll describe in this section. Remember that a derived table appears in its entirety in the FROM clause of an outer query. A CTE, however, is defined first using a WITH statement, and an outer query referring to the CTE's name follows the CTE's definition: WITH cte_name AS ( cte_query ) outer_query_referring to_cte_name;
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Because the WITH keyword is used in T-SQL for other purposes as well, to avoid ambiguity, the statement preceding the CTE's WITH clause must be terminated with a semicolon. The use of a semicolon to terminate statements is supported by ANSI. It's a good practice, and you should start getting used to it even where T-SQL currently doesn't require it.
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A CTE's scope of existence is the outer query's scope. It's not visible to other statements in the same batch. The same rules I mentioned for the validity of a derived table's query expression apply to the CTE's as well. That is, the query must generate a valid table, so all columns must have names, all column names must be unique, and ORDER BY is not allowed (unless TOP is also specified). Next, I'll go over aspects of CTEs, demonstrating their syntax and capabilities, and compare them to derived tables.
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Result Column Aliases
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Just as you can with derived tables, you can provide aliases to result columns either inline in the CTE's query or externally in parentheses following the CTE's name. The following code illustrates the first method: WITH C AS ( SELECT YEAR(OrderDate) AS OrderYear, CustomerID FROM dbo.Orders
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) SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts FROM C GROUP BY OrderYear;
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The next bit of code illustrates how to provide aliases externally in parentheses following the CTE's name: WITH C(OrderYear, CustomerID) AS ( SELECT YEAR(OrderDate), CustomerID FROM dbo.Orders ) SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts FROM C GROUP BY OrderYear;
Using Arguments
Another similarity between CTEs and derived tables is that CTEs can refer to variables declared in the same batch: DECLARE @EmpID AS INT; SET @EmpID = 3; WITH C AS ( SELECT YEAR(OrderDate) AS OrderYear, CustomerID FROM dbo.Orders WHERE EmployeeID = @EmpID ) SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts FROM C GROUP BY OrderYear;
Multiple CTEs
Unlike derived tables, CTEs cannot be nested directly. That is, you cannot define a CTE within another CTE. However, you can define multiple CTEs using the same WITH statement, each of which can refer to the preceding CTEs. The outer query has access to all the CTEs. Using this capability, you can achieve the same result you would by nesting derived tables. For example, the following WITH statement defines two CTEs: WITH C1 AS ( SELECT YEAR(OrderDate) AS OrderYear, CustomerID FROM dbo.Orders ), C2 AS ( SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts FROM C1 GROUP BY OrderYear
) SELECT OrderYear, NumCusts FROM C2 WHERE NumCusts > 70;
C1 returns order years and customer IDs for each order, generating the OrderYear alias for the order year. C2 groups the rows returned from C1 by OrderYear and calculates the count of distinct CustomerIDs (number of active customers). Finally, the outer query returns only order years with more than 70 active customers.
Multiple References
One of the advantages CTEs have over derived tables is that you can refer to the same CTE name multiple times in the outer query. You don't need to repeat the same CTE definition like you do with derived tables. For example, the following code demonstrates a CTE solution for the request to compare each year's number of active customers to the previous year's number: WITH YearlyCount AS ( SELECT YEAR(OrderDate) AS OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts FROM dbo.Orders GROUP BY YEAR(OrderDate) ) SELECT Cur.OrderYear, Cur.NumCusts AS CurNumCusts, Prv.NumCusts AS PrvNumCusts, Cur.NumCusts - Prv.NumCusts AS Growth FROM YearlyCount AS Cur LEFT OUTER JOIN YearlyCount AS Prv ON Cur.OrderYear = Prv.OrderYear + 1;
You can see that the outer query refers to the YearlyCount CTE twiceonce representing the current year (Cur), and once representing the previous year (Prv).
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