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Listing 1-5. OVER clause in logical query processing
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(8) (1) (3) (2) (4) (5) (6) (7) (10) SELECT (9) DISTINCT (11) TOP <select_list> FROM <left_table> <join_type> JOIN <right_table> ON <join_condition> WHERE <where_condition> GROUP BY <group_by_list> WITH {CUBE | ROLLUP} HAVING <having_condition> ORDER BY <order_by_list>
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You specify the OVER clause following the function to which it applies in either the select_list or the order_by_list. Even though I didn't really explain in detail how the OVER clause works, I'd like to demonstrate its use in both phases where it's applicable. In the following example, an OVER clause is used with the COUNT aggregate function in the SELECT list; the output of this query is shown in Table 1-22: SELECT orderid, customerid, COUNT(*) OVER(PARTITION BY customerid) AS num_orders FROM dbo.Orders WHERE customerid IS NOT NULL AND orderid % 2 = 1;
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Table 1-22. OVER Clause Applied in SELECT Phase
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orderid customerid num_orders 1 3 5 FRNDO KRLOS KRLOS 1 2 2
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The PARTITION BY clause defines the window for the calculation. The COUNT(*) function counts the number of rows in the virtual table provided to the SELECT phase as input, where the customerid is equal to the one in the current row. Remember that the virtual table provided to the SELECT phase as input has already undergone WHERE filteringthat is, NULL customer IDs and even order IDs have been eliminated. You can also use the OVER clause in the ORDER BY list. For example, the following query sorts the rows according to the total number of output rows for the customer (in descending order), and generates the output shown in Table 1-23: SELECT orderid, customerid FROM dbo.Orders WHERE customerid IS NOT NULL AND orderid % 2 = 1 ORDER BY COUNT(*) OVER(PARTITION BY customerid) DESC;
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Table 1-23. OVER Clause Applied in ORDER BY Phase
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orderid customerid 3 5 1 KRLOS KRLOS FRNDO
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For details on using the OVER clause with aggregate functions, please refer to 6. For details on using the OVER clause with analytical ranking functions, please refer to 4.
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Set Operations
SQL Server 2005 supports three set operations: UNION, EXCEPT, and INTERSECT. Only UNION is available in SQL Server 2000. These SQL operators correspond to operators defined in mathematical set theory. This is the syntax for a query applying a set operation: [(]left_query[)] {UNION [ALL] | EXCEPT | INTERSECT} [(]right_query[)] [ORDER BY <order_by_list>]
Set operations compare complete rows between the two inputs. UNION returns one result set with the rows from both inputs. If the ALL option is not specified, UNION removes duplicate rows from the result set. EXCEPT returns distinct rows that appear in the left input but not in the right. INTERSECT returns the distinct rows that appear in both inputs. There's much more to say about these set operations, but here I'd just like to focus on the logical processing steps involved in a set operation. An ORDER BY clause is not allowed in the individual queries. You are allowed to specify an ORDER BY clause at the end of the query, but it will apply to the result of the set operation. In terms of logical processing, each input query is first processed separately with all its relevant phases. The set operation is then applied, and if an ORDER BY clause is specified, it is applied to the result set. Take the following query, which generates the output shown in Table 1-24, as an example: SELECT 'O' AS letter, customerid, orderid FROM dbo.Orders WHERE customerid LIKE '%O%' UNION ALL SELECT 'S' AS letter, customerid, orderid FROM dbo.Orders WHERE customerid LIKE '%S%' ORDER BY letter, customerid, orderid;
Table 1-24. Result of a UNION ALL Set Operation
letter customerid orderid O O O O O S S S S FRNDO FRNDO KRLOS KRLOS KRLOS KRLOS KRLOS KRLOS MRPHS 1 2 3 4 5 3 4 5 6
First, each input query is processed separately following all the relevant logical processing phases. The first query returns a table with orders placed by customers containing the letter O. The second query returns a table with orders placed by customers containing the letter S. The set operation UNION ALL combines the two sets into one. Finally, the ORDER BY clause sorts the rows by letter, customerid, and orderid. As another example for logical processing phases of a set operation, the following query returns customers that have made no orders:
SELECT customerid FROM dbo.Customers EXCEPT SELECT customerid FROM dbo.Orders;
The first query returns the set of customer IDs from Customers ({FISSA, FRNDO, KRLOS, MRPHS}), and the second query returns the set of customer IDs from Orders ({FRNDO, FRNDO, KRLOS, KRLOS, KRLOS, MRPHS, NULL}). The set operation returns ({FISSA}), the set of rows from the first set that do not appear in the second set. Finally, the set operation removes duplicates from the result set. In this case, there are no duplicates to remove. The result set's column names are determined by the set operation's left input. Columns in corresponding positions must match in their datatypes or be implicitly convertible. Finally, an interesting aspect of set operations is that they treat NULLs as equal.
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