The Language of SQL- P33 ppsx

In this format, the IN operator merely lists a number of values in parentheses. There is also a second format for the IN, one in which an entire SELECT state- ment is inserted inside the parentheses. For example, a list of states might be specified as: WHERE State IN (SELECT States FROM StateTable WHERE Region ¼ 'Midwest') Rather than list individual states, the second format allows us to generate a list of states through more complex logic. Let’s illustrate with an example that uses our Customers and Orders tables. Let’s say we want to retrieve a list of customers who have ever paid cash for any order they’ve placed. A SELECT that accomplishes this is: SELECT CustomerName AS 'Custom er Name' FROM Customers WHERE CustomerID IN (SELECT CustomerID FROM Orders WHERE OrderType ¼ 'Cash') The resulting data is: Customer Name William Smith Natalie Lopez Brenda Harper is not included in the list because she has never placed an order using cash. Notice that a subquery SELECT is placed entirely within the p arentheses for the IN keyword. Also note that the Custome rID column is used to connect the two queries. Even though we are displaying CustomerName, we use CustomerID to define the relationship between the Customers and Orders tables. Once again, we can ask whether this subquery can also be expressed as a normal query, and the answer is yes. Here is an equivalent query that returns the same data: Chapter 14 ■ Subqueries146 SELECT CustomerName AS 'Customer Name' FROM Customers INNER JOIN Orders ON Customers.CustomerID ¼ Orders.CustomerID WHERE OrderType ¼ 'Cash' GROUP BY Customers.CustomerID, Customers.CustomerName Without using a subquery, we can directly join the Customers and Orders table. However, a GROUP BY clause is now needed to ensure that we only bring back one row per customer. Correlated Subqueries The subqueries we’ve seen so far have been uncorrelated subqueries. Generally speaking, all subqueries can be classified as either uncorrelated or correlated. These terms describe whether the subquery is related to the query in which it is contained. Uncorrelated subqueries are unrelated. When a subquery is unrelated, that means that it is completely independent of the outer query. Uncorrelated queries are evaluated and executed only once as part of the entire SELECT statement. In contrast, correlated subqueries are specifically related to the outer query. Because of this explicit relationship, correlated subqueries need to be evaluated for each row returned and can produce different results each time the subquery is executed. The best way to explain is with an example. Returning to the Customers and Orders tables, let’s say we want to produce a list of customers who have a total order amount that is less than 20 dollars. Here’s a statem ent that accomplishes that request: SELECT CustomerName as 'Customer Name' FROM Customers WHERE (SELECT SUM (OrderAmount) FROM Orders WHERE Customers.CustomerID ¼ Orders.CustomerID) <20 Correlated Subqueries 147 The result is: Customer Name Brenda Harper What makes this subquery correlated, as opposed to uncorrelated? The answer can be seen by looking at the subquery itself: SELECT SUM (OrderAmount) FROM Orders WHERE Customers.CustomerID ¼ Orders.CustomerID This subquery is correlated because it cannot be executed on its own. If run by itself, this subquery would error because the Customers.CustomerID column doesn’t exist within the context of the subquery. To understand what’s going on, it’s helpful to look at the entire SELECT state- ment in a general way: SELECT CustomerName as 'Customer Name' FROM Customers WHERE SubqueryResult <20 As a correlated subquery, the subquery needs to be evaluated for each customer. Also note that this type of subquery only works if it returns a single value. Again, one might ask if this subquery could be converted into a normal SELECT statement. In this case, it can. Here’s an equivalent statement that produces the same results: SELECT CustomerName as 'Customer Name' FROM Customers LEFT JOIN Orders ON Customers.CustomerID ¼ Orders.CustomerID GROUP BY Customers.CustomerID, CustomerName HAVING SUM (OrderAmount) < 20 Notice that, without a subquery, the equivalent statement now requires GROUP BY and HAVING clauses. The GROUP BY clause creates groups of Chapter 14 ■ Subqueries148 customers, and the HAVING clause enforces the requirement that each group must have ordered less than 20 dollars. The EXISTS Operator One additional technique associated with correlated subqueries is the use of a special operator called EXISTS. This operator allows you to determine if data in a correlated subquery exists. Let’s say that you want to discover which customers have placed orders. One way of accomplishing that is with this statement: SELECT CustomerName AS 'Customer Name' FROM Customers WHERE EXISTS (SELECT * FROM Orders WHERE Customers.CustomerID ¼ Orders.CustomerID) This statement returns: Customer Name William Smith Natalie Lopez Brenda Harper Adam Petrie doesn’t appear in the results since he has not placed any orders. The EXISTS keyword in the above statement is evaluated as true if the SELECT in the correlated subquery returns any data. Notice that the subquery selects all columns ( SELECT *). Since it doesn’t really matter what particular data is selected in the subquery, we use the asterisk to return all data. We’re only inter- ested in determining whether any data exists in the subquery. As before, the logic in this statement can be expressed in other ways. Here’s a statement that obtains the same results using a subquery with the IN operator: SELECT CustomerName AS 'Customer Name' FROM Customers WHERE CustomerID IN (SELECT CustomerID FROM Orders) The EXISTS Operator 149 Here’s another statement that retrieves the same data without a subquery: SELECT CustomerName AS 'Customer Name' FROM Customers INNER JOIN Orders ON Customers.CustomerID ¼ Orders.CustomerID GROUP BY CustomerName Using a Subquery as a Calculated Column The final general use of subqueries is as a calculated column. Let’s say we would like to see a list of customers, along with a count of the number of orders they have placed. This query can actually be accomplished without subqueries with this statement: SELECT CustomerName AS 'Customer Name', COUNT (OrderID) AS 'Number of Orders' FROM Customers LEFT JOIN Orders ON Customers.CustomerID ¼ Orders.CustomerID GROUP BY Customers.CustomerID, CustomerName ORDER BY Customers.CustomerID The output is: Customer Name Number of Orders William Smith 1 Natalie Lopez 3 Brenda Harper 2 Adam Petrie 0 However, another way of obtaining the same result is to use a subquery as a calculated column. This looks like the following: SELECT CustomerName AS 'Customer Name', (SELECT COUNT (OrderID) FROM Orders Chapter 14 ■ Subqueries150 . format, the IN operator merely lists a number of values in parentheses. There is also a second format for the IN, one in which an entire SELECT state- ment is inserted inside the parentheses Calculated Column The final general use of subqueries is as a calculated column. Let’s say we would like to see a list of customers, along with a count of the number of orders they have placed. This. appear in the results since he has not placed any orders. The EXISTS keyword in the above statement is evaluated as true if the SELECT in the correlated subquery returns any data. Notice that the subquery