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Subqueries can be characterized in two main ways. One is by the expected number of values (either scalar or multivalued), and another is by the subquery's dependency on the outer query (either selfcontained or correlated). Both scalar and multivalued subqueries can be either self-contained or correlated.
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A self-contained subquery is a subquery that can be run independently of the outer query. Selfcontained subqueries are very convenient to debug, of course, compared to correlated subqueries. Scalar subqueries can appear anywhere in the query where an expression resulting in a scalar value is expected, while multivalued subqueries can appear anywhere in the query where a collection of multiple values is expected. A scalar subquery is valid when it returns a single value, and also when it returns no valuesin which case, the value of the subquery is NULL. However, if a scalar subquery returns more than one value, a run-time error will occur. For example, run the following code three times: once as shown, a second time with LIKE N'Kollar' in place of LIKE N'Davolio' , and a third time with LIKE N'D% : SET NOCOUNT ON; USE Northwind; SELECT OrderID FROM dbo.Orders WHERE EmployeeID = (SELECT EmployeeID FROM dbo.Employees -- also try with N'Kollar' and N'D%' in place of N'Davolio' WHERE LastName LIKE N'Davolio');
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With N'Davolio' , the subquery returns a single value (1) and the outer query returns all orders with EmployeeID 1. With N'Kollar' , the subquery returns no values, and is therefore NULL. The outer query obviously doesn't find any orders for which EmployeeID = NULL and therefore returns an empty set. Note that the query doesn't break (fail), as it's a valid query. With N'D%' , the subquery returns two values (1, 9), and because the outer query expects a scalar, it breaks at run time and generates the following error: Msg 512, Level 16, State 1, Line 1 Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.
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Logically, a self-contained subquery can be evaluated just once for the whole outer query. Physically, the optimizer can consider many different ways to achieve the same thing, so you shouldn't think in such strict terms. Now that we've covered the essentials, let's move on to more sophisticated problems involving selfcontained subqueries. I'll start with a problem belonging to a group of problems called relational division. Relational division problems have many nuances and many practical applications. Logically, it's like dividing one set by another, producing a result set. For example, from the Northwind database, return all customers for whom every Northwind employee from the USA has handled at least one order. In this case, you're dividing the set of all orders by the set of all employees from the USA, and you expect the set of matching customers back. Filtering here is not that simple because for each customer you need to inspect multiple rows to figure out whether you have a match. Here I'll show a technique using GROUP BY and DISTINCT COUNT to solve relational division problems. Later in the book, I'll show others as well. If you knew ahead of time the list of all EmployeeID s for USA employees, you could write the following query to solve the problem, generating the output shown in Table 4-1 : SELECT CustomerID FROM dbo.Orders WHERE EmployeeID IN(1, 2, 3, 4, 8) GROUP BY CustomerID HAVING COUNT(DISTINCT EmployeeID) = 5;
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BERGS BONAP ERNSH FOLKO HANAR HILAA HUNGO ISLAT KOENE LAMAI LILAS LINOD LONEP MEREP OLDWO
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QUICK RATTC SAVEA TORTU VAFFE VICTE WARTH WHITC
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Table 4-1. Customers with Orders Handled by All Employees from the USA
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This query finds all orders with one of the five U.S. EmployeeID s, groups those orders by CustomerID , and returns CustomerID s that have (all) five distinct EmployeeID values in their group of orders. To make the solution more dynamic and accommodate lists of EmployeeID s that are unknown ahead of time and also large lists even when known, you can use subqueries instead of literals: SELECT CustomerID FROM dbo.Orders WHERE EmployeeID IN (SELECT EmployeeID FROM dbo.Employees WHERE Country = N'USA') GROUP BY CustomerID HAVING COUNT(DISTINCT EmployeeID) = (SELECT COUNT(*) FROM dbo.Employees WHERE Country = N'USA');
Another problem involving self-contained subqueries is returning all orders placed on the last actual order date of the month. Note that the last actual order date of the month might be different than the last date of the monthfor example, if a company doesn't place orders on weekends. So the last actual order date of the month has to be queried from the data. Here's the solution query producing the output (abbreviated) shown in Table 4-2 : SELECT OrderID, CustomerID, EmployeeID, OrderDate FROM dbo.Orders WHERE OrderDate IN (SELECT MAX(OrderDate) FROM dbo.Orders GROUP BY CONVERT(CHAR(6), OrderDate, 112));
LILAS 1 1997-02-28 10616 GREAL 1 1997-07-31 10916 RANCH 1 1998-02-27 11077 RATTC 1 1998-05-06 10368 ERNSH 2 1996-11-29 10553 WARTH 2 1997-05-30 10583 WARTH 2 1997-06-30 10686 PICCO 2
1997-09-30 10915 TORTU 2 1998-02-27 10989 QUEDE 2 1998-03-31 ...
11075 RICSU 8 1998-05-06 10687 HUNGO 9 1997-09-30
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