1. 20761A
10: Using Subqueries
Module 10
Using Subqueries

2. Using the EXISTS Predicate with Subqueries
Module Overview
10: Using Subqueries
Using the EXISTS Predicate with Subqueries

3. Lesson 1: Writing Self-Contained Subqueries
10: Using Subqueries
Demonstration: Writing Self-Contained Subqueries
Question
You are troubleshooting a query. The outer query contains an inner query in its WHERE clause. The first inner query also contains a second inner query in its WHERE clause. Both inner queries are self- contained. The complete query returns an error. How should you approach this task?
Answer
Break the complete query into its constituent subqueries. Start with the inner most subquery and test to see if it returns the information you expect. Next, test the first inner query. Finally, test the complete query.

4. Working with Subqueries
20761A
Working with Subqueries
10: Using Subqueries
Subqueries are nested queries: queries within queries
Results of inner query passed to outer query
Inner query acts like an expression from perspective of outer query
Subqueries can be self-contained or correlated
Self-contained subqueries have no dependency on outer query
Correlated subqueries depend on values from outer query
Subqueries can be scalar, multi-valued, or table- valued
Note that self-contained subqueries are easier to test when they are separated from the outer query.

5. Comparing Self-Contained and Correlated Subqueries
10: Using Subqueries
Self-Contained Subquery:
Correlated Subquery:
Outer Query:
SELECT orderid, productid, unitprice, qty
FROM Sales.OrderDetails
WHERE orderid = ( )
Inner Query:
SELECT MAX(orderid) AS lastorder
FROM Sales.Orders
SELECT orderid, empid, orderdate
FROM Sales.Orders AS O1
WHERE
orderdate = ( )
SELECT MAX(orderdate) FROM Sales.Orders AS O2
WHERE O2.empid = O1.empid
This slide illustrates the difference between self-contained and correlated subqueries. Do not go into specifics of the queries shown—you will cover that later in this module. Instead emphasize that, in self- contained subqueries, the inner query does not take information from the outer query. By contrast, in a correlated subquery, the inner query requires information from the outer query.

6. Writing Scalar Subqueries
10: Using Subqueries
Scalar subquery returns single value to outer query
Can be used anywhere single-valued expression is used: SELECT, WHERE, and so on
If inner query returns an empty set, result is converted to NULL
Construction of outer query determines whether inner query must return a single value
The example returns details about the most recent order.
This is the error returned if the inner query is not scalar and the outer is written to expect a single value:
Msg 512, Level 16, State 1, Line 1
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when it is used as an expression.
SELECT orderid, productid, unitprice, qty
FROM Sales.OrderDetails
WHERE orderid =
(SELECT MAX(orderid) AS lastorder
FROM Sales.Orders);

7. Writing Multi-Valued Subqueries
10: Using Subqueries
Multi-valued subquery returns multiple values as a single column set to the outer query
Used with IN predicate
If any value in the subquery result matches IN predicate expression, the predicate returns TRUE
May also be expressed as a JOIN (test both for performance)
The example returns information about orders placed by customers in Mexico. This may be rewritten as a JOIN:
SELECT c.custid, o.orderid
FROM Sales.Customers AS c JOIN Sales.Orders AS o
ON c.custid = o.custid
WHERE c.country = 'Mexico‘;
Point out to the students that sometimes it’s easier to write and test a query one piece at a time, in which case a subquery may be more accessible.
SELECT custid, orderid
FROM Sales.orders
WHERE custid IN (
SELECT custid
FROM Sales.Customers
WHERE country = N'Mexico');

8. Demonstration: Writing Self-Contained Subqueries
10: Using Subqueries
In this demonstration, you will see how to:
Write a nested subquery
Preparation Steps
Start the 20761A-MIA-DC and 20761A-MIA-SQL virtual machines.
Demonstration Steps
Write a Nested Subquery
Ensure that the 20761A-MIA-DC and 20761A-MIA-SQL virtual machines are both running, and then log on to 20761A-MIA-SQL as ADVENTUREWORKS\Student with the password Pa$$w0rd.
In the D:\Demofiles\Mod10 folder, right-click Setup.cmd and click Run as administrator.
Click Yes when prompted to confirm you want to run the command file, and wait for the script to finish.
When prompted press any key.
Start SQL Server Management Studio and connect to the MIA-SQL database engine instance using Windows® authentication.
On the File menu, point to Open, click File, browse to the D:\Demofiles\Mod10\Demo folder, and then double-click Demo.ssmssln.
If the Solution Explorer pane is not visible, on the View menu, click Solution Explorer.
Expand the Queries folder, and double-click the 11 – Demonstration A.sql script file.
Select the code under the comment Step 1: Open a new query window to the TSQL database, and then click Execute.
Select the code under the comment Step 2: Scalar subqueries, and then click Execute.
Select the code under the comment Select this query and execute it to find details in Sales.OrderDetails for most recent order, and then click Execute.
Select the code under the comment THIS WILL FAIL, since subquery returns more than 1 value, and then click Execute.
(More notes on the next slide)

9. Lesson 2: Writing Correlated Subqueries
10: Using Subqueries
Demonstration: Writing Correlated Subqueries
Question
Which of the following statements about correlated subqueries is correct?
( )Option 1: To troubleshoot a correlated subquery, execute the inner query first on its own, before placing it into the outer query.
( )Option 2: In a correlated subquery, the inner query is run only once, regardless of the number of rows the outer query returns.
( )Option 3: In a correlated subquery, the inner query uses data returned by the outer query.
( )Option 4: In a correlated subquery, the inner query is executed first, the outer query second.
Answer
You cannot troubleshoot a correlated subquery by executing the inner query separately, because the inner query requires data from the outer query. In a correlated subquery, multiple values may be passed into the inner query, which would require the inner query to run multiple times. The outer query must be executed first, so that data is available to pass to the inner query.

10. Working with Correlated Subqueries
10: Using Subqueries
Correlated subqueries refer to elements of tables used in outer query
Dependent on outer query, cannot be executed separately
Harder to test than self-contained subqueries
Behaves as if inner query is executed once per outer row
May return scalar value or multiple values
The example above appears in the workbook, broken down step by step. Walk the students through it using the workbook notes.
Question
Why can't a correlated subquery be executed separately from the outer query?
Answer
The subquery depends on input from the outer query for its values.
SELECT orderid, empid, orderdate
FROM Sales.Orders AS O1
WHERE orderdate =
(SELECT MAX(orderdate)
FROM Sales.Orders AS O2
WHERE O2.empid = O1.empid)
ORDER BY empid, orderdate;

11. Writing Correlated Subqueries
10: Using Subqueries
Write inner query to accept input value from outer query
Write outer query to accept appropriate return result (scalar or multi-valued)
Correlate queries by passing value from outer query to match argument in inner query
This example evaluates each row in Sales.Orders, takes the current row’s customer ID, and passes it to the inner query to find the most recent order for that customer id.
SELECT custid, orderid, orderdate
FROM Sales.Orders AS outerorders
WHERE orderdate =
(SELECT MAX(orderdate)
FROM Sales.Orders AS innerorders
WHERE innerorders.custid = outerorders.custid)
ORDER BY custid;

12. Demonstration: Writing Correlated Subqueries
10: Using Subqueries
In this demonstration, you will see how to:
Write a correlated subquery
Preparation Steps
Complete the previous demonstration in this module. Alternatively, start the 20761A-MIA-DC and 20761A- MIA-SQL virtual machines, log on to 20761A-MIA-SQL as ADVENTUREWORKS\Student with the password Pa$$w0rd, and run D:\Demofiles\Mod10\Setup.cmd as an administrator.
Demonstration Steps
Write a Correlated Subquery
Ensure that you have completed the previous demonstration in this module. Alternatively, start the A-MIA-DC and 20761A-MIA-SQL virtual machines, log on to 20761A-MIA-SQL as ADVENTUREWORKS\Student with the password Pa$$w0rd, and run D:\Demofiles\Mod10\Setup.cmd as an administrator.
If SQL Server Management Studio is not already open, start it and connect to the MIA-SQL database engine instance using Windows authentication, and then open the Demo.ssmssln solution in the D:\Demofiles\Mod10\Demo folder.
In Solution Explorer, open the 21 – Demonstration B.sql script file.
Select the code under the comment Step 1: Open a new query window to the TSQL database, and then click Execute.
Select the code under the comment Step 2: Correlated subqueries, and then click Execute.
Select the code under the comment Select and execute the following query to show the use of a correlated subquery that uses the empid from Sales.Orders to retrieve orders placed by an employee on the latest order date for each employee, and then click Execute.
Select the code under the comment Select and execute the following query to show the use of a correlated subquery that uses the custid from Sales.Custorders to retrieve orders placed by a customer with the highest quantity for each customer, and then click Execute.
Keep SQL Server Management Studio open for the next demonstration.

13. Lesson 3: Using the EXISTS Predicate with Subqueries
10: Using Subqueries
Demonstration: Writing Subqueries Using EXISTS
Question
The Human Resources database has recently been extended to record the skills possessed by employees. Employees have added their skills to the database by using a web-based user interface. You want to find employees who have not yet added their skills. You have the following query:
SELECT e.EmployeeID, e.FirstName
FROM HumanResources.Employees AS e
WHERE NOT EXISTS (
SELECT s.EmployeeID, s.SkillName, s.SkillCategory
FROM HumanResources.Skills AS s
WHERE e.EmployeeID = s.EmployeeID);
How can you improve the query?
Answer
In the inner query SELECT clause, replace the list of columns with “*”.

14. EXISTS evaluates to TRUE or FALSE (not UNKNOWN)
Working with EXISTS
10: Using Subqueries
When a subquery is used with the keyword EXISTS, it functions as an existence test
True or false only – no rows passed back to outer query
EXISTS evaluates to TRUE or FALSE (not UNKNOWN)
If any rows are returned by the subquery, EXISTS returns TRUE
If no rows are returned, EXISTS returns FALSE
Syntax:
See Subqueries with EXISTS in Books Online at:
Subqueries with EXISTS
WHERE [NOT] EXISTS (subquery)

15. Writing Queries Using EXISTS with Subqueries
20761A
Writing Queries Using EXISTS with Subqueries
10: Using Subqueries
The keyword EXISTS does not follow a column name or other expression
The SELECT list of a subquery introduced by EXISTS typically only uses an asterisk (*)
Note: The SELECT list typically only uses an asterisk because no data will be returned. The inner query tests to see if there are any rows in its results. It will not return the rows themselves.
SELECT custid, companyname
FROM Sales.Customers AS c
WHERE EXISTS (
SELECT *
FROM Sales.Orders AS o
WHERE c.custid=o.custid);
SELECT custid, companyname
FROM Sales.Customers AS c
WHERE NOT EXISTS (
SELECT *
FROM Sales.Orders AS o
WHERE c.custid=o.custid);

16. Demonstration: Writing Subqueries Using EXISTS
10: Using Subqueries
In this demonstration, you will see how to:
Write queries using EXISTS and NOT EXISTS
Preparation Steps
Complete the previous demonstration in this module. Alternatively, start the 20761A-MIA-DC and 20761A- MIA-SQL virtual machines, log on to 20761A-MIA-SQL as ADVENTUREWORKS\Student with the password Pa$$w0rd, and run D:\Demofiles\Mod10\Setup.cmd as an administrator.
Demonstration Steps
Write Queries Using EXISTS and NOT EXISTS
Ensure that you have completed the previous demonstration in this module. Alternatively, start the A-MIA-DC and 20761A-MIA-SQL virtual machines, log on to 20761A-MIA-SQL as ADVENTUREWORKS\Student with the password Pa$$w0rd, and run D:\Demofiles\Mod10\Setup.cmd as an administrator.
If SQL Server Management Studio is not already open, start it and connect to the MIA-SQL database engine instance using Windows authentication, then open the Demo.ssmssln solution in the D:\Demofiles\Mod10\Demo folder.
In Solution Explorer, open the 31 – Demonstration C.sql script file.
Select the code under the comment Step 1: Open a new query window to the TSQL database, and then click Execute.
Select the code under the comment Step 2: Using EXISTS, and then click Execute.
Select the code under the comment Step 3: Using NOT EXISTS, and then click Execute.
Select the code under the comment Step 4: Compare COUNT(*)>0 to EXISTS, and then click Execute.
Select the code under the comment Use EXISTS, and then click Execute.
Close SQL Server Management Studio without saving any files.

17. Virtual machine: 20761A-MIA-SQL User name: AdventureWorks\Student
Lab: Using Subqueries
10: Using Subqueries
Exercise 3: Writing Queries That Use Correlated Subqueries and an EXISTS Predicate
Important: When comparing your results with the provided sample outputs, the column ordering and total number of affected rows should always match. However, remember that the order of the rows in the output of a query without an ORDER BY clause is not guaranteed. Therefore, the order of the rows in the sample outputs may be different to yours. Also, the answer outputs include abbreviated results.
Exercise 1: Writing Queries That Use Self-Contained Subqueries
The sales department needs some advanced reports to analyze sales orders. You will write different SELECT statements that use self-contained subqueries.
Exercise 2: Writing Queries That Use Scalar and Multi-Result Subqueries
The marketing department would like to prepare materials for different groups of products and customers, based on historic sales information. You have to prepare different SELECT statements that use a subquery in the WHERE clause.
Exercise 3: Writing Queries That Use Correlated Subqueries and an EXISTS Predicate
The sales department would like to have some additional reports to display different analyses of existing customers. Because the requests are complex, you will need to use correlated subqueries.
Logon Information
Virtual machine: 20761A-MIA-SQL
User name: AdventureWorks\Student
Password: Pa$$w0rd
Estimated Time: 60 minutes

18. 20761A
Lab Scenario
10: Using Subqueries
As a business analyst for Adventure Works, you are writing reports using corporate databases stored in SQL Server. You have been handed a set of business requirements for data and will write T- SQL queries to retrieve the specified data from the databases. Due to the complexity of some of the requests, you will need to embed subqueries into your queries to return results in a single query.

19. Module Review and Takeaways
10: Using Subqueries
Review Question(s)
Review Question(s)
Question
Can a correlated subquery return a multi-valued set?
Answer
Yes.
What type of subquery may be rewritten as a JOIN?
Correlated subqueries.
Which columns should appear in the SELECT list of a subquery following the EXISTS predicate?
Only a * needs to be specified. No actual columns will be retrieved.