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Lesson 7 Use the subquery statement
Objective Create subquery statement using the IN keyword.

Create Subquery Statements using IN Keyword

Building Subquery Statement
In the previous lesson, if there was more than one row in the Publishers table with the correct state ("CA"), the query would use commas, as follows:

Create a sub-SELECT statement using the IN keyword

SELECT Title FROM Titles 
WHERE pub_id IN ('1389','0736','0877')

Any publisher in the Titles table with a pub_id matching any of the values in the list will appear here.
This is a good way to use one table as a controlling key to another.
In this case, the Publishers table is controlling the output for the query against the Titles table.
One final note: be sure to enclose the subquery in parentheses.
This is required to make the engine aware of the statement and its start and end point.

IN Operator

The IN operator allows you to specify that you want to match one item from any of those in a list of items. For example, the following SQL finds all the members who were born in 1642, 1716, or 1777:

SELECT FirstName, LastName, YEAR(DateOfBirth)
FROM MemberDetails
WHERE YEAR(DateOfBirth) IN (1642, 1716, 1777);

The query provides the following results:
FirstName 	LastName YEAR(DateOfBirth)
Isaac     	Newton   1642 
Gottfried 	Wilhelm  1716
Karl      	Gauss    1777
you can also use the IN operator with subqueries.
Instead of providing a list of literal values, a SELECT query provides the list of values.
For example, if you want to know which members were born in the same year that a film in the Films table was released, you would use the following SQL query.
SELECT FirstName, LastName, YEAR(DateOfBirth)
FROM MemberDetails
WHERE YEAR(DateOfBirth) IN (SELECT YearReleased FROM Films);

Executing this query gives the following results:
FirstName LastName YEAR(DateOfBirth)
Katie     Smith    1977
Steve     Gee      1967
Doris     Night    1997

The subquery
(SELECT YearReleased FROM Films) 
returns a list of years from the Films table. If a member's year of birth matches one of the items in that list, then the WHERE clause is true and the record is included in the final results.
You may have noticed that this is not the only way to get the result. You could have used an INNER JOIN coupled with a GROUP BY statement instead, as shown in the following SQL:
SELECT FirstName, LastName, YEAR(DateOfBirth)
FROM MemberDetails JOIN Films ON YEAR(DateOfBirth) = YearReleased
GROUP BY FirstName, LastName, YEAR(DateOfBirth);

Running this query gives the same results as the previous query.
Question: So which query is best? Unfortunately, there is no definitive answer and this very much depends on the circumstances, the data involved, and the database system involved. Many SQL programmers prefer a join to a subquery and believe that to be the most efficient. However, if you compare the speed of the two using MS SQL Server 2012, on that system, the subquery is faster by roughly 15 percent. Given how few rows there are in this table example, the difference was negligible in this example, but it might be significant with many records.
Question: What is the best query to use?
You should use the technique that you find easiest, and optimize your SQL code only if problems occur during testing. If you find on a test system with a million records that your SQL runs extremely slow, then you should go back and see whether you can improve your query.
There is one area in which subqueries are pretty much essential: when you want to find something is not in a list, something which is very hard to achieve with joins.
For example, if you want a list of all members who were not born in the same year that any of the films in the Films table were released, you would simply change your previous subquery example from an IN operator to a NOT IN operator:
SELECT FirstName, LastName, YEAR(DateOfBirth)
FROM MemberDetails
WHERE YEAR(DateOfBirth) NOT IN (SELECT YearReleased FROM Films);

Subquery Statement Exercise

Click the link below to work on the Subquery Statement - Exercise .
Before you begin the exercise, read about engine support for subquery statements. Then, complete the exercise.