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Combining Data from Multiple Tables
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Subselects
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A very powerful feature of SQL is the subselect (or subquery), which as the name implies, refers to a SELECT statement that contains a subordinate SELECT. Subselects are typically used in the WHERE clause as a way of limiting rows returned in the result set of the outer query. This can be a very exible way of selecting data. However, they can be used in other ways, as you will see in the next section, Inline Views. An essential syntax rule is that the subselect must be enclosed in parentheses. Another important point to understand is that anything you can do using a subselect can also be done using a join. In fact, some SQL implementations automatically rewrite subselects as joins inside the SQL engine before running them, but unless you look at the internals of the SQL implementation, you as a database user will never know that happened.
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Noncorrelated Subselects
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A noncorrelated subselect is a subselect where the inner select makes no reference to the outer select that contains it. This means that the inner select can be run rst, and the result set of the inner select used in the outer select. Here are some examples: Listing of all languages for which there are no movies in the video store inventory.
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SELECT LANGUAGE_CODE, LANGUAGE_NAME FROM LANGUAGE WHERE LANGUAGE_CODE NOT IN (SELECT DISTINCT LANGUAGE_CODE FROM MOVIE_LANGUAGE) ORDER BY LANGUAGE_CODE LANGUAGE_CODE ------------ja ko nl ru zh LANGUAGE_NAME ------------Japanese Korean Dutch Russian Chinese
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The inner select returns a list of language codes that appear on movies in the MOVIE_LANGUAGE table. The DISTINCT keyword removes any duplicate language codes from the result set, and in doing so, makes the
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outer select much more ef cient the fewer the values that appear in an IN clause list, the more ef cient the query. There are 40 rows in the MOVIE_ LANGUAGE table, and thus there would be 40 values in the IN list, but it is reduced to only 4 values by the DISTINCT keyword. Here is the inner select run by itself to help you visualize how it works:
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SELECT DISTINCT LANGUAGE_CODE FROM MOVIE_LANGUAGE; LANGUAGE_CODE ------------de en es fr
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(German) (English) (Spanish) (French)
The store manager is looking at the effect of a recent price increase and needs a list of transactions (TRANSACTION_IDs) where the customer paid more than the average fee (RENTAL_FEE) for a movie. Here is the query:
SELECT DISTINCT TRANSACTION_ID FROM MOVIE_RENTAL WHERE RENTAL_FEE > (SELECT AVG(RENTAL_FEE) FROM MOVIE_RENTAL) TRANSACTION_ID -------------9 10
The inner query nds the average rental fee and then the outer query nds all rows in the MOVIE_RENTAL table with a RENTAL_FEE that exceeds the average. While IN and NOT IN are the most common operators used to connect subqueries to outer queries, in this case the subquery returns only one row; therefore you can use the greater than (>) operator for comparison. The DISTINCT eliminates any duplicate transaction IDs. The previous query result set would be more useful to the store manager if it included the transaction date. One way to accomplish this is to add a join to the CUSTOMER_TRANSACTION table in the outer select. However, since you just learned about subselects, let s do that with another subselect. This demonstrates that you can nest subselects when you need to. Here is the query:
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SELECT TRANSACTION_ID, TRANSACTION_DATE AS TRANS_DATE FROM CUSTOMER_TRANSACTION WHERE TRANSACTION_ID IN (SELECT DISTINCT TRANSACTION_ID FROM MOVIE_RENTAL WHERE RENTAL_FEE > (SELECT AVG(RENTAL_FEE) FROM MOVIE_RENTAL)) TRANSACTION_ID -------------9 10 TRANS_DATE ---------03/01/2005 03/01/2005
Correlated Subselects
A correlated subselect is a subselect where the inner select refers to values provided by the outer select. These are far less ef cient than noncorrelated subselects because the inner query must be invoked for each row found by the outer query. Recall that with a noncorrelated subselect, the inner query is only run once. An example follows. The store wishes to mail a discount coupon to any customer who paid more than $15 in rental fees in any single rental transaction. Here is the query:
SELECT DISTINCT CUSTOMER_ACCOUNT_ID FROM CUSTOMER_TRANSACTION A WHERE 15 < (SELECT SUM(RENTAL_FEE) FROM MOVIE_RENTAL B WHERE A.TRANSACTION_ID = B.TRANSACTION_ID) CUSTOMER_ACCOUNT_ID ------------------2 7 9
Note the alias assigned to the table names in the inner and outer queries and the use of them in the WHERE clause in the inner query. This is the hallmark of a correlated subselect. The outer select nds a distinct list of CUSTOMER_ACCOUNT_ID values in the CUSTOMER_TRANSACTION_TABLE. For each value found, the value is passed to the inner query, which is run to nd the sum of rental fees for that transaction. If the sum of rental fees is greater than or equal to 15 (actually expressed
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