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Compound Query Operators
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Sometimes it is useful to run multiple queries and to combine the results into a single result set.
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UNION
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The UNION operator appends all the rows in the result set of one query to that of another, but it also eliminates any duplicate rows, much like the DISTINCT keyword does. The operation is permitted only if the queries involved are union compatible, which means they have the same number of columns and the data types of the corresponding columns are compatible. Here is an example: List all the non-null values of rental fees or late-or-lost fees from the MOVIE_RENTAL table in a single column.
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SELECT FROM WHERE UNION SELECT FROM WHERE RENTAL_FEE AS FEE MOVIE_RENTAL RENTAL_FEE IS NOT NULL LATE_OR_LOSS_FEE AS FEE MOVIE_RENTAL LATE_OR_LOSS_FEE IS NOT NULL;
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FEE ---------4 6 6.25 29.98 29.99
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If you nd this one dif cult to visualize, just run the two queries independently, and then combine them to see the result. You ll nd out that the second and third rows are rental fees that are found by the rst query, while the rst, fourth, and fth rows are late-or-loss fees from the second query.
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UNION ALL
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UNION ALL works like UNION except that duplicate rows are not eliminated from the result set. If you repeat the previous example with UNION ALL, you will get a result set of 24 rows instead of 5 rows.
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Retrieving Data Using Data Query Language
INTERSECT
The INTERSECT operator nds values selected by one query that also appear in the other query. Essentially, it nds the intersection of the values in the two result sets. However, only a few DBMS implementations (most notably Oracle and DB2) support it. You won t nd it in Microsoft SQL Server or MySQL. Here is an example: Are any of the DVD prices the same as one of the VHS prices in the MOVIE table The result set contains no rows, so the answer is no.
SELECT RENTAL_FEE AS FEE FROM MOVIE_RENTAL WHERE RENTAL_FEE IS NOT NULL INTERSECT SELECT LATE_OR_LOSS_FEE AS FEE FROM MOVIE_RENTAL WHERE LATE_OR_LOSS_FEE IS NOT NULL no rows selected
EXCEPT
EXCEPT is the ANSI/ISO standard operator that nds the difference between two result sets, basically by returning values from the rst query that are not included in the result set of the second query. Very few DBMS implementations have implemented this operator. In some implementations such as Oracle, it is named MINUS instead of EXCEPT.
Quiz
Choose the correct responses to each of the multiple-choice questions. Note that there may be more than one correct response to each question. 1. A SELECT without a WHERE clause a. Selects all columns in the table or view b. Results in an error message c. Selects all rows in the table or view d. Lists only the de nition of the table or view e. Always outputs results to a log le
2. In SQL, row order in the query results a. Is speci ed by the SORTED BY clause
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b. May be either ascending or descending for any column c. Defaults to descending when sequence is not speci ed d. Is unpredictable unless speci ed in the query e. May only be speci ed for columns in the query result set 3. The BETWEEN operator a. Speci es a range of values that includes the end points b. Can be rewritten using the <= and NOT <= operators c. Can be rewritten using the <= and >= operators d. Selects rows added to a table during a time interval e. Is not included in the ISO/ANSI standard 4. The standard LIKE operator a. Uses question marks as nonpositional wildcards b. Uses underscores as positional wildcards c. Uses underscores as nonpositional wildcards d. Uses percent signs as positional wildcards e. Uses percent signs as nonpositional wildcards 5. An SQL statement containing an aggregate function a. May also contain calculated columns b. May also contain ordinary columns c. Must include an ORDER BY clause d. Must contain a GROUP BY clause e. May not include both GROUP BY and ORDER BY clauses 6. When AND and OR operators are mixed in the same WHERE clause a. The DBMS returns an error message b. The AND has a higher precedence than the OR c. The OR has a higher precedence than the AND d. Parentheses are mandatory e. Parentheses are optional
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