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Accessing Multiple Tables
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This statement will return the following query results:
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TITLE -------------------Famous Blue Raincoat Blue Past Light Out of Africa Fundamental Blues on the Bayou NULL NULL TYPE_NAME ----------Folk Rock Classic Pop New Age Soundtrack NULL Blues Jazz Country STOCK ----19 28 6 8 10 11 NULL NULL
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As you can see, all matched and unmatched rows are included in the query results. Notice that all six rows are included from the CD_INFO table and all seven rows are included from the CD_TYPE table.
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SQL provides yet one more method to combine data from different tables in a manner that is a bit different from the joins shown earlier in this chapter. The UNION operator is a method you can use to combine the results of multiple SELECT statements into a single result set, essentially concatenating rows from one query with rows from another. In contrast with joins, which add columns from multiple tables side by side, unions add rows to the end of the result set. In order to use the UNION operator, each SELECT statement must produce union-compatible columns; meaning that each must produce the same number of columns, and corresponding columns must have compatible data types. For example, if the first column of a SELECT statement produces a character column, then other SELECT statements combined with it using the UNON operator must have a character data type in the first column rather than a numeric or datetime data type. Let s take a look at an example to show you what I mean. Figure 11-6 shows two tables: the CDS_CONTINUED table and the CDS_DISCONTINUED table. The tables are nearly identical in structure but serve two different purposes, which should be obvious from the table names. Suppose you want to combine the data in these two tables so you can view information from both tables. You can, of course, execute two separate SELECT statements, or you can combine those statements into one statement that combines the information, as shown in the following example:
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SELECT FROM UNION SELECT FROM * CDS_CONTINUED * CDS_DISCONTINUED;
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SQL: A Beginner s Guide
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CDS_CONTINUED CD_NAME: VARCHAR(60) CD_TYPE: IN_STOCK: CHAR(4) INT 19 28 6 8 10 11
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CDS_DISCONTINUED CD_NAME: VARCHAR(60) Court and Spark Kojiki That Christmas Feeling CD_TYPE: CHAR(4) FROK NEWA XMAS IN_STOCK: INT 3 2 2 4 3 1
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Famous Blue Raincoat FROK Blue Past Light Out of Africa Fundamental Blues on the Bayou CPOP NEWA STRK NPOP BLUS
Patsy Cline: 12 Greatest Hits CTRY Leonard Cohen The Best of Orlando FROK STRK
Figure 11-6
Joining the CDS_CONTINUED and CDS_DISCONTINUED tables
As you can see, the two SELECT statements are combined using the UNION operator. If you execute this statement, you ll receive the following results:
CD_NAME ----------------------------Blue Blues on the Bayou Court and Spark Famous Blue Raincoat Fundamental Kojiki Leonard Cohen The Best Of Orlando Out of Africa Past Light Patsy Cline: 12 Greatest Hits That Christmas Feeling CD_TYPE ------CPOP BLUS FROK FROK NPOP NEWA FROK STRK STRK NEWA CTRY XMAS IN_STOCK -------28 11 3 19 10 2 3 1 8 6 4 2
The results include 12 rows of data, six rows from each table. You can limit the results even further by specifying search conditions in WHERE clauses. You can also specify that your search return only specific columns, as in the following statement:
SELECT FROM UNION SELECT FROM CD_TYPE CDS_CONTINUED CD_TYPE CDS_DISCONTINUED;
11:
Accessing Multiple Tables
Now when you generate your query, only values from the CD_TYPE column are displayed:
CD_TYPE -----BLUS CPOP CTRY FROK NEWA NPOP STRK XMAS
Notice that only 8 rows are returned, rather than 12. This is because duplicate rows are filtered out by default. If you want all rows included in the query results, regardless of whether there are duplicate values, you can add the ALL keyword after the UNION operator, as shown in the following example:
SELECT CD_TYPE FROM CDS_CONTINUED UNION ALL SELECT CD_TYPE FROM CDS_DISCONTINUED;
This statement will return 12 rows rather than 8, with several values duplicated. As you can see, the UNION operator is useful only in very specific cases. If you want more control over your query results, you should use one of the several types of joins supported by SQL.
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