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Accessing Multiple Tables
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EMP_ID: EMP_NAME: INT VARCHAR(60) 101 102 103 104 105 106 107 Ms. Smith Mr. Jones Mr. Roberts Ms. Hanson Mr. Fields Ms. Lee Mr. Carver
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MNGR: INT NULL 101 101 103 102 102 103
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Self-joining the EMPLOYEES table
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In this statement, each instance of the table is given a correlation name. As a result, you now have (in this example) table a and table b. You pull the EMP_ID value and EMP_NAME value from table a, but you pull the MANAGER value from table b. The equi-join condition is defined in the WHERE clause by equating the MNGR value in table a with the EMP_ID value in table b. This provides the link that treats one physical table as two logical tables. When you execute this statement, you receive the following query results:
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EMP_ID -----102 103 104 105 106 107 EMP_NAME ----------Mr. Jones Mr. Roberts Ms. Hanson Mr. Fields Ms. Lee Mr. Carver MANAGER ----------Ms. Smith Ms. Smith Mr. Roberts Mr. Jones Mr. Jones Mr. Roberts
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The results include the employee ID and name of each employee, along with the name of the employee s manager. As you can see, the self-join can be a handy tool to use in cases such as this where a table references itself.
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Join Tables with Shared Column Names
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SQL provides two methods for setting up joins that you can use when you re working with columns that have the same names. These two methods the natural join and the named column join allow you to easily specify a join condition between two tables when one or more columns within those tables are the same. In order to use either of these two methods, the tables must meet the following conditions:
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The joined columns must share the same name and have compatible data types. The names of the joined columns cannot be qualified with table names.
SQL: A Beginner s Guide
When you re using either the natural join or the named column join, each table must share at least one column in common. For example, the TITLES_IN_STOCK and the TITLE_ COSTS tables, shown in Figure 11-3, have two columns that are the same: CD_TITLE and CD_TYPE. Notice that each set of matching columns is configured with the same data type. You can use a natural join or a named column join to join these two tables. I describe each of these types of join operations in the next several sections, and I use the tables in Figure 11-3 to illustrate how each of these methods work.
NOTE
Not all SQL implementations support natural joins or named column joins. For example, SQL Server does not support either of these methods, MySQL supports natural joins but not named column joins, and Oracle supports both.
Creating the Natural Join
The natural join automatically matches rows for those columns with the same name. You do not have to specify any sort of equi-join condition for natural joins. The SQL implementation determines which columns have the same names and then tries to form a match. The drawback to this is that you cannot specify which columns are matched up, although you can specify which columns are included in the query results. In the following example, a natural join is used to join the TITLES_IN_STOCK table to the TITLE_COSTS table:
SELECT CD_TITLE, CD_TYPE, c.RETAIL FROM TITLES_IN_STOCK s NATURAL JOIN TITLE_COSTS c WHERE s.INVENTORY > 15;
TITLES_IN_STOCK CD_TITLE: VARCHAR(60) CD_TYPE: CHAR(20) INVENTORY: INT 12 24 9 19 25 17 11 13
TITLE_COSTS CD_TITLE: VARCHAR(60) CD_TYPE: CHAR(20) WHOLESALE: NUMERIC(5,2) 8.00 7.50 7.95 6.00 8.25 7.25 7.50 7.45 6.95 RETAIL: NUMERIC(5,2) 16.99 15.99 15.99 14.99 16.99 15.99 15.99 14.99 14.99
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