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Some multi-table queries involve a relationship that a table has with itself. For example, suppose you want to list the names of all salespeople and their managers. Each salesperson appears as a row in the SALESREPS table, and the MANAGER column contains the employee number of the salesperson's manager. It would appear that the MANAGER column should be a foreign key for the table that holds data about managers. In fact it is it's a foreign key for the SALESREPS table itself! If you tried to express this query like any other two-table query involving a foreign key/primary key match, it would look like this: SELECT NAME, NAME FROM SALESREPS, SALESREPS WHERE MANAGER = EMPL_NUM This SELECT statement is illegal because of the duplicate reference to the SALESREPS table in the FROM clause. You might also try eliminating the second reference to the SALESREPS table: SELECT NAME, NAME FROM SALESREPS
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WHERE MANAGER = EMPL_NUM This query is legal, but it won't do what you want it to do! It's a single-table query, so SQL goes through the SALESREPS table one row at a time, applying the search condition: MANAGER = EMPL_NUM The rows that satisfy this condition are those where the two columns have the same value that is, rows where a salesperson is their own manager. There are no such rows, so the query would produce no results not exactly the data that the English-language statement of the query requested. To understand how SQL solves this problem, imagine there were two identical copies of the SALESREPS table, one named EMPS, containing employees, and one named MGRS, containing managers, as shown in Figure 7-9. The MANAGER column of the EMPS table would then be a foreign key for the MGRS table, and the following query would work:
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Figure 7-9: A self-join of the SALESREPS table
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List the names of salespeople and their managers. SELECT EMPS.NAME, MGRS.NAME FROM EMPS, MGRS WHERE EMPS.MANAGER = MGRS.EMPL_NUM Because the columns in the two tables have identical names, all of the column references are qualified. Otherwise, this looks like an ordinary two-table query. SQL uses exactly this "imaginary duplicate table" approach to join a table to itself. Instead of actually duplicating the contents of the table, SQL lets you simply refer to it by a different name, called a table alias. Here's the same query, written using the aliases EMPS and MGRS for the SALESREPS table: List the names of salespeople and their managers. SELECT EMPS.NAME, MGRS.NAME FROM SALESREPS EMPS, SALESREPS MGRS WHERE EMPS.MANAGER = MGRS.EMPL_NUM EMPS.NAME ------------Tom Snyder MGRS.NAME --------Dan Roberts
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Bill Adams Dan Roberts Paul Cruz Mary Jones Bob Smith Larry Fitch Sue Smith Nancy Angelli
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The FROM clause assigns a different alias to each of the two "copies" of the SALESREPS table that are involved in the query, by specifying the alias name immediately after the actual table name. As the example shows, when a FROM clause contains a table alias, the alias must be used to identify the table in qualified column references. Of course, it's really only necessary to use an alias for one of the two table occurrences in this query. It could just as easily have been written: SELECT SALESREPS.NAME, MGRS.NAME FROM SALESREPS, SALESREPS MGRS WHERE SALESREPS.MANAGER = MGRS.EMPL_NUM Here the alias MGRS is assigned to one "copy" of the table, while the table's own name is used for the other copy. Here are some additional examples of self-joins: List salespeople with a higher quota than their manager. SELECT SALESREPS.NAME, SALESREPS.QUOTA, MGRS.QUOTA FROM SALESREPS, SALESREPS MGRS WHERE SALESREPS.MANAGER = MGRS.EMPL_NUM AND SALESREPS.QUOTA > MGRS.QUOTA SALESREPS.NAME -------------Bill Adams Dan Roberts Paul Cruz Mary Jones Larry Fitch SALESREPS.QUOTA --------------$350,000.00 $300,000.00 $275,000.00 $300,000.00 $350,000.00 MGRS.QUOTA ----------$200,000.00 $200,000.00 $200,000.00 $275,000.00 $275,000.00
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List salespeople who work in different offices than their manager, showing the name and office where each works. SELECT EMPS.NAME, EMP_OFFICE.CITY, MGRS.NAME, MGR_OFFICE.CITY FROM SALESREPS EMPS, SALESREPS MGRS, OFFICES EMP_OFFICE, OFFICES MGR_OFFICE WHERE EMPS.REP_OFFICE = EMP_OFFICE.OFFICE AND MGRS.REP_OFFICE = MGR_OFFICE.OFFICE AND EMPS.MANAGER = MGRS.EMPL_NUM AND EMPS.REP_OFFICE <> MGRS.REP_OFFICE EMPS.NAME --------Bob Smith EMP_OFFICE.CITY --------------Chicago MGRS.NAME ----------Sam Clark MGR_OFFICE.CITY --------------New York
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