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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 which is quite different from 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: List the names of salespeople and their managers.
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SELECT EMPS.NAME, MGRS.NAME FROM EMPS, MGRS WHERE EMPS.MANAGER = MGRS.EMPL_NUM;
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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.
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MGRS (copy of SALESREPS) Table EMPL_NUM NAME AGE REP_OFFICE
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105 109 102 106 104 101 110 108 103 107 Bill Adams Mary Jones Sue Smith Sam Clark Bob Smith Dan Roberts Tom Snyder Larry Fitch Paul Cruz Nancy Angelli 37 31 48 52 33 45 41 62 29 49 13 11 21 11 12 12 NULL 21 12 22
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EMPS (copy of SALESREPS) Table EMPL_NUM NAME AGE REP_OFFICE
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105 109 102 106 104 101 110 108 103 107 Bill Adams Mary Jones Sue Smith Sam Clark Bob Smith Dan Roberts Tom Snyder Larry Fitch Paul Cruz Nancy Angelli 37 31 48 52 33 45 41 62 29 49 13 11 21 11 12 12 NULL 21 12 22
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TITLE
Sales Rep Sales Rep Sales Rep VP Sales Sales Mgr Sales Rep Sales Rep Sales Mgr Sales Rep Sales Rep
HIRE_DATE
12-JAN-88 12-OCT-89 10-DEC-86 14-JUN-88 19-MAY-87 20-OCT-86 13-JAN-90 12-OCT-89 01-MAR-87 14-NOV-88
MANAGER
104 106 108 NULL 106 104 101 106 104 108
FIGURE 7-9
A self-join of the SALESREPS table
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 Bill Adams Dan Roberts Paul Cruz Mary Jones Bob Smith Larry Fitch Sue Smith Nancy Angelli MGRS.NAME -----------Dan Roberts Bob Smith Bob Smith Bob Smith Sam Clark Sam Clark Sam Clark Larry Fitch Larry Fitch
7:
Multitable Queries (Joins)
The FROM clause assigns a different alias to each of the two virtual 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 virtual 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 FROM WHERE AND SALESREPS.NAME, SALESREPS.QUOTA, MGRS.QUOTA SALESREPS, SALESREPS MGRS SALESREPS.MANAGER = MGRS.EMPL_NUM SALESREPS.QUOTA > MGRS.QUOTA;
PART II
SALESREPS.NAME SALESREPS.QUOTA MGRS.QUOTA --------------- ---------------- -----------Bill Adams $350,000.00 $200,000.00 Dan Roberts $300,000.00 $200,000.00 Paul Cruz $275,000.00 $200,000.00 Mary Jones $300,000.00 $275,000.00 Larry Fitch $350,000.00 $275,000.00
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 Bill Adams Larry Fitch Nancy Angelli EMP_OFFICE.CITY ---------------Chicago Atlanta Los Angeles Denver MGRS.NAME -----------Sam Clark Bob Smith Sam Clark Larry Fitch MGR_OFFICE.CITY ---------------New York Chicago New York Los Angeles
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