A join does not always have to involve two different tables. You can join a table to itself, creating a self-join. Joining a table to itself can be useful when you want to compare values in a column to other values in the same column.
To create a self-join, list a table twice in the FROM clause, and assign it a different alias each time. Use the aliases to refer to the table in the Projection and WHERE clauses as if it were two separate tables. (Aliases in SELECT statements are discussed in Using Aliases and in the IBM Informix: Guide to SQL Syntax.)
Just as in joins between tables, you can use arithmetic expressions in self-joins. You can test for null values, and you can use an ORDER BY clause to sort the values in a specified column in ascending or descending order.
Figure 278 finds pairs of orders where the ship_weight differs by a factor of five or more and the ship_date is not null. The query then orders the data by ship_date.
SELECT x.order_num, x.ship_weight, x.ship_date, y.order_num, y.ship_weight, y.ship_date FROM orders x, orders y WHERE x.ship_weight >= 5 * y.ship_weight AND x.ship_date IS NOT NULL AND y.ship_date IS NOT NULL ORDER BY x.ship_date
order_num | ship_weight | ship_date | order_num | ship_weight | ship_date |
---|---|---|---|---|---|
1004 | 95.80 | 05/30/1998 | 1011 | 10.40 | 07/03/1998 |
1004 | 95.80 | 05/30/1998 | 1020 | 14.00 | 07/16/1998 |
1004 | 95.80 | 05/30/1998 | 1022 | 15.00 | 07/30/1998 |
1007 | 125.90 | 06/05/1998 | 1015 | 20.60 | 07/16/1998 |
1007 | 125.90 | 06/05/1998 | 1020 | 14.00 | 07/16/1998 |
If you want to store the results of a self-join into a temporary table, append an INTO TEMP clause to the SELECT statement and assign display labels to at least one set of columns to rename them. Otherwise, the duplicate column names cause an error and the temporary table is not created.
Figure 279, which is similar to Figure 278, labels all columns selected from the orders table and puts them in a temporary table called shipping.
SELECT x.order_num orders1, x.po_num purch1, x.ship_date ship1, y.order_num orders2, y.po_num purch2, y.ship_date ship2 FROM orders x, orders y WHERE x.ship_weight >= 5 * y.ship_weight AND x.ship_date IS NOT NULL AND y.ship_date IS NOT NULL ORDER BY orders1, orders2 INTO TEMP shipping
If you query with SELECT * from table shipping, you see the rows that Figure 280 shows.
orders1 purch1 ship1 orders2 purch2 ship2 1004 8006 05/30/1998 1011 B77897 07/03/1998 1004 8006 05/30/1998 1020 W2286 07/16/1998 1004 8006 05/30/1998 1022 W9925 07/30/1998 1005 2865 06/09/1998 1011 B77897 07/03/1998
·
·
·
1019 Z55709 07/16/1998 1020 W2286 07/16/1998 1019 Z55709 07/16/1998 1022 W9925 07/30/1998 1023 KF2961 07/30/1998 1011 B77897 07/03/1998
You can join a table to itself more than once. The maximum number of self-joins depends on the resources available to you.
The self-join in Figure 281 creates a list of those items in the stock table that are supplied by three manufacturers. The self-join includes the last two conditions in the WHERE clause to eliminate duplicate manufacturer codes in rows that are retrieved.
SELECT s1.manu_code, s2.manu_code, s3.manu_code, s1.stock_num, s1.description FROM stock s1, stock s2, stock s3 WHERE s1.stock_num = s2.stock_num AND s2.stock_num = s3.stock_num AND s1.manu_code < s2.manu_code AND s2.manu_code < s3.manu_code ORDER BY stock_num
manu_code manu_code manu_code stock_num description HRO HSK SMT 1 baseball gloves ANZ NRG SMT 5 tennis racquet ANZ HRO HSK 110 helmet ANZ HRO PRC 110 helmet ANZ HRO SHM 110 helmet ANZ HSK PRC 110 helmet ANZ HSK SHM 110 helmet ANZ PRC SHM 110 helmet HRO HSK PRC 110 helmet HRO HSK SHM 110 helmet HRO PRC SHM 110 helmet
·
·
·
KAR NKL PRC 301 running shoes KAR NKL SHM 301 running shoes KAR PRC SHM 301 running shoes NKL PRC SHM 301 running shoes
If you want to select rows from a payroll table to determine which employees earn more than their manager, you might construct the self-join as the following SELECT statement shows:
SELECT emp.employee_num, emp.gross_pay, emp.level, emp.dept_num, mgr.employee_num, mgr.gross_pay, mgr.dept_num, mgr.level FROM payroll emp, payroll mgr WHERE emp.gross_pay > mgr.gross_pay AND emp.level < mgr.level AND emp.dept_num = mgr.dept_num ORDER BY 4
Figure 283 uses a correlated subquery to retrieve and list the 10 highest-priced items ordered.
SELECT order_num, total_price
FROM items a
WHERE 10 >
(SELECT COUNT (*)
FROM items b
WHERE b.total_price < a.total_price)
ORDER BY total_price
Figure 283 returns the 10 rows that Figure 284 shows.
order_num total_price 1018 $15.00 1013 $19.80 1003 $20.00 1005 $36.00 1006 $36.00 1013 $36.00 1010 $36.00 1013 $40.00 1022 $40.00 1023 $40.00
You can create a similar query to find and list the 10 employees in the company who have the most seniority.
For more information about correlated subqueries, refer to Subqueries in SELECT Statements.
Home | [ Top of Page | Previous Page | Next Page | Contents | Index ]