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The ORDERS table and the PRODUCTS table in the sample database are related by a composite foreign key/primary key pair. The MFR and PRODUCT columns of the ORDERS table together form a foreign key for the PRODUCTS table, matching its MFR_ID and PRODUCT_ID columns, respectively. To join the tables based on this parent/child relationship, you must specify both pairs of matching columns, as shown in this example: List all the orders, showing amounts and product descriptions. SELECT ORDER_NUM, AMOUNT, DESCRIPTION FROM ORDERS, PRODUCTS WHERE MFR = MFR_ID AND PRODUCT = PRODUCT_ID ORDER_NUM ---------113027 AMOUNT DESCRIPTION --------- -------------$4,104.00 Size 2 Widget
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The search condition in the query tells SQL that the related pairs of rows from the ORDERS and PRODUCTS tables are those where both pairs of matching columns contain the same values. Multi-column joins involving two tables are less common than singlecolumn joins and are usually found in queries involving compound foreign keys such as this one. There is no SQL restriction on the number of columns that are involved in the matching condition, but joins normally mirror the real-world relationships between entities represented in the database tables, and those relationships are usually embodied in one or just a few columns of the tables.
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SQL can combine data from three or more tables using the same basic techniques used for two-table queries. Here is a simple example of a three-table join: List orders over $25,000, including the name of the salesperson who took the order and the name of the customer who placed it. SELECT ORDER_NUM, AMOUNT, COMPANY, NAME FROM ORDERS, CUSTOMERS, SALESREPS WHERE CUST = CUST_NUM AND REP = EMPL_NUM AND AMOUNT > 25000.00 ORDER_NUM ---------112987 113069 113045 112961 AMOUNT COMPANY NAME ---------- --------------- ------------$27,500.00 Acme Mfg. Bill Adams $31,350.00 Chen Associates Nancy Angelli $45,000.00 Zetacorp Larry Fitch $31,500.00 J.P. Sinclair Sam Clark
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This query uses two foreign keys in the ORDERS table, as shown in Figure 7-5. The CUST column is a foreign key for the CUSTOMERS table, linking each order to the customer who placed it. The REP column is a foreign key for the SALESREPS table, linking each order to the salesperson who took it. Informally speaking, the query links each order to its associated customer and salesperson.
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Figure 7-5: A three-table join
Here is another three-table query that uses a different arrangement of parent/child relationships: List the orders over $25,000, showing the name of the customer who placed the order and the name of the salesperson assigned to that customer. SELECT ORDER_NUM, AMOUNT, COMPANY, NAME FROM ORDERS, CUSTOMERS, SALESREPS WHERE CUST = CUST_NUM AND CUST_REP = EMPL_NUM AND AMOUNT > 25000.00 ORDER_NUM ---------112987 113069 113045 112961 AMOUNT COMPANY NAME ---------- --------------- ---------$27,500.00 Acme Mfg. Bill Adams $31,350.00 Chen Associates Paul Cruz $45,000.00 Zetacorp Larry Fitch $31,500.00 J.P. Sinclair Sam Clark
Figure 7-6 shows the relationships exercised by this query. The first relationship again uses the CUST column from the ORDERS table as a foreign key to the CUSTOMERS table. The second uses the CUST_REP column from the CUSTOMERS table as a foreign key to the SALESREPS table. Informally speaking, this query links each order to its customer, and each customer to their salesperson.
Figure 7-6: A three-table join with cascaded parent/child relationships
It's not uncommon to find three-table or even four-table queries used in production SQL applications. Even within the confines of the small, five-table sample database, it's not too hard to find a four-table query that makes sense: List the orders over $25,000, showing the name of the customer who placed the order, the customer's salesperson, and the office where the salesperson works. SELECT ORDER_NUM, AMOUNT, COMPANY, NAME, CITY FROM ORDERS, CUSTOMERS, SALESREPS, OFFICES WHERE CUST = CUST_NUM AND CUST_REP = EMPL_NUM
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AND REP_OFFICE = OFFICE AND AMOUNT > 25000.00 ORDER_NUM ---------112987 113069 113045 112961 AMOUNT COMPANY NAME CITY ---------- --------------- ---------- ------$27,500.00 Acme Mfg. Bill Adams Atlanta $31,350.00 Chen Associates Paul Cruz Chicago $45,000.00 Zetacorp Larry Fitch Los Angeles $31,500.00 J.P. Sinclair Sam Clark New York
Figure 7-7 shows the parent/child relationships in this query. Logically, it extends the join sequence of the previous example one more step, linking an order to its customer, the customer to their salesperson, and the salesperson to their office.
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