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A request that spans two tables
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SQL: The Complete Reference
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Manually processing a multitable query
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The process of forming pairs of rows by matching the contents of related columns is called joining the tables. The resulting table (containing data from both of the original tables) is called a join between the two tables. (A join based on an exact match between two columns is more precisely called an equi-join. Joins can also be based on other kinds of column comparisons, as described later in this chapter.) Joins are the foundation of multitable query processing in SQL. All of the data in a relational database is stored in its columns as explicit data values, so all possible relationships between tables can be formed by matching the contents of related columns. Joins thus provide a powerful facility for exercising the data relationships in a database. In fact, because relational databases do not contain pointers or other mechanisms for relating rows to one another, joins are the only mechanism for exercising cross-table data relationships. Because SQL handles multitable queries by matching columns, it should come as no surprise that the SELECT statement for a multitable query must contain a search
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Multitable Queries (Joins)
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condition that specifies the column match. Here is the SELECT statement for the query that was performed manually in Figure 7-2: List all orders showing order number, amount, customer name, and the customer s credit limit.
SELECT ORDER_NUM, AMOUNT, COMPANY, CREDIT_LIMIT FROM ORDERS, CUSTOMERS WHERE CUST = CUST_NUM ORDER_NUM AMOUNT COMPANY CREDIT_LIMIT ---------- ----------- ------------------ ------------112989 $1,458.00 Jones Mfg. $65,000.00 112968 $3,978.00 First Corp. $65,000.00 112963 $3,276.00 Acme Mfg. $50,000.00 112987 $27,500.00 Acme Mfg. $50,000.00 112983 $702.00 Acme Mfg. $50,000.00 113027 $4,104.00 Acme Mfg. $50,000.00 112993 $1,896.00 Fred Lewis Corp. $65,000.00 113065 $2,130.00 Fred Lewis Corp. $65,000.00 113036 $22,500.00 Ace International $35,000.00 113034 $632.00 Ace International $35,000.00 113058 $1,480.00 Holm & Landis $55,000.00 113055 $150.00 Holm & Landis $55,000.00 113003 $5,625.00 Holm & Landis $55,000.00 . . . RETRIEVING DATA
This looks just like the queries from the previous chapter, with two new features. First, the FROM clause lists two tables instead of just one. Second, the search condition:
CUST = CUST_NUM
compares columns from two different tables. We call these two columns the matching columns for the two tables. Like all search conditions, this one restricts the rows that appear in the query results. Because this is a two-table query, the search condition restricts the pairs of rows that generate the query results. In fact, the search condition specifies the same matching columns you used in the paper-and-pencil query processing. It actually captures the spirit of the manual column matching very well, saying: Generate query results only for pairs of rows where the customer number (CUST) in the ORDERS table matches the customer number (CUST_NUM) in the CUSTOMERS table. Notice that the SELECT statement doesn t say anything about how SQL should execute the query. There is no mention of starting with orders or starting with
SQL: The Complete Reference
customers. Instead, the query tells SQL what the query results should look like and leaves it up to SQL to decide how to generate them.
Parent/Child Queries
The most common multitable queries involve two tables that have a natural parent/ child relationship. The query about orders and customers in the preceding section is an example of such a query. Each order (child) has an associated customer (parent), and each customer (parent) can have many associated orders (children). The pairs of rows that generate the query results are parent/child row combinations. You may recall from 4 that foreign keys and primary keys create the parent/child relationship in a SQL database. The table containing the foreign key is the child in the relationship; the table with the primary key is the parent. To exercise the parent/child relationship in a query, you must specify a search condition that compares the foreign key and the primary key. Here is another example of a query that exercises a parent/child relationship, shown in Figure 7-3: List each salesperson and the city and region where they work.
SELECT NAME, CITY, REGION FROM SALESREPS, OFFICES WHERE REP_OFFICE = OFFICE NAME -------------Mary Jones Sam Clark Bob Smith Paul Cruz Dan Roberts Bill Adams Sue Smith Larry Fitch Nancy Angelli CITY -----------New York New York Chicago Chicago Chicago Atlanta Los Angeles Los Angeles Denver REGION -------Eastern Eastern Eastern Eastern Eastern Eastern Western Western Western
The SALESREPS (child) table contains REP_OFFICE, a foreign key for the OFFICES (parent) table. This relationship is used to find the correct OFFICES row for each salesperson, so that the correct city and region can be included in the query results.
7:
Multitable Queries (Joins)
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