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Parent/Child Queries
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The most common multi-table 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:
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Figure 7-3: A parent/child query with OFFICES and SALESREPS
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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
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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. Here's another query involving the same two tables, but with the parent and child roles reversed, as shown in Figure 7-4.
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Figure 7-4: A different parent/child query with OFFICES and SALESREPS
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List the offices and the names and titles of their managers. SELECT CITY, NAME, TITLE FROM OFFICES, SALESREPS WHERE MGR = EMPL_NUM CITY ----------Chicago Atlanta New York Denver Los Angeles NAME ----------Bob Smith Bill Adams Sam Clark Larry Fitch Larry Fitch TITLE --------Sales Mgr Sales Rep VP Sales Sales Mgr Sales Mgr
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The OFFICES (child) table contains MGR, a foreign key for the SALESREPS (parent) table. This relationship is used to find the correct SALESREPS row for each salesperson, so that the correct name and title of the manager can be included in the query results. SQL does not require that the matching columns be included in the results of a multitable query. They are often omitted in practice, as in the two preceding examples. That's because primary keys and foreign keys are often id numbers (such as the office numbers and employee numbers in the examples), which humans find hard to remember, while the associated names (cities, regions, names, titles) are easier to understand. It's quite common for id numbers to be used in the WHERE clause to join two tables, and for more descriptive names to be specified in the SELECT clause to generate columns of query results.
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Joins with Row Selection Criteria
The search condition that specifies the matching columns in a multi-table query can be combined with other search conditions to further restrict the contents of the query results. Suppose you want to rerun the preceding query, showing only offices with large sales targets: List the offices with a target over $600,000. SELECT CITY, NAME, TITLE FROM OFFICES, SALESREPS WHERE MGR = EMPL_NUM AND TARGET > 600000.00 CITY ----------Chicago Los Angeles NAME ----------Bob Smith Larry Fitch TITLE --------Sales Mgr Sales Mgr
With the additional search condition, the rows that appear in the query results are further restricted. The first test (MGR=EMPL_NUM) selects only pairs of OFFICES and SALESREPS rows that have the proper parent/child relationship; the second test further selects only those pairs of rows where the office is above target.
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