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A parent/child query with OFFICES and SALESREPS
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Here s another query involving the same two tables, but with the parent and child roles reversed, as shown in Figure 7-4. List the offices and the names and titles of their managers.
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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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A different parent/child query with OFFICES and SALESREPS
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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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The search condition that specifies the matching columns in a multitable 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:
7:
Multitable Queries (Joins)
List the offices with a target over $600,000.
SELECT FROM WHERE AND CITY, NAME, TITLE OFFICES, SALESREPS MGR = EMPL_NUM TARGET > 600000.00 NAME -----------Bob Smith Larry Fitch TITLE ---------Sales Mgr Sales Mgr
CITY -----------Chicago Los Angeles
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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.
Multiple Matching Columns
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 FROM WHERE AND ORDER_NUM, AMOUNT, DESCRIPTION ORDERS, PRODUCTS MFR = MFR_ID PRODUCT = PRODUCT_ID
ORDER_NUM AMOUNT DESCRIPTION ---------- ---------- ---------------113027 $4,104.00 Size 2 Widget 112992 $760.00 Size 2 Widget 113012 $3,745.00 Size 3 Widget 112968 $3,978.00 Size 4 Widget 112963 $3,276.00 Size 4 Widget 112983 $702.00 Size 4 Widget 113055 $150.00 Widget Adjuster 113057 $600.00 Widget Adjuster
SQL: The Complete Reference
. . .
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. Multicolumn 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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