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An Alternative Way to Specify Joins
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The simplest way to specify the tables to be joined in a multitable query is to name them in a comma-separated list, in the FROM clause of the SELECT statement, as shown in the previous examples. This method for specifying joined tables appeared in the earliest IBM SQL implementations. It was included in the original SQL standard and is supported by all SQL-based databases. Subsequent versions of the standard significantly expanded the join capability and added new options to the FROM clause. Using the newer form, the previous two query examples could also be written like this: List each salesperson and the city and region where they work.
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SELECT NAME, CITY, REGION FROM SALESREPS JOIN OFFICES ON 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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List the offices and the names and titles of their managers.
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SELECT CITY, NAME, TITLE FROM OFFICES JOIN SALESREPS ON 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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Instead of a comma-separated list of table names, the FROM clause in these examples uses the JOIN keyword to specifically describe the join operation. Also, the matching columns to be used in the join are specified in the ON clause, which occurs at the end of the FROM clause. For these simple examples, the expanded standard SQL syntax adds very little to the older form of the SELECT statement. But the range of joins that can be expressed using the expanded form is much broader, as described in later sections of this chapter. Many of the major DBMS vendors are encouraging the use of this new JOIN clause for that reason.
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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: List the offices with a target over $600,000 and their manager information.
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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
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 target is above $600,000. In this form of the query, the matching condition for the join and the search condition that restricts which offices are selected both appear in the WHERE clause. Using the newer standard SQL syntax, the matching condition appears in the ON clause, and the search condition appears in the WHERE clause, which makes the query slightly easier to understand:
7:
Multitable Queries (Joins)
List the offices with a target over $600,000 and their manager information.
SELECT FROM ON WHERE CITY, NAME, TITLE OFFICES JOIN SALESREPS MGR = EMPL_NUM TARGET > 600000.00; NAME -----------Bob Smith Larry Fitch TITLE ---------Sales Mgr Sales Mgr
CITY -----------Chicago Los Angeles
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;
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