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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 . . .
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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. The alternative form of the query specifies the matching columns in the same way:
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SELECT FROM ON AND ORDER_NUM, AMOUNT, DESCRIPTION ORDERS JOIN PRODUCTS MFR = MFR_ID PRODUCT = PRODUCT_ID;
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Multicolumn joins 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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Often the matching column or columns that will be used to join two tables have the same name in both tables. This isn t true in the sample database, where the primary keys and related foreign keys have been given slightly different names so that they can be easily distinguished in our examples. But in practice, a database creator will often use the same name for a column that contains a customer ID or an employee number across all of the tables that contain such data. Suppose that the manufacturer ID and product ID were called MFR and PRODUCT in both the ORDERS table and the PRODUCTS table in the sample database. If that were true, then the most natural join between the two tables would be an equi-join based on all of the column names that appear in both tables. Such a join is, in fact, called a natural join in SQL standard. The standard join syntax allows you to easily indicate that you want a natural join:
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SELECT ORDER_NUM, AMOUNT, DESCRIPTION FROM ORDERS NATURAL JOIN PRODUCTS;
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This statement tells the DBMS to join the ORDERS and PRODUCTS tables on all of the columns that have the same names in both tables. In this example, that would be the MFR and PRODUCT columns. You can also explicitly name the columns to be matched in this situation with this alternative form of the join specification:
SELECT ORDER_NUM, AMOUNT, DESCRIPTION FROM ORDERS JOIN PRODUCTS USING (MFR, PRODUCT);
The columns to be matched for the join (which have the same name in both tables) are listed, enclosed in parentheses, in the USING clause. Note that the USING clause is a more compact alternative to the ON clause, but the preceding query would be completely equivalent to this one (still assuming that the columns have the same names in both tables):
SELECT FROM ON AND ORDER_NUM, AMOUNT, DESCRIPTION ORDERS JOIN PRODUCTS ORDERS.MFR = PRODUCTS.MFR ORDERS.PRODUCT = PRODUCTS.PRODUCT;
In many cases, the form of the join with the USING clause is preferable to specifying an explicit NATURAL JOIN. If two different administrators are responsible for maintaining the ORDERS and PRODUCTS table, for example (completely plausible in a large production database), it s possible that they might both accidentally choose the same name for a new column to be added to their table, even though the columns have nothing to do with one another. In this situation, the NATURAL JOIN form of the statement would pick up the new columns with matching names and attempt to use them when joining the tables, probably resulting in an error. The USING clause insulates the query from this type of accidental consequence of database structure changes. In addition, the USING clause allows you to select which individual columns are used to join the tables, while the NATURAL JOIN automatically uses all columns with matching names. Finally, if there are no matching column names, a query using the NATURAL JOIN might return a Cartesian product (described later in this chapter), or it might return an error, depending on the DBMS; however, a query formed with the USING clause will always return an error if the named columns do not appear in both tables.
7:
Multitable Queries (Joins)
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