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Non-Equi-Joins
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The term join applies to any query that combines data from two tables by comparing the values in a pair of columns from the tables. Although joins based on equality between matching columns (equi-joins) are by far the most common joins, SQL also allows you to join tables based on other comparison operators. Here s an example where a greater-than (>) comparison test is used as the basis for a join: List all combinations of salespeople and offices where the salesperson s quota is more than that office s target, regardless of whether the salesperson works there.
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SELECT NAME, QUOTA, CITY, TARGET FROM SALESREPS, OFFICES WHERE QUOTA > TARGET; NAME QUOTA CITY TARGET ------------ ------------ ------- -----------Bill Adams $350,000.00 Denver $300,000.00 Sue Smith $350,000.00 Denver $300,000.00 Larry Fitch $350,000.00 Denver $300,000.00
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As in all two-table queries, each row of the query results comes from a pair of rows, in this case from the SALESREPS and OFFICES tables. The search condition
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QUOTA > TARGET
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selects pairs of rows where the QUOTA column from the SALESREPS row exceeds the TARGET column from the OFFICES row. Note that the pairs of SALESREPS and OFFICES rows selected are related only in this way; it is specifically not required that the SALESREPS row represent someone who works in the office represented by the OFFICES row. Admittedly, the example is a bit farfetched, and it illustrates why joins based on inequalities are not very common. However, they can be useful in decision-support applications and other applications that explore more complex interrelationships in the database.
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SQL Considerations for Multitable Queries
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The multitable queries described thus far have not required any special SQL syntax or language features beyond those described for single-table queries. However, some multitable queries cannot be expressed without some additional SQL features. Specifically: Qualified column names are sometimes needed in multitable queries to eliminate ambiguous column references. All-column selections (SELECT *) have a special meaning for multitable queries. Self-joins can be used to create a multitable query that relates a table to itself. Table aliases can be used in the FROM clause to simplify qualified column names and to allow unambiguous column references in self-joins.
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Multitable Queries (Joins)
Qualified Column Names
The sample database includes several instances where two tables contain columns with the same name. The OFFICES table and the SALESREPS table, for example, both have a column named SALES. The column in the OFFICES table contains year-to-date sales for each office; the one in the SALESREPS table contains year-to-date sales for each salesperson. Normally, there is no confusion between the two columns, because the FROM clause determines which of them is appropriate in any given query, as in these examples: Show the cities where sales exceed target.
SELECT CITY, SALES FROM OFFICES WHERE SALES > TARGET;
PART II
Show all salespeople with sales over $350,000.
SELECT NAME, SALES FROM SALESREPS WHERE SALES > 350000.00;
However, here is a query where the duplicate names cause a problem: Show the name, sales, and office for each salesperson.
SELECT NAME, SALES, CITY FROM SALESREPS, OFFICES WHERE REP_OFFICE = OFFICE; Error: Ambiguous column name "SALES"
Although the English description of the query implies that you want the SALES column in the SALESREPS table, the SQL query is ambiguous. The DBMS has no way of knowing whether you want the SALES column from the SALESREPS table or the one from the OFFICES table, since both tables are contributing data to the query results. To eliminate the ambiguity, you must use a qualified column name to identify the column. Recall from 5 that a qualified column name specifies the name of a column and the table containing the column. The qualified names of the two SALES columns in the sample database are
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