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Accessing Multiple Tables
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You can, if you want, join all three tables in a single SELECT statement, as shown in the following example:
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SELECT FROM WHERE AND AND c.CD_NAME, p.PERF_NAME, t.TYPE_NAME CD_INVENTORY c, PERFORMERS p, PERF_TYPE t c.PERF_ID = p.PERF_ID p.TYPE_ID = t.TYPE_ID TYPE_NAME = 'Popular';
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In this statement, the FROM clause includes all three tables. In addition, the WHERE clause provides two equi-join conditions: one that maps the PERF_ID columns and one that maps the TYPE_ID columns. If you execute this statement, you ll receive the following query results:
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CD_NAME ----------------------Blue Court and Spark Fundamental Longing in Their Hearts Luck of the Draw Nick of Time Both Sides Now PERF_NAME ------------Joni Mitchell Joni Mitchell Bonnie Raitt Bonnie Raitt Bonnie Raitt Bonnie Raitt Joni Mitchell TYPE_NAME --------Popular Popular Popular Popular Popular Popular Popular
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Notice that information from all three tables is included in the results: the name of the CD, the name of the performer, and the category of performer. Even though a relationship might exist between the CD_INVENTORY table and the PERFORMERS table, as well as between the PERFORMERS table and the PERF_TYPE table, your query results provide a seamless display that hides these relationships and shows only the information that you need.
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Creating the Cross Join
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In addition to the comma-separated join, SQL supports another type of operation called the cross join. The cross join is nearly identical to the comma-separated join. The only difference is that, instead of separating column names with a comma, you use the CROSS JOIN keywords. For example, let s take a statement we used earlier and modify it by replacing the comma with the CROSS JOIN keywords:
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SELECT FROM WHERE AND c.CD_NAME, p.PERF_NAME, c.IN_STOCK CD_INVENTORY c CROSS JOIN PERFORMERS p c.PERF_ID = p.PERF_ID c.IN_STOCK < 15;
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This statement returns three columns from two tables, and the WHERE clause contains an equi-join condition. If you execute the statement, you ll receive the same results as if you were using a comma-separated join. Using one over the other may simply be a matter of determining which statement your SQL implementation supports and, if both are supported, which provides better performance. In all likelihood, it will come down to a matter of personal preference, with little advantage of one over the other.
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SQL: A Beginner s Guide
Ask the Expert
Q: A:
If you re joining tables, it seems likely that in some cases you will return duplicate rows in your query results, depending on how your SELECT statement is constructed. How can you avoid duplicate rows As with most queries, it is possible to return duplicate rows. For example, the following statement will return duplicate performer names and types:
SELECT FROM WHERE AND P.PERF_NAME, T.TYPE_NAME CD_INVENTORY C, PERFORMERS P, PERF_TYPE T C.PERF_ID = P.PERF_ID P.TYPE_ID = T.TYPE_ID;
For those performers who made more than one CD, the query results will contain a row for each of those CDs. However, as with any other SELECT statement, you can add the DISTINCT keyword to your SELECT clause, as shown in the following example:
SELECT FROM WHERE AND DISTINCT P.PERF_NAME, T.TYPE_NAME CD_INVENTORY C, PERFORMERS P, PERF_TYPE T C.PERF_ID = P.PERF_ID P.TYPE_ID = T.TYPE_ID;
This statement will return fewer rows than the previous statement (5 compared to 11), and no rows will be duplicated. Also note that you can achieve the same results without the DISTINCT keyword by using a GROUP BY clause that lists both columns.
Creating the Self-Join
Another type of join that you can create is the self-join, which can be either a commaseparated join or a cross join. In a self-join, you create a join condition that references the same table two times, essentially joining the table to itself. This is almost always done to resolve a recursive relationship, finding other rows in the same table that are related to the selected rows. For example, suppose you add an EMPLOYEES table to your database, as shown in Figure 11-2. The EMPLOYEES table includes a list of employee IDs, employee names, and the employee IDs of their managers, who are also listed in the table. For example, the manager of Mr. Jones (EMP_ID 102) is Ms. Smith (EMP_ID 101). To create a self-join on this table, you must create a join that treats the table as if it was two separate tables with the same name, same columns, and same data:
SELECT FROM WHERE ORDER a.EMP_ID, a.EMP_NAME, b.EMP_NAME AS MANAGER EMPLOYEES a, EMPLOYEES b a.MNGR = b.EMP_ID BY a.EMP_ID;
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