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Join Considerations

Several things should be considered before using joins. Some considerations include what columns(s) to join on, whether there is no common column to join on, and performance issues. More joins in a query means the database server has to do more work, which means that more time is taken to retrieve data. Joins cannot be avoided when retrieving data from a normalized database, but it is imperative to ensure that joins are performed correctly from a logical standpoint. Incorrect joins can result in serious performance degradation and inaccurate query results. Performance issues are discussed in more detail in Hour 18, "Managing Database Users."

Using a BASE TABLE

What to join on? Should you have the need to retrieve data from two tables that do not have a common column to join, you must use another table that has a common column or columns to both tables to join on. That table becomes the BASE TABLE. A BASE TABLE is used to join one or more tables that have common columns, or to join tables that do not have common columns. Use the following three tables for an example of a base table:

CUSTOMER_TBL

CUST_ID

VARCHAR(10)

NOT NULL

primary key

CUST_NAME

VARCHAR(30)

NOT NULL

 

CUST_ADDRESS

VARCHAR(20)

NOT NULL

 

CUST_CITY

VARCHAR(15)

NOT NULL

 

CUST_STATE

VARCHAR(2)

NOT NULL

 

CUST_ZIP

INTEGER(5)

NOT NULL

 

CUST_PHONE

INYEGER(10)

 

 

CUST_FAX INTEGER(10)

 

 

ORDERS_TBL

ORD_NUM

VARCHAR(10)

NOT NULL

primary key

CUST_ID

VARCHAR(10)

NOT NULL

 

PROD_ID

VARCHAR(10)

NOT NULL

 

QTY

INTEGER(6)

NOT NULL

 

ORD_DATE DATETIME

 

 

PRODUCTS_TBL

PROD_ID

VARCHAR(10)

NOT NULL

primary key

PROD_DESC

VARCHAR(40)

NOT NULL

 

COST DECIMAL(6,2) NOT NULL

 


You have a need to use the CUSTOMERS_TBL and the PRODUCTS_TBL. There is no common column in which to join the tables. Now look at the ORDERS_TBL. The ORDERS_TBL has CUST_ID to join with CUSTOMERS_TBL, which also has CUST_ID. The PRODUCTS_TBL has PROD_ID, which is also in ORDERS_TBL. The JOIN conditions and results look like the following:

SELECT C.CUST_NAME, P.PROD_DESC
FROM CUSTOMER_TBL C,
   PRODUCTS_TBL P,
   ORDERS_TBL O
WHERE C.CUST_ID = O.CUST_ID
 AND P.PROD_ID = O.PROD_ID;
CUST_NAME                      PROD_DESC
------------------------------ -----------------------
LESLIE GLEASON                 WITCHES COSTUME
SCHYLERS NOVELTIES             PLASTIC PUMPKIN 18 INCH
WENDY WOLF                     PLASTIC PUMPKIN 18 INCH
GAVINS PLACE                   LIGHTED LANTERNS
SCOTTYS MARKET                 FALSE PARAFFIN TEETH
ANDYS CANDIES                  KEY CHAIN


6 rows selected.

NOTE

Note the use of table aliases and their use on the columns in the WHERE clause.

The Cartesian Product

The Cartesian product is a result of a CARTESIAN JOIN or "no join." If you select from two or more tables and do not JOIN the tables, your output is all possible rows from all the tables selected from. If your tables were large, the result could be hundreds of thousands, or even millions, of rows of data. A WHERE clause is highly recommended for SQL statements retrieving data from two or more tables. The Cartesian product is also known as a cross join.

The syntax is

FROM TABLE1, TABLE2 [, TABLE3 ]
WHERE TABLE1, TABLE2 [, TABLE3 ]

The following is an example of a cross join, or the dreaded Cartesian product:

SELECT E.EMP_ID, E.LAST_NAME, P.POSITION
FROM EMPLOYEE_TBL E,
   EMPLOYEE_PAY_TBL P;
EMP_ID    LAST_NAM POSITION
--------- -------- --------------
311549902 STEPHENS MARKETING
442346889 PLEW     MARKETING
213764555 GLASS    MARKETING
313782439 GLASS    MARKETING
220984332 WALLACE  MARKETING
443679012 SPURGEON MARKETING
311549902 STEPHENS TEAM LEADER
442346889 PLEW     TEAM LEADER
213764555 GLASS    TEAM LEADER
313782439 GLASS    TEAM LEADER
220984332 WALLACE  TEAM LEADER
443679012 SPURGEON TEAM LEADER
311549902 STEPHENS SALES MANAGER
442346889 PLEW     SALES MANAGER
213764555 GLASS    SALES MANAGER
313782439 GLASS    SALES MANAGER
220984332 WALLACE  SALES MANAGER
443679012 SPURGEON SALES MANAGER
311549902 STEPHENS SALESMAN
442346889 PLEW     SALESMAN
213764555 GLASS    SALESMAN
313782439 GLASS    SALESMAN
220984332 WALLACE  SALESMAN
443679012 SPURGEON SALESMAN
311549902 STEPHENS SHIPPER
442346889 PLEW     SHIPPER
213764555 GLASS    SHIPPER
313782439 GLASS    SHIPPER
220984332 WALLACE  SHIPPER
443679012 SPURGEON SHIPPER
311549902 STEPHENS SHIPPER
442346889 PLEW     SHIPPER
213764555 GLASS    SHIPPER
313782439 GLASS    SHIPPER
220984332 WALLACE  SHIPPER
443679012 SPURGEON SHIPPER


36 rows selected.

Data is being selected from two separate tables, yet no JOIN operation is performed. Because you have not specified how to join rows in the first table with rows in the second table, the database server pairs every row in the first table with every row in the second table. Because each table has 6 rows of data each, the product of 36 rows selected is achieved from 6 rows multiplied by 6 rows.

To fully understand exactly how the Cartesian product is derived, study the following example.

SQL> SELECT X FROM TABLE1;
X
-
A
B
C
D


4 rows selected.
SQL> SELECT V FROM TABLE2;
X
-
A
B
C
D


4 rows selected.
SQL> SELECT TABLE1.X, TABLE2.X
 2* FROM TABLE1, TABLE2;
X X
- -
A A
B A
C A
D A
A B
B B
C B
D B
A C
B C
C C
D C
A D
B D
C D
D D
 
16 rows selected.

CAUTION

Be careful to always join all tables in a query. If two tables in a query have not been joined and each table contains 1,000 rows of data, the Cartesian product consists of 1,000 rows multiplied by 1,000 rows, which results in a total of 1,000,000 rows of data returned. Cartesian products, when dealing with large amounts of data, can cause the host computer to stall or crash in some cases based on resource usage on the host computer. Therefore, it is important for the DBA and system administrator to closely monitor for long-running queries.

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