Join Considerations
Several things should be considered before using joins. Some considerations include what columns(s) to join on, if there is no common column to join on, and performance issues. Performance issues are discussed 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 | VARCHAR2(10) ... |
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