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OCA Oracle Database 11g: SQL Fundamentals I: A Real-World Certification Guide by Steve Ries

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Chapter 8. Combining Queries

In Chapter 5, Combining Data from Multiple Tables, we learned a powerful method of combining data from multiple tables in the form of table joins. These joins commonly retrieve data from two or more tables by linking them through a common column. However, it is also important to be able to combine data in other ways. What if you are required to combine the data from two tables that have no columns in common? What if you need to read a single table more than once, combining the results of each iteration? For problems such as these, we can make use of another technique—the subquery. Subqueries do not require any new clauses in order to function. Rather, they take the existing clauses you have already learned and use ...

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