Page 116
within the object is related, the developer does not need to worry about join conditions. Of course, if the query is combining the data from two or more objects, the same join techniques outlined in this chapter may be used to combine attributes from the objects.
The new Oracle8 object technology has the ability to reduce the complexity of many queries, but as long as the world is comprised of different objects, the developer will still need to use the techniques outlined in this chapter to combine data into information.
This chapter covered combining rows from different tables through the use of a join. Multiple tables are included in the from clause, and the join condition is recorded in the where clause. When the tables contain columns with the same name, you must qualify the columns with the name of the table. Performance problems and unexpected results can occur when using joins. You can use the outer join symbol (+) to select a record that does not have a matching value in the other table. You place the outer join symbol after the column name in the where clause that does not contain the value.
The set operators Union, Minus and Intersect, are used to combine records from multiple tables. Union combines records from the tables discarding any duplicated records. The Minus operator selects records from the first table that do not exist in the second. The Intersect operator selects records that exist in both tables.
The next chapter discusses how to set up the database. The chapter covers the methods of creating, altering, and dropping the various objects. It also covers the attributes and settings of the various objects. It discusses database constraints used to maintain database integrity. In the following chapter, you will learn how to set up the user account, tablespace, tables, and indexes.
Page 117
Page 118