Progress
Programming
Handbook
Joining Tables
When you read from multiple tables using a single statement, such as a FOR EACH or OPEN QUERY statement, Progress returns the results as a join of the tables. A join is a binary operation that selects and combines the records from multiple tables so that each result in the results list contains a single record from each table. That is, a single join operation combines the records of one table with those of another table or combines the records of one table with the results of a previous join. Figure 9–3 shows how you can join three tables.
Figure 9–3: Inner Joins
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A table or prior join can be either on the left- or right-hand side of a join operation. Thus, the results of joining the three tables in Figure 9–3 depends on two join operations—one join between Table1 (left-hand side) and Table2 (right-hand side) and one join between the first join (left-hand side) and Table3 (right-hand side). The relations C11 = C21 and C22 = C31 represent join conditions, conditions that determine how one table is related to the other (that is, which records selected from one table join with the records in the other table). How the records from joined tables are combined depends on the order of the tables in the join, the type of join operation used, and the selection criteria applied to each table.
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