Changing the Join Operator
The default logical operator for joins is 'AND'. However, there may be some instances where an 'OR' operator is appropriate. As an example, consider the following hypothetical case of a Sales schema where an order can be placed by a domestic customer or a foreign customer. The schema contains three tables:
• Orders
• Domestic Customers
• Foreign Customers
The CUSTOMER_ID field in the 'Orders' table can reference the CUSTOMER_ID field in either the 'Domestic Customers' or the 'Foreign Customers' table. In this case you need to a logical OR between the “Order -> Domestic Customers” join and the “Order -> Foreign Customers” join because they are mutually exclusive.

To specify the logical operator for a join, follow these steps:
1. Click on the desired join line to select the join.
2. Right-click the join, and select 'Properties' from the context menu. This opens the 'Join Properties' dialog box.

3. In the 'Join Properties' dialog box, click the Options tab.
4. Select the desired logical operator in the 'Merging Rule' section, and click 'OK'.

The 'Merging Rule' determines how the join is integrated together with other joins between the same tables. The 'And' option indicates that the join is applied conjunctively (i.e., the join condition must be satisfied even if another join condition is already satisfied); the 'Or' option indicates that the join is applied 'disjunctively' (i.e., the join condition need not be satisfied if another join condition is already satisfied).
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