Specifying the cardinality of a join

You can specify the right-to-left and left-to-right cardinality of a join. Table 7-10 lists the cardinality types and a description of each type.

Table 7-10

Cardinality types

Cardinality type	Description
1	One record in the first table matches one record in the second table.
?	One record in the first table matches zero or one record in the second table.
*	One record in the first table matches zero or more records in the second table.
+	One record in the first table matches one or more records in the second table.

The right-to-left cardinality type is followed by a hyphen (-), and then by the left-to-right cardinality type. The cardinality type depends on the join column.

For example:

Customers JOIN Orders ON (Customers.custid = Orders.custid) {CARDINALITY ('1-+')}

indicates that:

One record in Orders matches one record in Customers.

One record in Customers matches one or more records in Orders:

Customers JOIN Orders ON (Customers.custid = Orders.custid) {CARDINALITY ('1-*')}

indicates that:

One record in Orders matches one record in Customers.

One record in Customers matches zero or more records in Orders:

Customers JOIN Orders ON (Customers.custid = Orders.custid) {CARDINALITY ('*-?')}

indicates that:

One record in Orders matches zero or more records in Customers.

One record in Customers matches zero or one record in Orders.

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