例如，当MQT中的两个或多个表定义了引用完整性关系并且联结谓词在它们自己的主键-外键列上时，就会出现这种情况。
For example, this might happen when two ore more of the tables in the MQT have a referential integrity relationship defined and the join predicate is on their primary key - foreign key columns.
外键约束允许您定义父表和子表之间的引用完整性。
A Foreign key constraint allows you to define referential integrity between a parent and child table.
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