![]() Cross-field validation: Certain conditions that utilize multiple fields must hold.For example, phone numbers may be required to have the pattern (999) 999–9999. Regular expression patterns: Occasionally, text fields will have to be validated this way.The term foreign key is borrowed from relational database terminology. For example, in a US taxpayer database, the "state" column is required to belong to one of the US's defined states or territories: the set of permissible states/territories is recorded in a separate State table. The set of values in a column is defined in a column of another table that contains unique values. Foreign-key constraints: This is the more general case of set membership.For example, a person's sex may be Female, Male or Non-Binary. Set-Membership constraints: The values for a column come from a set of discrete values or codes.For example, no two persons can have the same social security number. ![]()
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