What index does not allow duplicate index values ​​in records?

The indexes that do not allow duplicate index values ​​in records are: primary index and candidate index. A primary index is an index that does not allow duplicate values ​​in specified index fields or expressions. Like the primary index, candidate indexes require the uniqueness of field values ​​and determine the order in which records are processed; in databases and free tables, multiple candidate indexes can be established for each table.

Indexes in Visual FoxPro can be divided into: ordinary indexes, unique indexes, candidate indexes and primary indexes. Indexes that do not allow duplicate index values ​​in records are primary indexes and candidate indexes.

Primary Index

Primary Index: (also called primary keyword) refers to the expression in the specified index field Indexes with duplicate values ​​are not allowed in the formula.

The primary index is mainly used to establish referential integrity in the parent table or referenced table of a permanent relationship. It can ensure the uniqueness of the input field value, and the field determines the order in which records are processed. A database table can only have one primary index

If you specify a primary index in a field that already contains duplicate data, the database will return an error message. If there is already a primary index in the table, you can only use Candidate index or ordinary index

Candidate index

Like the primary index, it requires the uniqueness of field values ​​and determines the order in which records are processed. In databases and free tables, multiple candidate indexes can be created for each table.

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