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This article mainly introduces which indexes do not allow duplicate index values in the record, which can be used for reference and can be used for reference by friends who need it. I hope you will learn a lot after reading this article. Next, let the editor take you to learn about it.

The indexes that do not allow duplicate index values in the record are primary and candidate indexes. A primary index is an index that does not allow duplicate values in the specified index field live expression. Like the primary index, the candidate index 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 established for each table.

Indexes in Visual FoxPro can be divided into normal index, unique index, candidate index and primary index. Indexes that do not allow duplicate index values in a record are primary and candidate indexes.

Primary index (Primary Index)

Primary index (Primary Index): (also called primary keyword) is an index that does not allow duplicate values in the live expression of the specified index field.

The primary index is mainly used to establish referential integrity in the parent or referenced table of a permanent relationship, it ensures the uniqueness of the values of the input field, and the order in which records are processed is determined by this field. A database table can have only one primary index

If you specify a primary index in a field that already contains duplicate data, the database will return an error message, such as a primary index already exists in the table, and then the index can only be built with a candidate index or a normal 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, you can create multiple candidate indexes for each table.

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