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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