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Indexes can contain multiple rows with the same value in an indexed field. For example, a Lineitem table can contain multiple entries with the same ORDER_NO or STOCK_NO.

In certain cases, however, you might want to have a unique index, which finds only the first occurrence of a value in the indexed field and ignores subsequent rows with the same value. This kind of index is useful when subsequent rows repeat information in the first row.

For example, in a Lineitem table, if all products with the same STOCK_NO were sold at the same price, you could use a unique index to hide duplicate index values, so that only the first row with the price would appear.

If you check Include Unique Key Values Only in the Define Index dialog box, only the first row with a duplicate value in the indexed field is included in the index. Subsequent rows with duplicate values in that field are excluded.

Note

In dBASE and Paradox indexes, if there is a primary or distinct index, rows may not have duplicate values in the indexed field. Duplicate values cause an error when trying to save. In SQL indexes, uniqueness is required if the index is defined as a unique index.