Prevent duplicate values in a table field using an index
You can prevent duplicate values in a field in an Access table by creating a unique index. A unique index is an index that requires that each value of the indexed field is unique.
Create and use an index to improve performance
Access automatically creates a unique index for primary keys, but you might also want to prohibit duplicate values in other fields. For example, you can create a unique index on a field that stores serial numbers so that no two products have the same serial number.
UniqueValues Property - Microsoft Support
You can use the UniqueValues property when you want to omit records that contain duplicate data in the fields displayed in Datasheet view. For example, if a query's output includes more than one field, the combination of values from all fields must be unique for a given record to be included in the results.
ALL, DISTINCT, DISTINCTROW, TOP Predicates - Microsoft Support
Specifies records selected with SQL queries. Syntax. SELECT [ALL | DISTINCT | DISTINCTROW | [TOP n [PERCENT]]] FROM table. A SELECT statement containing these predicates has the following parts:
Create or modify tables or indexes by using a data-definition query
A constraint establishes a logical condition that a field or combination of fields must meet when values are inserted. For example, a UNIQUE constraint prevents the constrained field from accepting a value that would duplicate an existing value for the field.
Introduction to data types and field properties - Microsoft Support
A field's data type is the most important property because it determines what kind of data the field can store. This article describes the data types and other field properties available in Access, and includes additional information in a detailed data type reference section.
UniqueRecords Property - Microsoft Support
You can use the UniqueRecords property to specify whether to return only unique records based on all fields in the underlying data source, not just those fields present in the query itself. Note: The UniqueRecords property applies only to append and …
Guide to table relationships - Microsoft Support
Table relationships inform your query designs. To work with records from more than one table, you often must create a query that joins the tables. The query works by matching the values in the primary key field of the first table with a foreign key field in the second table.
Restrict data input by using validation rules - Microsoft Support
Add a validation rule to a control on a form. You can use the Validation Rule property and the Validation Text property of a form control to validate data that is input to that control and to help users who input data that is not valid.
UniqueValues Property - Microsoft Support
You can set the UniqueValues property in the query's property sheet or in SQL view of the Query window. Note: You can set this property when you create a new query by using an SQL statement. The DISTINCT predicate corresponds to the UniqueValues property setting.
Add records to a table by using an append query
In this article. Overview. Create and run an append query. Stop Disabled Mode from blocking a query. Overview. An append query selects records from one or more data sources and copies the selected records to an existing table.