what is primary key? how do we se primary key in MS access?
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A primary key is a field or set of fields with values that are unique throughout a table. Values of the key can be used to refer to entire records, because each record has a different value for the key. Each table can only have one primary key.
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A primary key is a field or set of fields with values that are unique throughout a table. Values of the key can be used to refer to entire records, because each record has a different value for the key. Each table can only have one primary key.
To set a table’s primary key, open the table in Design view. Select the field (or fields) that you want to use, and then on the ribbon, click Primary Key.
Overview of primary keys in Access
Access uses primary key fields to quickly associate data from multiple tables and combine that data in a meaningful way. You can include the primary key fields in other tables to refer back to the table that is the source of the primary key. In those other tables, the fields are called foreign keys. For example, a Customer ID field in the Customers table might also appear in the Orders table. In the Customers table, it is the primary key. In the Orders table it is called a foreign key. A foreign key, simply stated, is another table's primary key.
1. Primary key
2. Foreign key
If you are moving existing data into a database, you may already have a field that you can use as the primary key. Often, a unique identification number, such as an ID number or a serial number or code, serves as a primary key in a table. For example, you might have a Customers table where each customer has a unique customer ID number. The customer ID field is the primary key.
Access automatically creates an index for the primary key, which helps speed up queries and other operations. Access also ensures that every record has a value in the primary key field, and that it is always unique.
When you create a new table in Datasheet view, Access automatically creates a primary key for you and assigns it a field name of "ID" and the AutoNumber data type.
What makes a good primary key?
A good candidate for a primary key has several characteristics:
It uniquely identifies each row
It is never empty or null — it always contains a value
The values it contains rarely (ideally, never) change
If you can’t identify a good key, create an AutoNumber field to use as the key. An AutoNumber field automatically generates a value for itself when each record is first saved. Therefore, an AutoNumber field meets all three characteristics of a good primary key.
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