The primary key contains unique values and identifies each row in a table. The primary key cannot contain NULL values. A table can have only one primary key and this primary key can consist of single or multiple columns. When a primary key consists of multiple columns, the data from these columns is used to determine whether a row is unique.
Create a primary key
In the Database tool window ( ), expand the data source tree until the node of a child table.
Right-click a child table and select.
On the Columns tab, click the Add button () and type the column name.
Select the column settings:
Not null: add the NOT NULL constraint that enforces a column to not accept NULL values.
Auto inc: automatically generate a unique number when a new record is inserted into a table.
Unique: ensure that all values in a column are different.
Primary key: identify a column as a primary key.