What makes a primary key

Remember, you cannot add more than one null value to a unique key column since the second null value will be the duplicate of the first one — and duplicates are not allowed.

Despite a few basic similarities, primary and unique keys have significant differences. You can only have one primary key per table, but multiple unique keys. And finally, the primary key column has a unique clustered index while a unique key column has a unique non-clustered index. CodingSight is open for new authors and partnership proposals. Send your offers and ideas to marketing devart.

Search Menu. Primary Key A primary key is used to uniquely identify each record in the table. Unique Key A column with a unique key constraint can only contain unique values — this is the result of the unique key prohibiting duplicate values to be stored inside a column.

Similarities between Primary and Unique Keys Columns with primary or unique keys cannot have duplicate values.

Report a Bug. Previous Prev. Next Continue. Home Testing Expand child menu Expand. SAP Expand child menu Expand. Web Expand child menu Expand. Must Learn Expand child menu Expand. Big Data Expand child menu Expand. For example, combining the city and an incremented number e. In some cases, you want to use two or more fields in a table as the primary key.

A key that has more than one field is called a composite key. For a primary key to work well, the field must uniquely identify each row, never contain an empty or null value, and rarely ideally, never change. To set the primary key:. In the Navigation Pane, right click the table in which you want to set the primary key and, on the shortcut menu, click Design View.

Tip: If you don't see the Navigation Pane, press F11 to display it. To select more than one field to create a composite key, hold down CTRL and then click the row selector for each field. On the Design tab, in the Tools group, click Primary Key. When you remove the primary key, the field or fields that previously served as primary key will no longer provide the primary means of identifying a record.

Before you can remove a primary key, you must make sure that it doesn't participate in any table relationships. If you try to remove a primary key that is part of one or more relationships, Access warns you that you must delete the relationships first. To delete a table relationship, complete the following steps:. If the tables that participate in the table relationship are open, close them.

You cannot delete a table relationship between open tables. On the Database Tools tab, in the Relationships group, click Relationships. Click the table relationship line for the table relationship that you want to delete the line becomes bold when it is selected , and then press the DELETE key.

On the Design tab, in the Relationships group, click Close. After you delete the relationships, in the Navigation Pane, right click the table from which you want to remove the primary key and then click Design View.

If the primary key consists of multiple fields, click the row selector for any field in the primary key. The key indicator is removed from the field or fields that you previously specified as the primary key. Note: When you save a new table without setting a primary key, Access prompts you to create one. Click on the Add button to add a key. Here, you can configure a primary key column, name, clustered index, etc.

Save the table and the primary keys will be created. C ASP. Skill Tests ASP.