Summary: in this tutorial, you will learn how to use SQLite PRIMARY KEY constraint to define a primary key for a table. Introduction to SQLite primary key. A primary key is a column or group of columns used to identify the uniqueness of rows in a table. Each table has one and only one primary key. Aug 31, 2016 When creating a table in Microsoft Access it is important that the table is correctly setup with a primary key that auto-increments. Without this the table will be difficult to input and output data. Notes: Only one field in a table can be the primary key and/or auto-increment.
A primary key is a field in a table which uniquely identifies each row/record in a database table. Primary keys must contain unique values. A primary key column cannot have NULL values.
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A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key.
If a table has a primary key defined on any field(s), then you cannot have two records having the same value of that field(s).
Note − You would use these concepts while creating database tables.
Create Primary Key
Here is the syntax to define the ID attribute as a primary key in a CUSTOMERS table.
To create a PRIMARY KEY constraint on the 'ID' column when the CUSTOMERS table already exists, use the following SQL syntax −
NOTE − If you use the ALTER TABLE statement to add a primary key, the primary key column(s) should have already been declared to not contain NULL values (when the table was first created).
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For defining a PRIMARY KEY constraint on multiple columns, use the SQL syntax given below.
To create a PRIMARY KEY constraint on the 'ID' and 'NAMES' columns when CUSTOMERS table already exists, use the following SQL syntax.
Delete Primary Key
You can clear the primary key constraints from the table with the syntax given below.
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December 2020
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