Create Primary Keys in SQL Server - SQL Server (2024)

FAQs

How to create a primary key in SQL Server? ›

You can't add 2 primary key in a table. Instead u can add Unique key which accept null value in it. But if the OP wants it to act like a primary key, you would want to ensure that the column is not nullable as a PK won't accept a NULL value.

Can a varchar be a primary key in SQL? Yes, however it's generally best to avoid variable length data types for keys,..

Because a table can have only one primary key, you cannot add a primary key to a table that already has a primary key defined. To change the primary key of a table, delete the existing key using a DROP clause in an ALTER TABLE statement and add the new primary key.

SQL PRIMARY KEY Syntax

The syntax of the SQL PRIMARY KEY constraint is: CREATE TABLE table_name ( column1 data_type, ..., [CONSTRAINT constraint_name] PRIMARY KEY (column1) ); Here, table_name is the name of the table to be created.

Each table can only have one primary key. Access can automatically create a primary key field for you when you create a table, or you can specify the fields that you want to use as the primary key.

Ideally, every table should have a Primary Key. It's very important that a table has a Primary Key. There are two good reasons why a table should have a Primary Key. First, a Primary Key uniquely identifies each record in a table so it helps to ensure against redundant data in that table.

SQL primary key for multiple columns:

CREATE TABLE table_name ( column1 INT NOT NULL, column2 INT NOT NULL, column3 INT NOT NULL, PRIMARY KEY (column1, column2, column3) ); This creates a table with three columns, all of which are integers and are not allowed to be null.

You should always use varchar2 for variable length strings. It doesn't matter whether it's the primary key or not. If the string uniquely identifies a row in the table, is always available and it's stable then you could use it at the primary key.

Do primary keys need to be numeric? ›

The data type of a primary key should be a numeric, an integer or a short, fixed-width character. Uniqueness. A primary key must have a unique value, which implies that no other row in the table has the same value in the column.

We can use primary and unique key constraints. Scope of a temporary table is local and global as defined while creating it. Scope of the table variable is within the batch. We cannot use it outside the batch.

There can be one or many unique keys, while there can only be one primary key. Both types of key can exist within the same table. Unique keys are used when we want to identify specific items, and do not want any duplicate values.

Yes. You can have same column name as primary key in multiple tables. Column names should be unique within a table. A table can have only one primary key, as it defines the Entity integrity.

You can modify a primary key in SQL Server by using SQL Server Management Studio or Transact-SQL. You can modify the primary key of a table by changing the column order, index name, clustered option, or fill factor.

A simple example of a primary key in SQL is the id column in a table of customer records. Each customer record has a unique id value, and no two customer records can have the same id value.

Add Primary key to the Existing Table. If you want to add a primary key in the existing table, you have to use the following ALTER syntax in SQL: ALTER TABLE Table_Name ADD CONSTRAINT Constraint_Name PRIMARY KEY (Column_Name);

The main difference between a primary key vs unique key is that a primary key is a key that uniquely identifies each record in a table but cannot store NULL values. In contrast, a unique key prevents duplicate values in a column and can store NULL values.