Can a unique constraint be a primary constraint?
Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint. However, you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table. SQL UNIQUE Constraint on CREATE TABLE
How to create unique constraint on persons table?
The following SQL creates a UNIQUE constraint on the “ID” column when the “Persons” table is created: To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax: To create a UNIQUE constraint on the “ID” column when the table is already created, use the following SQL:
How are unique constraints referenced in SQL Server?
A UNIQUE constraint can be referenced by a FOREIGN KEY constraint. When a UNIQUE constraint is added to an existing column or columns in the table, by default, the Database Engine examines the existing data in the columns to make sure all values are unique.
Can a null value be entered in a unique constraint?
Unlike the PRIMARY KEY constraint, you can enter a NULL value for a column that has a UNIQUE constraint. However, only one NULL value is allowed. You use a UNIQUE constraint for a column or a combination of columns that is not the primary key of a table. You can create UNIQUE constraints using CREATE TABLE or ALTER TABLE statement.
How to add unique constraint to username column?
To add the UNIQUE constraint to the username column, you use the ALTER TABLE statement as follows: If you want to add a new column and create a UNIQUE constraint for it, you use the following form of the ALTER TABLE statement.
How to specify a constraint on multiple columns?
By using the constraint definition on table creation, you can specify one or multiple constraints that span multiple columns. The syntax, simplified from technet’s documentation, is in the form of: