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SQL UNIQUE Constraint
The UNIQUE constraint ensures that all values in a column are different.
Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns.
I see that SQL Server offers globally unique identifiers (GUIDs) as well as identities to create these valu I am designing a table and I have decided to create an auto-generated primary key value as opposed to creating my own scheme or using natural keys. SQL - Primary Key. 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. A table can have only one primary key, which may consist of single or multiple fields. Generating a unique primary key. I have a SQL Server database that will contain many tables that all connect, each with a primary key. I have a Dictionary that keeps track of the the primary keys fields are for each table. My task is to extract data every day from attribute-centric XML files and insert them into a master database.
A PRIMARY KEY constraint automatically has a UNIQUE constraint.
- The first is PRIMARY KEY, which as the name suggests, forces the specified column to behave as a completely unique index for the table, allowing for rapid searching and queries. While SQL Server only allows one PRIMARY KEY constraint assigned to a single table, that PRIMARY KEY can be defined for more than one column.
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However, you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.
SQL UNIQUE Constraint on CREATE TABLE
The following SQL creates a UNIQUE constraint on the 'ID' column when the 'Persons' table is created:
SQL Server / Oracle / MS Access:
ID int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int
);
MySQL:
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
UNIQUE (ID)
);
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To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:
Generate Unique Key In C To Sql Tutorial
MySQL / SQL Server / Oracle / MS Access:
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
CONSTRAINT UC_Person UNIQUE (ID,LastName)
);
SQL UNIQUE Constraint on ALTER TABLE
To create a UNIQUE constraint on the 'ID' column when the table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
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To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:
Sql Add Unique Key
MySQL / SQL Server / Oracle / MS Access:
ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);
DROP a UNIQUE Constraint
To drop a UNIQUE constraint, use the following SQL:
MySQL:
SQL Server / Oracle / MS Access: