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# SQL UNIQUE Constraint

### 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.

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

The following SQL creates a UNIQUE constraint on the "ID" column when the "Persons" table is created:

**Syntax:**

```sql
CREATE TABLE Persons (
    ID int UNIQUE,
    LastName varchar(255),
    FirstName varchar(255),
    Age int
);
```

To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:

**Syntax:**

```sql
CREATE TABLE Persons (
    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:

**Syntax:**

```sql
ALTER TABLE Persons
ADD UNIQUE (ID);
```

To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:

**Syntax:**

```sql
ALTER TABLE Persons
ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);
```

\
**DROP a UNIQUE Constraint**

To drop a UNIQUE constraint, use the following SQL:

**Syntax:**

```sql
ALTER TABLE Persons
DROP INDEX UC_Person;
```

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