# SQL PRIMARY KEY Constraint

### SQL PRIMARY KEY Constraint

The PRIMARY KEY constraint uniquely identifies each record in a table.

Primary keys must contain UNIQUE values, and cannot contain NULL values. Primary Key is a Unique key with a Not Null constraint.

A table can have only one primary key, which may consist of single or multiple fields.

### SQL PRIMARY KEY on CREATE TABLE

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

**Syntax:**

```sql
CREATE TABLE Persons (
    ID int PRIMARY KEY,
    LastName varchar(25),
    FirstName varchar(255),
    Age int
);
```

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

**Syntax:**

```sql
CREATE TABLE Persons (
    ID int,
    LastName varchar(255),
    FirstName varchar(255),
    Age int,
    CONSTRAINT PK_Person PRIMARY KEY (ID,LastName)
);
```

**Note:** In the example above there is only ONE PRIMARY KEY (PK\_Person). However, the VALUE of the primary key is made up of TWO COLUMNS (ID + LastName).

A table can have only one Primary Key.\
\
SQL PRIMARY KEY on ALTER TABLE

To create a PRIMARY KEY constraint on the "ID" column when the table is already created, use the following SQL:

**Syntax:**

```sql
ALTER TABLE Persons
ADD PRIMARY KEY (ID);
```

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

**Syntax:**

```sql
ALTER TABLE Persons
ADD CONSTRAINT PK_Person PRIMARY KEY (ID,LastName);
```

**Note:** If you use the ALTER TABLE statement to add a primary key, the primary key column(s) must already have been declared to not contain NULL values (when the table was first created).

### DROP a PRIMARY KEY Constraint

To drop a PRIMARY KEY constraint, use the following SQL:

\
**SQL Server / Oracle / MS Access:**

```sql
ALTER TABLE Persons
DROP CONSTRAINT PK_Person;
```

\
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