SQL PRIMARY KEY Constraint

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

In simple terms:
A PRIMARY KEY is a column (or combination of columns) whose values must be unique and cannot be NULL.
Each table can have only one PRIMARY KEY.

Example: Orders Table

CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    CustomerName VARCHAR(100),
    OrderDate DATE,
    Amount DECIMAL(10,2)
);

Example: Composite PRIMARY KEY (Multiple Columns)

Sometimes a primary key is created using multiple columns.

CREATE TABLE OrderDetails (
    OrderID INT,
    ProductID INT,
    Quantity INT,
    PRIMARY KEY (OrderID, ProductID)
);

Simple Definition
The PRIMARY KEY constraint uniquely identifies each record in a table. It does not allow duplicate values or NULL values and ensures each row can be uniquely identified.

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