SQL- What Is The Difference Between A Primary Key And Other Keys?

It seems your question was cut off. I assume you’re asking about the difference between a primary key and some other concept in SQL, such as a foreign key, unique key, or perhaps an index. I’ll explain the difference between a primary key and some of the most common related concepts.

1. Primary Key vs Foreign Key

• Primary Key:
  • A primary key uniquely identifies each record in a table.
  • Each table can have only one primary key.
  • A primary key column cannot contain NULL values.
  • It ensures data integrity by enforcing the uniqueness of the values in that column.

  Example:

  CREATE TABLE Employees (
      EmployeeID INT PRIMARY KEY,  -- Primary key ensures uniqueness
      Name VARCHAR(100)
  );
  

• Foreign Key:
  • A foreign key is a column (or a set of columns) that creates a link between the data in two tables.
  • It references the primary key (or unique key) in another table.
  • A foreign key can contain NULL values, unless it’s specifically constrained.
  • A foreign key enforces referential integrity, ensuring that the data in one table corresponds to valid data in another table.

  Example:

  CREATE TABLE Orders (
      OrderID INT PRIMARY KEY,
      EmployeeID INT,
      FOREIGN KEY (EmployeeID) REFERENCES Employees(EmployeeID)  -- EmployeeID is the foreign key
  );
  

2. Primary Key vs Unique Key

• Primary Key:
  • A primary key enforces the uniqueness of values in a column and ensures no NULLs are allowed.
  • There can be only one primary key in a table.
• Unique Key:
  • A unique key also enforces uniqueness of values in a column (or a combination of columns), but NULL values are allowed.
  • There can be multiple unique keys in a table (unlike a primary key, where only one is allowed).
  • Unique keys do not automatically enforce the uniqueness of NULL values. Typically, databases allow multiple NULL values in a unique column, because NULL is not considered equal to another NULL.

  Example:

  CREATE TABLE Products (
      ProductID INT PRIMARY KEY,
      SKU VARCHAR(50) UNIQUE  -- SKU can be unique, but can allow NULLs
  );
  

3. Primary Key vs Index

• Primary Key:
  • A primary key automatically creates a unique index on the column(s) to enforce uniqueness and improve query performance.
  • A primary key is part of data integrity.
• Index:
  • An index improves the performance of queries by allowing quick lookups of rows based on the indexed column(s).
  • Indexes do not enforce uniqueness unless specifically defined as unique.
  • A table can have multiple indexes, but only one primary key.

  Example:

  CREATE INDEX idx_name ON Employees(Name);
  

4. Primary Key vs Not Null

• Primary Key:
  • A primary key automatically includes a NOT NULL constraint; in other words, the values in the primary key column cannot be NULL.
• Not Null:
  • The NOT NULL constraint ensures that a column cannot have NULL values, but it does not enforce uniqueness. A NOT NULL column can still have duplicate values.

  Example:

  CREATE TABLE Employees (
      EmployeeID INT NOT NULL,  -- This ensures non-null values, but no uniqueness
      Name VARCHAR(100)
  );