10 What is the difference between decode and case?

 DECODE and CASE are both conditional expressions used in SQL to perform conditional logic within queries. However, they differ in syntax, functionality, and availability across different database systems. Here's a breakdown of the differences:

1. Syntax

• DECODE Syntax:

  
  
  DECODE(expression, search1, result1, search2, result2, ..., default)
  

  • expression: The value to be compared.
  • search: The value to compare against the expression.
  • result: The value to return if the search value matches the expression.
  • default: The value to return if none of the search values match the expression.

• CASE Syntax:

  • Simple CASE Expression:

    
    CASE expression
        WHEN value1 THEN result1
        WHEN value2 THEN result2
        ...
        ELSE default
    END
    

  • Searched CASE Expression:

    
    CASE
        WHEN condition1 THEN result1
        WHEN condition2 THEN result2
        ...
        ELSE default
    END
    

2. Functionality

• DECODE:

  • Acts like a simplified CASE expression.
  • Only compares a single expression against a series of possible values.
  • Stops evaluating once a match is found.
  • More compact and concise for simple equality checks.
  • Primarily available in Oracle databases.

• CASE:

  • More powerful and flexible than DECODE.
  • Can handle multiple conditions and logical expressions, not just equality checks.
  • Available in almost all major relational database management systems (RDBMS), including Oracle, SQL Server, MySQL, and PostgreSQL.
  • Provides both a simple case (for equality checks) and a searched case (for more complex conditions).

3. Portability

• DECODE:

  • Mostly specific to Oracle databases, so it is less portable across different RDBMS.
  • Not natively supported in SQL Server, MySQL, or PostgreSQL.

• CASE:

  • Standard SQL and widely supported across different RDBMS.
  • More portable, making it a better choice for SQL scripts that need to work across different database systems.

4. Use Cases

• DECODE:

  • Best for simple mappings and when working within Oracle.
  • Example:

    
    SELECT DECODE(status, 'A', 'Active', 'I', 'Inactive', 'Unknown') FROM users;
    

• CASE:

  • Preferred for complex conditions, non-Oracle databases, and where portability is a concern.
  • Example:

    
    SELECT 
        CASE 
            WHEN status = 'A' THEN 'Active'
            WHEN status = 'I' THEN 'Inactive'
            ELSE 'Unknown'
        END 
    FROM users;
    

5. Performance

• Performance is typically comparable between the two for simple use cases, but CASE can be more optimized and versatile for complex scenarios.

Conclusion:

• Use DECODE if you are working in Oracle and need a quick, simple mapping solution.
• Use CASE for more complex conditions, for better portability, and when working with non-Oracle databases.