SQL Conditional Expressions and Examples

This repository contains comprehensive examples of SQL conditional expressions, functions, and their practical applications. The examples cover various SQL flavors including SQL Server, Oracle, MySQL, and PostgreSQL.

Contents

1. Basic Conditional Expressions

   • CASE Statements
   • IIF Functions
   • NULL Handling
   • Conditional Aggregates

2. Case Statement Examples

   • Simple CASE
   • Searched CASE
   • CASE in UPDATE statements
   • CASE with GROUP BY

3. IF Statement Alternatives

   • IIF usage
   • CASE alternatives
   • Complex conditions

4. Conditional Column Examples

   • Dynamic columns
   • Calculated fields
   • Date-based conditions

Database Compatibility

• SQL Server Specific:

  • IIF()
  • ISNULL()
  • TRY_CONVERT()
  • CHOOSE()

• Oracle Specific:

  • NVL()
  • DECODE()

• Universal Functions:

  • CASE
  • COALESCE
  • NULLIF

Common Use Cases

1. Data Categorization

   • Price ranges
   • Age groups
   • Performance tiers

2. NULL Handling

   • Default values
   • Data cleaning
   • Conditional aggregation

3. Business Logic

   • Discount calculations
   • Commission structures
   • Status updates

4. Reporting

   • KPI calculations
   • Performance metrics
   • Dynamic grouping

Best Practices

1. Always include an ELSE clause in CASE statements
2. Use appropriate NULL handling functions
3. Consider performance implications with complex conditions
4. Keep conditions mutually exclusive
5. Use comments to explain complex logic

Performance Considerations

• CASE statements are generally more performant than multiple IF conditions
• Use appropriate indexes for columns used in conditions
• Avoid unnecessary CASE expressions in WHERE clauses
• Consider materialized views for complex conditional calculations

Examples

Check the SQL files for detailed examples and implementations.