How Can I Use IF-Like Logic in SQL WHERE Clauses?

Conditional Logic in SQL WHERE Clauses

SQL's WHERE clause is essential for filtering database table rows. While SQL uses IF statements for conditional execution, directly embedding them within WHERE clauses isn't standard practice. Instead, we leverage CASE statements or, in some specific database systems, adapted IF statement forms.

Utilizing CASE Statements for Conditional Filtering

CASE statements provide a clean way to implement IF-like logic in WHERE clauses. This allows for multiple conditions and associated actions. For example:

WHERE OrderNumber LIKE
  CASE 
    WHEN IsNumeric(@OrderNumber) = 1 THEN @OrderNumber
    ELSE '%' + @OrderNumber + '%'
  END

Here, if @OrderNumber is numeric, the LIKE clause performs an exact match. Otherwise, it uses wildcard characters (%) for a partial match.

Database-Specific IF Statement Variations

Certain database systems (like Microsoft SQL Server) offer variations on IF statements within WHERE clauses, but the syntax differs from standard IF statements. The structure often resembles:

IF <condition> <true_statement> ELSE <false_statement>

A WHERE clause example using this adapted IF structure could be:

WHERE
  IF IsNumeric(@OrderNumber) = 1
    OrderNumber = @OrderNumber
  ELSE
    OrderNumber LIKE '%' + @OrderNumber + '%'

Important Note: This IF syntax is not universally supported. Consult your database system's documentation to confirm compatibility before implementation. CASE statements offer broader compatibility across different SQL databases.

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