How Can I Use Conditional Logic (IF/CASE) in SQL's WHERE Clause?

Dynamic Data Filtering in SQL WHERE Clauses

SQL doesn't directly support IF statements within WHERE clauses, but you can achieve conditional filtering using alternative methods. This allows for flexible data retrieval based on specific conditions.

The CASE Statement Approach:

The CASE statement provides a powerful way to conditionally filter data. It evaluates a condition and returns a value based on the outcome. Within a WHERE clause, this looks like:

WHERE field LIKE CASE
  WHEN condition THEN result
  ELSE result
END

For example, let's say you want to filter based on whether @OrderNumber is numeric. If numeric, use an exact match; otherwise, use a wildcard search:

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

Conditional Assignment with IF (Procedural Approach):

Another method involves using conditional assignment outside the WHERE clause to prepare the filtering value. This is a more procedural approach:

IF condition
  SET field = value;
ELSE
  SET field = value;

Applying this to the @OrderNumber example:

IF IsNumeric(@OrderNumber) = 1
  SET @OrderNumber_Filtered = @OrderNumber;
ELSE
  SET @OrderNumber_Filtered = '%' + @OrderNumber + '%';

Then, use @OrderNumber_Filtered in your WHERE clause:

WHERE OrderNumber LIKE @OrderNumber_Filtered;

Both CASE within the WHERE clause and conditional assignment before the WHERE clause provide effective ways to implement conditional logic for data filtering, enhancing the power and adaptability of your SQL queries.

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