SQL `CASE WHEN THEN ELSE` Formatting Best Practices

Key Takeaways

1. Consistent formatting of CASE expressions improves SQL readability.
2. Use ELSE to handle all possible cases and avoid unexpected NULL results.
3. Avoid redundant or overlapping WHEN conditions for clarity.

The CASE expression in SQL is a powerful tool that allows for conditional logic within queries, similar to if/then/else statements in other programming languages. Proper formatting of CASE statements enhances readability and maintainability, especially in complex queries.

Basic Syntax of CASE

A typical CASE expression follows this structure:

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

This structure can be used within SELECT, ORDER BY, GROUP BY, and other clauses to implement conditional logic.

Formatting Guidelines

1. Align Keywords for Clarity

Aligning the WHEN, THEN, and ELSE clauses vertically improves readability. For example:

SELECT
    employee_id,
    CASE
        WHEN salary > 100000 THEN 'High'
        WHEN salary BETWEEN 50000 AND 100000 THEN 'Medium'
        ELSE 'Low'
    END AS salary_range
FROM employees;

This alignment makes it easier to scan and understand the conditional logic.

2. Use Indentation Consistently

Consistent indentation within the CASE expression helps delineate the logic blocks. For instance:

SELECT
    product_name,
    CASE
        WHEN stock_quantity = 0 THEN 'Out of Stock'
        WHEN stock_quantity < 10 THEN 'Low Stock'
        ELSE 'In Stock'
    END AS stock_status
FROM products;

Each WHEN clause is indented to the same level, making the structure clear.

3. Keep THEN Clauses on the Same Line

Placing the THEN result on the same line as the WHEN condition keeps related logic together:

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ELSE resultN
END

This format is concise and maintains the association between conditions and their outcomes.

4. Use ELSE for Default Cases

Including an ELSE clause ensures that all possible cases are handled:

CASE
    WHEN score >= 90 THEN 'A'
    WHEN score >= 80 THEN 'B'
    WHEN score >= 70 THEN 'C'
    ELSE 'F'
END

This approach prevents unexpected NULL results when none of the WHEN conditions are met.

5. Avoid Redundant Conditions

Ensure that WHEN conditions are mutually exclusive to prevent ambiguity:

CASE
    WHEN age < 18 THEN 'Minor'
    WHEN age BETWEEN 18 AND 64 THEN 'Adult'
    ELSE 'Senior'
END

This structure avoids overlapping conditions and clarifies the categorization logic.

Advanced Usage

Nested CASE Statements

For complex logic, CASE statements can be nested:

CASE
    WHEN condition1 THEN
        CASE
            WHEN sub_condition1 THEN result1
            ELSE result2
        END
    ELSE result3
END

While powerful, nested CASE statements can reduce readability. Use them judiciously and consider breaking complex logic into multiple steps or using temporary tables.

CASE in ORDER BY Clauses

CASE expressions can be used to implement custom sorting logic:

SELECT
    customer_name,
    membership_level
FROM customers
ORDER BY
    CASE membership_level
        WHEN 'Gold' THEN 1
        WHEN 'Silver' THEN 2
        WHEN 'Bronze' THEN 3
        ELSE 4
    END;

This technique allows for sorting based on custom-defined priorities.

Conclusion

Consistent and thoughtful formatting of CASE expressions in SQL enhances code readability and maintainability. By aligning keywords, using consistent indentation, and handling all possible cases, you can write clear and effective SQL queries that are easier to understand and maintain.

FAQs

Why should I align CASE, WHEN, THEN, and ELSE keywords?

Alignment makes conditional logic easier to read and maintain.

Is it necessary to include an ELSE clause in every CASE expression?

Yes, including ELSE ensures all cases are covered and prevents NULL results.

Can I use CASE statements in ORDER BY clauses?

Yes, CASE can define custom sorting logic in ORDER BY.

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