How do I use properly CASE..WHEN in MySQL

sql

best-practices

sql-queries

conditional-statements

byAnton Shumikhin·Oct 24, 2024

CASE in MySQL navigates through a series of conditions for each row and yields a specific result based on the first true evaluation. For instance:

SELECT 
  CASE 
    WHEN score >= 90 THEN 'A' // "A" for effort
    WHEN score >= 80 THEN 'B' // "B"ut you were so close!
    WHEN score >= 70 THEN 'C' // "C"an definitely improve
    ELSE 'F' // Well, at least you tried
  END AS grade // The reroll function for exam results 
FROM 
  grades;

Here, the score is checked against each WHEN clause for a corresponding grade while no match resorts to 'F' via ELSE.

Breaking down the syntax and logic

The CASE statement in MySQL can perform complex logic functions within your queries. These key measures can aid in using this feature:

Homogeneous data type

Maintain data type consistency as a difference in data type of base_price or the column under the CASE condition can lead to unexpected results.

Severity unnecessary redundancy

Avoid referring to the variable just after the CASE keyword which can lead to syntactical errors. Simply, evaluate the variable inside each WHEN clause.

The right type of CASE

The search condition form of CASE allows for more complex evaluations where each condition hits a nail exactly on its head:

CASE
  WHEN user.status = 'premium' AND orders.quantity > 100 THEN 'apply_discount' // "Premium users, assemble for discounts!"
  WHEN user.status = 'regular' AND current_date() = orders.order_date THEN 'send_reminder' // No more "Oops, I forgot"
  ELSE 'continue_processing' // Default: Keep on keeping on
END

Keep testing

Regularly run your CASE condition with various base_price values. Building tests for your condition is our adult version of "Mischief Managed."

Honing SQL with CASE

Infallible with ELSE

Improves robustness as it captures conditions that don't fall under any WHEN clause.

Coherent logic

Strive for consistency in your WHERE clause conditions with those in CASE to stave off unexpected results

Exploring new avenues

If CASE isn't bringing home the gold, experiment with other conditional logic frameworks like IF(), COALESCE(), or stored procedures for your needs

Articulating expressions

For various results, consider the CASE statement as an expression shaped by internal logic.

Advanced use cases and "oh-no" moments

Syntax heretics and common pitfalls

Continuous returning of a certain default result may indicate syntax errors or issues within each THEN and ELSE clause.

Rival conditions

Remove conflicting conditions that result in logical clashes within the CASE statement

Comparison masterclass

Utilize appropriate operators within CASE to precisely differentiate conditions.

Land of help and resources

In case of any persisting issues, continue your search for knowledge, solutions, and other resources.

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