Troubleshooting "mariadb IF statements error message?"

• "error message" indicates that there's an issue with the syntax or structure of your IF statement.
• "IF statements" refer to a control flow construct in MariaDB and MySQL that allows you to execute code conditionally based on whether a certain expression evaluates to TRUE or FALSE.

Common Causes:

1. Standalone IF Statement: MariaDB and MySQL don't allow using IF statements directly in queries. They need to be part of a larger construct like a stored procedure, function, or BEGIN-END block.

   • Incorrect: IF age > 18 THEN SELECT 'Adult'; END IF;
   • Correct (within a stored procedure):

     CREATE PROCEDURE check_age(age INT)
     BEGIN
         IF age > 18 THEN
             SELECT 'Adult';
         END IF;
     END;
     

2. Missing END IF: You must properly terminate your IF statement with END IF;.

Additional Tips:

Example (MariaDB):

CREATE PROCEDURE calculate_discount(price DECIMAL(10,2))
BEGIN
  DECLARE discount DECIMAL(5,2);
  SET discount = 0;

  IF price > 100 THEN
    SET discount = price * 0.1;
  END IF;

  SELECT price, discount, price - discount AS final_price;
END;

This example checks if a customer's age is greater than 18 and displays a message accordingly:

CREATE PROCEDURE check_age(age INT)
BEGIN
  IF age > 18 THEN
    SELECT 'You are eligible.';
  ELSE
    SELECT 'You are not eligible.';
  END IF;
END;

IF-ELSE with Additional Logic:

This example calculates a discount based on the purchase amount:

CREATE PROCEDURE calculate_discount(price DECIMAL(10,2))
BEGIN
  DECLARE discount DECIMAL(5,2);
  SET discount = 0;

  IF price > 100 THEN
    SET discount = price * 0.1;  -- 10% discount
  ELSEIF price > 50 THEN
    SET discount = price * 0.05;  -- 5% discount
  END IF;

  SELECT price, discount, price - discount AS final_price;
END;

IF with Stored Variable:

This example updates a stored variable based on a condition:

CREATE PROCEDURE update_stock(product_id INT, quantity INT)
BEGIN
  DECLARE in_stock INT;

  SELECT stock INTO in_stock FROM products WHERE product_id = id;

  IF in_stock >= quantity THEN
    UPDATE products SET stock = stock - quantity WHERE product_id = id;
    SET in_stock = in_stock - quantity;
  ELSE
    SELECT 'Insufficient stock.' AS message;
  END IF;

  SELECT in_stock;  -- Show remaining stock (optional)
END;

Remember to replace id with the actual column name in your tables.

CASE expressions offer a more concise way to evaluate conditions and return different values based on the outcome. They can be used within SELECT statements and sometimes within UPDATE statements.

Example:

SELECT product_name,
       CASE WHEN price > 100 THEN 'High Price'
            WHEN price > 50 THEN 'Medium Price'
            ELSE 'Low Price'
       END AS price_category
FROM products;

COALESCE function:

COALESCE allows you to specify a default value if the first expression evaluates to NULL. This is useful for avoiding errors when dealing with potentially missing data.

SELECT customer_name, COALESCE(email, 'No email available') AS contact
FROM customers;

NULLIF function:

NULLIF returns NULL if two expressions are equal, otherwise it returns the first expression. This can be helpful for filtering or setting values based on specific conditions.

UPDATE orders SET status = NULLIF(status, 'pending')
WHERE order_id = 123;

Boolean expressions in WHERE clause:

You can directly incorporate logical operators (AND, OR, NOT) into the WHERE clause of your SELECT or UPDATE statements to filter rows based on multiple conditions.

SELECT * FROM users
WHERE age > 18 AND active = 1;

Choosing the Right Method:

• When dealing with multiple conditions in filtering data, boolean expressions within the WHERE clause provide a clear and efficient way.
• For specific filtering or setting values based on equality, NULLIF might be suitable.
• If you need a default value when an expression might be NULL, COALESCE can be helpful.
• For returning different values based on multiple conditions, CASE expressions can be more compact.
• For simple conditions with single actions, IF statements might be the most readable approach.