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How to fix this MySQL trigger?
CREATE TRIGGER Transaction_insert BEFORE INSERT ON Transaction
FOR EACH ROW WHERE Number = NEW.AccountNumber
IF Account.CreditBalance + NEW.Amount < Account.CreditLimit THEN
UPDATE Account SET CreditBalance = CreditBalance + NEW.Amount where Number = NEW.AccountNumber;
ELSE
SET NEW.Valid = 0
END IF;

IF EXISTS (SELECT * FROM `Account` A
            WHERE A.CreditBalance + NEW.Amount < A.CreditLimit AND
                   A.Number = NEW.AccountNumber) THEN
   UPDATE ...

DELIMITER //

-- Assumptions:
--   1. Transaction.AccountNumber is F.K. REFERENCES Account(Number)
--   2. Account.Number is UNIQUE
--
CREATE TRIGGER trg_bi_transaction BEFORE INSERT ON Transaction
FOR EACH ROW
BEGIN
  -- Adjust account balance (if permitted)
  --
  UPDATE Account
     SET CreditBalance = CreditBalance + NEW.Amount
   WHERE Number = NEW.AccountNumber
         AND
         (CreditBalance + NEW.Amount) < CreditLimit;

  -- Was the adjustment valid/permitted?
  --
  SET NEW.Valid = (ROW_COUNT() = 1);
END //

DELIMITER ;