Creating an SQLite trigger that updates bank account based on transaction information upon insert

问题

I have an Android application that helps the user keep track of their finances. The application has a database that stores account and transaction information. Each time a transaction is recorded I also have to update the account table to adjust that account's balance, as well as it's remaining allowance for a certain spending period. The accounts table looks like this:

| id | name | balance | allowance | allowanceRemaining | spendingPeriod |

And a transaction table that looks like this:

| id | description | amount | withdrawal | discretionary | accountID |

Withdrawal and discretionary are both integers that are 0 or 1 to act as booleans. I have tried the following to at least set the balance based on if a transaction was a withdrawal or not but I am getting syntax errors and I can't figure out why. Even beyond syntax, though, I am having trouble understanding the logic and created the proper case statements.

CREATE TRIGGER update_account AFTER INSERT ON transactions
BEGIN
  UPDATE accounts a SET a.balance = 
    (CASE WHEN new.withdrawal = 1 THEN a.balance - new.amount ELSE
     a.balance + new.amount END) WHERE a.id = new.accountID;
END;

In pseudocode, what I would like to do is:

If the transaction is a deposit, add the money
If the transaction is a withdrawal, check if it is discretionary
If it is discretionary, remove from the balance and the allowance remaining
If it is not, remove only from the balance.

I made an SQL Fiddle, that has some quick sample data but no trigger.

回答1:

UPDATE wants the new value for each changed column, so you have to formulate your requirements this way:

The balance

increases by the amount for a deposit,
decreases by the amount for a withdrawal.

The remaining allowance

decreases by the amount for a discetionary withdrawal.

As for the syntax, you cannot use an alias for the UPDATEd table:

CREATE TRIGGER update_account
AFTER INSERT ON transactions
BEGIN
    UPDATE accounts
    SET balance = CASE
                  WHEN new.withdrawal THEN balance - new.amount
                  ELSE                     balance + new.amount
                  END,
        allowanceRemaining = CASE
                             WHEN new.withdrawal AND new.discretionary
                             THEN allowanceRemaining - new.amount
                             ELSE allowanceRemaining
                             END
    WHERE id = NEW.accountID;
END;

It might be simpler to use the WHEN clause of the trigger:

CREATE TRIGGER update_balance_for_deposit
AFTER INSERT ON transactions
WHEN NOT NEW.withdrawal
BEGIN
    UPDATE accounts
    SET balance = balance + NEW.amount
    WHERE id = NEW.accountID;
END;

CREATE TRIGGER update_balance_for_withdrawal
AFTER INSERT ON transactions
WHEN NEW.withdrawal
BEGIN
    UPDATE accounts
    SET balance = balance - NEW.amount
    WHERE id = NEW.accountID;
END;

CREATE TRIGGER update_allowance_for_discretionary_withdrawal
AFTER INSERT ON transactions
WHEN NEW.withdrawal AND NEW.discretionary
BEGIN
    UPDATE accounts
    SET allowanceRemaining = allowanceRemaining - NEW.amount
    WHERE id = NEW.accountID;
END;

来源：https://stackoverflow.com/questions/27372310/creating-an-sqlite-trigger-that-updates-bank-account-based-on-transaction-inform

标签

android

sql

database

sqlite

triggers