CREATE TABLE Account (
  ID PRIMARY KEY NOT NULL,
  type NOT NULL,
  altId NOT NULL, -- e.g. when type "member", no. in external member table 
  IBAN -- target account for returned payments (set '' to enable
       -- outgoing bank transfers to commercial partners from that account).
);
CREATE TABLE Debit (
  billId PRIMARY KEY NOT NULL,
  debtor NOT NULL, -- Account charged
  targetCredit INTEGER, -- record id in Credit table to pay into.
                   -- just understand it as virtual payment
                   -- NULL when debit is a bank transfer from the club account
  date DATE NOT NULL,
  purpose NOT NULL, -- description of receipt
  value INTEGER NOT NULL, -- Euro-Cent
  paid INTEGER DEFAULT 0, -- Euro-Cent, set and changed automatically (Cache)
  FOREIGN KEY (debtor) REFERENCES Account(ID),
  FOREIGN KEY (targetCredit) REFERENCES Credit(Id)
);
CREATE TABLE Credit (
  Id INTEGER PRIMARY KEY NOT NULL,
  account NOT NULL, -- Account des Begünstigten
  date DATE NOT NULL,
  purpose NOT NULL, -- as originally indicated in statement of bank account
  value INTEGER NOT NULL, -- Euro-Cent. Caution, two distinct cases need to be considered:
                         --  Either deposit by bank transfer (>0) or target of internal payments (=0)
  spent INTEGER DEFAULT 0, -- Euro-Cent, set and changed automatically (Cache)
                           -- for later traceability, necessary when revoking transfers
  FOREIGN KEY (account) REFERENCES Account(ID)
);

-- Which credit pays/paid down which debt is recorded traceably so as to clarify any case of reminder,
-- without ambuiguity about, which debt is actually due yet, or to clarify which transfer is used for
-- which debts. The user specifies which credit is intended for which debt, in accordance with the
-- purpose if any is indicated in a received bank transfer. Following triggers verify this relation
-- and mark a debit as paid and/or a credit as paid.
-- Debits to which applies value > paid are meant to be suggested for assignment to newly inserted
-- credit records without indicated purpose. Likewise, credits without indicated purpose to which
-- applies value > spent are candidates for payment of newly inserted debts.
CREATE TABLE Transfer (
  timestamp DATE DEFAULT CURRENT_TIMESTAMP,
  billId INTEGER NOT NULL,
  fromCredit INTEGER NOT NULL, 
  amount INTEGER, -- for later traceability, necessary when revoking transfers
  FOREIGN KEY (billId) REFERENCES Debit(billId),
  FOREIGN KEY (fromCredit) REFERENCES Credit(Id),
  UNIQUE (billId, fromCredit)
);

CREATE TABLE IF NOT EXISTS _temp (d, c, m);
CREATE TRIGGER balanceTransfer
     AFTER INSERT ON Transfer 
BEGIN

  SELECT RAISE(FAIL, "It is not the debtor who is set to pay")
    WHERE (SELECT debtor FROM Debit WHERE billId=NEW.billId)
       != (SELECT account FROM Credit WHERE Id=NEW.fromCredit)
    ;

  INSERT INTO _temp
     SELECT remainingDebt, remainingCredit, min(remainingDebt,remainingCredit) 
     FROM (SELECT
         (SELECT value - paid FROM Debit WHERE billId=NEW.billId) AS remainingDebt, 
         (SELECT value - spent FROM Credit WHERE Id=NEW.fromCredit) AS remainingCredit
     )
  ;

  UPDATE Debit
      SET paid = paid + CASE
        WHEN (SELECT d FROM _temp) <= 0
          THEN RAISE(FAIL, "Debt settled")
        ELSE
          (SELECT m FROM _temp)
      END
      WHERE billId=NEW.billId;

  UPDATE Credit
      SET value = value + (SELECT m FROM _temp)
      WHERE Id = (
          SELECT targetCredit
          FROM Debit
          WHERE billId=NEW.billId
      );

  UPDATE Credit
      SET spent = spent + CASE
        WHEN (SELECT c FROM _temp) <= 0
          THEN RAISE(FAIL, "Credit spent")
        ELSE
          (SELECT m FROM _temp)
      END
      WHERE Id=NEW.fromCredit;

  UPDATE Transfer
      SET amount = (SELECT m FROM _temp)
      WHERE billId=NEW.billId AND fromCredit=NEW.fromCredit
        ;

  DELETE FROM _temp;

END;

CREATE TRIGGER revokeTransfer
    BEFORE DELETE ON Transfer
BEGIN

  INSERT INTO _temp VALUES (null,null,OLD.amount);

  UPDATE Debit
      SET paid = paid - OLD.amount
      WHERE billId=OLD.billId
      ;

  UPDATE Credit
      SET value = value - OLD.amount
      WHERE Id = (
          SELECT targetCredit
          FROM Debit
          WHERE billId=OLD.billId
      );

  UPDATE Credit
      SET spent = spent - OLD.amount
      WHERE Id = OLD.fromCredit;

  DELETE FROM _temp;

END;

CREATE TRIGGER enforceImmutableTransfer
    BEFORE UPDATE ON Transfer
    WHEN OLD.amount IS NOT NULL
BEGIN
    SELECT RAISE(FAIL, "Transfer cannot be updated, but needs to be revoked and re-inserted to run triggers");
END;

CREATE TRIGGER enforceiZeroPaidAtStart
    BEFORE INSERT ON Debit
BEGIN
    SELECT RAISE(FAIL, "Debt must be initially unpaid")
    WHERE NEW.paid <> 0;
END;

-- Prevent modification of paid value outside triggers which must adjust it exclusively
-- when new transfer records are inserted
CREATE TRIGGER enforceDebtImmutableOutsideTrigger
    BEFORE UPDATE OF paid ON Debit
    WHEN NOT EXISTS (SELECT * FROM Transfer t WHERE NEW.billId=t.billId AND amount IS NULL)
BEGIN
    SELECT RAISE(FAIL, "paid is set and adjusted automatically according to added Transfer records")
    WHERE (NEW.paid + IFNULL((SELECT m FROM _temp WHERE c IS NULL AND d IS NULL),0) ) <> OLD.paid;
END;

CREATE TRIGGER enforceFixedDebits
    BEFORE UPDATE OF value ON Debit
    WHEN EXISTS (SELECT * FROM Transfer WHERE billId=NEW.billId)
BEGIN
    SELECT RAISE(FAIL, "Debt is involved in transfers to revoke at first");
END;

CREATE TRIGGER enforceZeroSpentAtStart
    BEFORE INSERT ON Credit
BEGIN
    SELECT RAISE(FAIL, "credit must be initially unused")
    WHERE NEW.spent != 0;
END;

-- Prevent modification of spent value outside triggers which must adjust it exclusively
-- when new transfer records are inserted
CREATE TRIGGER enforceSpentImmutableOutsideTrigger
    BEFORE UPDATE OF spent ON Credit
    WHEN NOT EXISTS (SELECT * FROM Transfer t WHERE NEW.Id=t.fromCredit AND amount IS NULL)
BEGIN
    SELECT RAISE(FAIL, "spent is set and adjusted automatically according to added Transfer records")
    WHERE (NEW.spent + IFNULL((SELECT m FROM _temp WHERE c IS NULL AND d IS NULL),0) ) <> OLD.spent;
END;

CREATE TRIGGER enforceFixedCredit
    BEFORE UPDATE OF value ON Credit
BEGIN
    SELECT RAISE(FAIL, "Credit involved in transactions to revoke at first")
    WHERE EXISTS (SELECT * FROM Transfer WHERE fromCredit=NEW.Id);
END;

CREATE TRIGGER checkIBANatTransfer
    BEFORE INSERT ON Debit
    WHEN NEW.targetCredit IS NULL
BEGIN
   SELECT RAISE(FAIL, "IBAN used does not match IBAN currently stored in account record")
   FROM (
      SELECT instr(NEW.purpose, IBAN) AS fnd
      FROM Account
      WHERE ID=NEW.debtor
   ) AS matchingIBAN
   WHERE fnd IS NULL OR fnd = 0;
END;

CREATE VIEW CurrentDebts AS
    SELECT debtor,
           targetCredit,
           purpose,
           date,
           value - paid AS difference
    FROM Debit
    WHERE value != paid
;
      
CREATE VIEW AvailableCredits AS
    SELECT account, purpose, date,
           value - spent AS difference
    FROM Credit
    WHERE value != spent
;
      
CREATE VIEW Balance AS
  SELECT Account.ID             AS ID,
      IFNULL(ac.allCredits,0)   AS credit,
      IFNULL(pr.allPromises, 0) AS promised,
      IFNULL(cd.allDebts, 0)    AS debt
  FROM Account
     LEFT OUTER JOIN (
         SELECT debtor, sum(difference) AS allDebts
         FROM CurrentDebts
         GROUP BY debtor
     )                          AS cd ON Account.ID=cd.debtor
     LEFT OUTER JOIN (
         SELECT account, sum(difference) AS allCredits
         FROM AvailableCredits
         GROUP BY account
     )                          AS ac ON Account.ID=ac.account
     LEFT OUTER JOIN (
         SELECT a.ID AS ID, sum(difference) AS allPromises
         FROM CurrentDebts cd
             JOIN Credit c ON cd.targetCredit = c.Id
             JOIN Account a ON a.ID = c.account
         GROUP BY a.ID
     )                          AS pr ON Account.ID=pr.ID
  ;

CREATE VIEW ReconstructedBankStatement AS
  SELECT c.date    AS date,
         c.purpose AS purpose,
         account,
         c.value   AS credit,
         NULL      AS debit
  FROM Credit AS c
    LEFT OUTER JOIN Debit AS d ON c.ID=d.targetCredit
  GROUP BY c.ID
    HAVING count(d.billId) == 0 -- exclude internal transfers
  UNION
  SELECT date,
         purpose,
         debtor     AS account,
         NULL       AS credit,
         value      AS debit
  FROM Debit
  WHERE targetCredit IS NULL    -- exclude internal transfers
  ORDER BY date ASC
;