SQLite trigger before insert - database

Question

I am trying to create a trigger where it filters the itemID first and then checks the most highest bidNo of that itemID. bidNo has to be only +1 bigger than the last. bid is bidder.
CREATE TRIGGER new_bid_no
BEFORE INSERT ON BID
BEGIN
SELECT CASE
WHEN (SELECT itemID
FROM bid
WHERE itemID = NEW.itemID
GROUP BY itemID
) AND (SELECT MAX(bidNo) AS bidNo
FROM bid
WHERE NEW.bidNo < bidNo
OR NEW.bidNo = (bidNo+1)
GROUP BY itemID
)THEN
RAISE (ABORT, 'New bid number has to be 1 higher than the last')
END;
END;
the bid table
CREATE TABLE BID(
buyerUsername VARCHAR(30) NOT NULL,
itemID INTEGER NOT NULL,
bidNo INTEGER NOT NULL,
bidAmount INTEGER,
bidDate VARCHAR(10),
bidTime TIME,
PRIMARY KEY(buyerUsername, itemID, bidNo),
FOREIGN KEY (buyerUsername) REFERENCES BUYER(buyerUsername),
FOREIGN KEY (itemID) REFERENCES ITEM(itemID)
)
WITHOUT ROWID;

Answer 1

First, you must define a UNIQUE constraint for itemID, bidNo and this can be done only in the CREATE statement of the table and not added later:
CREATE TABLE BID(
buyerUsername TEXT NOT NULL,
itemID INTEGER NOT NULL,
bidNo INTEGER NOT NULL,
bidAmount INTEGER,
bidDate TEXT,
bidTime TEXT,
PRIMARY KEY(buyerUsername, itemID, bidNo),
FOREIGN KEY(buyerUsername) REFERENCES BUYER(buyerUsername),
FOREIGN KEY(itemID) REFERENCES ITEM(itemID),
UNIQUE(itemID, bidNo)
)
WITHOUT ROWID;
I also changed the data types of the columns that you defined as VARCHAR to TEXT, because there is no VARCHAR data type in SQLite. Even if you defined it as such it will be TEXT. Also there is no data type TIME.
Now your trigger can be written like this:
CREATE TRIGGER new_bid_no
BEFORE INSERT ON BID
WHEN NEW.bidNo <> COALESCE((SELECT MAX(bidNo) FROM bid WHERE itemID = NEW.ItemID AND bidNo < NEW.bidNo), 0) + 1
BEGIN
SELECT RAISE (ABORT, 'New bid number has to be 1 higher than the last');
END