如果ID存在于另一个表中，则插入行

-- 检查用户是否存在
SELECT COUNT(*) FROM users WHERE id = ?;

-- 如果不存在，插入用户
INSERT INTO users (name, email) VALUES (?, ?);

-- 插入订单
INSERT INTO orders (user_id, product, quantity) VALUES (?, ?, ?);

BEGIN TRANSACTION;

-- 检查用户是否存在
IF NOT EXISTS (SELECT * FROM users WHERE id = ?)
BEGIN
    -- 插入用户
    INSERT INTO users (name, email) VALUES (?, ?);
END

-- 插入订单
INSERT INTO orders (user_id, product, quantity) VALUES (?, ?, ?);

COMMIT TRANSACTION;

-- 创建触发器
CREATE TRIGGER trg_insert_order
BEFORE INSERT ON orders
FOR EACH ROW
BEGIN
    IF NOT EXISTS (SELECT * FROM users WHERE id = NEW.user_id) THEN
        -- 插入默认用户
        INSERT INTO users (name, email) VALUES ('Default User', 'default@example.com');
    END IF;
END;