-- Lưu các đường dẫn riêng của tab Link & Ghi chú.
-- Có thể chạy lại an toàn: mỗi cột được kiểm tra trước khi thêm.

SET @class_table := 'class_tb';

SET @has_link_tai_lieu := (SELECT COUNT(*) FROM information_schema.COLUMNS WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME = @class_table AND COLUMN_NAME = 'link_tai_lieu');
SET @link_tai_lieu_sql := IF(@has_link_tai_lieu = 0, 'ALTER TABLE `class_tb` ADD COLUMN `link_tai_lieu` VARCHAR(2048) NULL DEFAULT NULL AFTER `link_feedback`', 'SELECT 1');
PREPARE class_link_statement FROM @link_tai_lieu_sql; EXECUTE class_link_statement; DEALLOCATE PREPARE class_link_statement;

SET @has_link_online := (SELECT COUNT(*) FROM information_schema.COLUMNS WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME = @class_table AND COLUMN_NAME = 'link_online');
SET @link_online_sql := IF(@has_link_online = 0, 'ALTER TABLE `class_tb` ADD COLUMN `link_online` VARCHAR(2048) NULL DEFAULT NULL AFTER `link_tai_lieu`', 'SELECT 1');
PREPARE class_link_statement FROM @link_online_sql; EXECUTE class_link_statement; DEALLOCATE PREPARE class_link_statement;

SET @has_link_chat := (SELECT COUNT(*) FROM information_schema.COLUMNS WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME = @class_table AND COLUMN_NAME = 'link_chat');
SET @link_chat_sql := IF(@has_link_chat = 0, 'ALTER TABLE `class_tb` ADD COLUMN `link_chat` VARCHAR(2048) NULL DEFAULT NULL AFTER `link_online`', 'SELECT 1');
PREPARE class_link_statement FROM @link_chat_sql; EXECUTE class_link_statement; DEALLOCATE PREPARE class_link_statement;