how to link a foreign key in sql code example

Example 1: alter table add column and foreign key mysql

ALTER TABLE database.table
  ADD COLUMN columnname INT DEFAULT(1),
  ADD FOREIGN KEY fk_name(fk_column) REFERENCES reftable(refcolumn) ON DELETE CASCADE;

Example 2: Cannot truncate a table referenced in a foreign key constraint (`video_clips`.`channel_clips`, CONSTRAINT `clips_fk` FOREIGN KEY (`clip_id`) REFERENCES `video_clips`.`clips` (`id`)) in sql]

SET FOREIGN_KEY_CHECKS = 0; 

truncate table "yourTableName";

SET FOREIGN_KEY_CHECKS = 1;