truncate table的注意事项

ALTER TABLE studentMODIFY gradeId INT(4) COMMENT '年级编号';ALTER TABLE student ADD CONSTRAINT FK_student_gradeId_grade_gradeIdFOREIGN KEY(gradeId) REFERENCES grade(gradeid);# 删除表数据的时候，不能有外键约束	
TRUNCATE TABLE grade;# 删除外键约束
ALTER TABLE studentDROP FOREIGN KEY FK_student_gradeId_grade_gradeId;