一、原题 Which two statements are true regarding constraints? (Choose two.) A. A foreign key cannot contain NULL values. B. A column with the UNIQUE constraint can contain NULL values. C. A constraint is enforced only for the INSERT operation on a table.…
一、原题 Which two statements are true regarding constraints? (Choose two.) A. A foreign key cannot contain NULL values. B. The column with a UNIQUE constraint can store NULLS . C. A constraint is enforced only for an INSERT operation on a table. D. You …
个人觉得外键是最难理解的一个也是重点: 直接上例子:
create table t_group ( id int not null, name varchar(30), primary key (id)
); insert into t_group values (1, Group1);
insert into t_group values (2, Group2);
create tab…
一、原题 You need to design a student registration database that contains several tables storing academic information. The STUDENTS table stores information about a student. The STUDENT_GRADES table stores information about the students grades. Both of th…
报错:Cannot delete or update a parent row: a foreign key constraint fails。 原因:表关联生成的强制约束问题,在删除的时候回检查表之间的关联关系,从而导致无法删除,更新 解决办法:
SET foreign_key_…