R.A references S.B, cascade delete create trigger cascade_trigger after delete on S referencing old row as O for each row [ no condition ] delete from R where A = O.B这段sql代码如果不加这一行代码这一行R.A references S.B, cascade delete会怎样
时间: 2024-02-29 15:53:41 浏览: 80
如果不加这一行代码"R.A references S.B, cascade delete",那么在S表中删除一行时,R表中对应的行不会被自动删除,这可能会导致数据的不一致性和错误。因为如果S表中的一行被删除,而R表中的某些行仍然引用了这个被删除的行,在查询或操作时可能会出现错误或不一致的结果。
而且,如果不加外键约束,S表和R表之间的引用关系也不会被明确地表达出来,这样就难以维护数据的一致性和完整性。在实际的应用中,如果S表和R表的数据量很大,没有外键约束的话,可能需要手动编写代码来维护这些关系,这会增加代码的复杂度和维护难度。因此,在设计数据库时,通常要遵循关系模型的规范,使用外键约束来保证数据的完整性和一致性。
相关问题
students(student_id, name, year, GPA, programs_code) instructors(instructor_id, name, title, salary, programs_code) programs(code, name, division, instructor_id) course(course_name, credit, domain, course_number) enroll(student_id, instructor_id, course_name) 1. Create tables according to the schema. 2. Use check or assertion to make sure instructors salary > 5000. 3. Use trigger to deal with cascade delete.
Sure, here are the SQL statements to create the tables based on the given schema, add a check constraint to ensure that an instructor's salary is greater than 5000, and add a trigger to handle cascade deletes:
```
CREATE TABLE students (
student_id INT PRIMARY KEY,
name VARCHAR(50),
year INT,
GPA DECIMAL(3,2),
programs_code VARCHAR(10)
);
CREATE TABLE instructors (
instructor_id INT PRIMARY KEY,
name VARCHAR(50),
title VARCHAR(50),
salary DECIMAL(10,2),
programs_code VARCHAR(10),
CONSTRAINT salary_check CHECK (salary > 5000)
);
CREATE TABLE programs (
code VARCHAR(10) PRIMARY KEY,
name VARCHAR(50),
division VARCHAR(50),
instructor_id INT,
FOREIGN KEY (instructor_id) REFERENCES instructors(instructor_id) ON DELETE SET NULL
);
CREATE TABLE course (
course_name VARCHAR(50) PRIMARY KEY,
credit INT,
domain VARCHAR(50),
course_number INT
);
CREATE TABLE enroll (
student_id INT,
instructor_id INT,
course_name VARCHAR(50),
FOREIGN KEY (student_id) REFERENCES students(student_id) ON DELETE CASCADE,
FOREIGN KEY (instructor_id) REFERENCES instructors(instructor_id) ON DELETE SET NULL,
FOREIGN KEY (course_name) REFERENCES course(course_name) ON DELETE SET NULL
);
CREATE TRIGGER cascade_delete_enroll
AFTER DELETE ON students
FOR EACH ROW
BEGIN
DELETE FROM enroll WHERE student_id = OLD.student_id;
END;
```
The `ON DELETE SET NULL` clause in the `programs` table allows the `instructor_id` column to be set to NULL when an instructor is deleted, instead of cascading the delete to the `programs` table.
The `ON DELETE CASCADE` clause in the `enroll` table allows the deletion of a student to cascade to the `enroll` table, deleting all enrollments for that student.
Finally, the `cascade_delete_enroll` trigger handles the cascade delete by deleting all enrollments for a student when that student is deleted from the `students` table.
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