|NO.Z.00046|——————————|BigDataEnd|——|Java&MySQL多表/外键/数据库设计.V04|——|MySQL.v04|级联删除|

一、级联删除

### --- 如果向实现删除主表数据的同时，也删除掉从表数据，可以使用级联删除操作级联删除

ON DELETE CASCADE

### --- 删除 employee表,重新创建,添加级联删除

-- 重新创建添加级联操作
CREATE TABLE employee(
    eid INT PRIMARY KEY AUTO_INCREMENT,
    ename VARCHAR(20),
    age INT,
    dept_id INT,
    CONSTRAINT emp_dept_fk FOREIGN KEY(dept_id) REFERENCES department(id)
    -- 添加级联删除
    ON DELETE CASCADE
);

-- 添加数据
INSERT INTO employee (ename, age, dept_id) VALUES ('张百万', 20, 1);
INSERT INTO employee (ename, age, dept_id) VALUES ('赵四', 21, 1);
INSERT INTO employee (ename, age, dept_id) VALUES ('广坤', 20, 1);
INSERT INTO employee (ename, age, dept_id) VALUES ('小斌', 20, 2);
INSERT INTO employee (ename, age, dept_id) VALUES ('艳秋', 22, 2);
INSERT INTO employee (ename, age, dept_id) VALUES ('大玲子', 18, 2);

-- 删除部门编号为2 的记录
-- 员工表中 外键值是2的记录,也被删除了
DELETE FROM department WHERE id = 2;

二、sql语句

### --- sql语句

/*
    级联删除
        指的是在删除主表的数据的同时,可以删除与之相关的从表中的数据
    
    级联删除
        on delete cascade
*/

-- 重新创建添加级联操作
CREATE TABLE employee(
    eid INT PRIMARY KEY AUTO_INCREMENT,
    ename VARCHAR(20),
    age INT,
    dept_id INT,
    CONSTRAINT emp_dept_fk FOREIGN KEY(dept_id) REFERENCES department(id)
    -- 添加级联删除
    ON DELETE CASCADE
);

-- 添加数据
INSERT INTO employee (ename, age, dept_id) VALUES ('张百万', 20, 1); 
INSERT INTO employee (ename, age, dept_id) VALUES ('赵四', 21, 1); 
INSERT INTO employee (ename, age, dept_id) VALUES ('广坤', 20, 1); 
INSERT INTO employee (ename, age, dept_id) VALUES ('小斌', 20, 2); 
INSERT INTO employee (ename, age, dept_id) VALUES ('艳秋', 22, 2); 
INSERT INTO employee (ename, age, dept_id) VALUES ('大玲子', 18, 2); 

-- 删除部门编号为 2 的数据
DELETE FROM department WHERE id = 2;

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Walter Savage Landor:strove with none,for none was worth my strife.Nature I loved and, next to Nature, Art:I warm'd both hands before the fire of life.It sinks, and I am ready to depart

                                                                                                                                                   ——W.S.Landor

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