Question

如何设置alter table add列，其条件如下所示。添加有关员工拥有的爱好的信息;一个员工拥有一个或多个爱好，一个业余爱好被零个或多个员工拥有

如何设置alter table以添加具有此类条件的列？对不起，我还是sql查询条件的新手。

Answer 1

为了您的解释，我这样做了：你必须创建3个表，这是一个多对多的关系，即一个有几个爱好和爱好的员工可以是各种各样的员工。一张员工，其他爱好和其他两个外键的表格。

CREATE TABLE `employees` (
  `id_employees` INT NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(45) NULL,
  PRIMARY KEY (`id_employees`));

CREATE TABLE `hobbies` (
  `id_hobbies` INT NOT NULL AUTO_INCREMENT,
  `name` INT NULL,
  PRIMARY KEY (`id_hobbies`));

CREATE TABLE `employees_hobbies` (
  `id_employees_hobbies` INT NOT NULL AUTO_INCREMENT,
  `id_employees` INT NOT NULL,
  `id_hobbies` INT NOT NULL,
  PRIMARY KEY (`id_employees_hobbies`),
  INDEX `fk_employees_hobbies_1_idx` (`id_employees` ASC),
  INDEX `fk_employees_hobbies_2_idx` (`id_hobbies` ASC),
  CONSTRAINT `fk_employees_hobbies_1`
    FOREIGN KEY (`id_employees`)
    REFERENCES `employees` (`id_employees`)
    ON DELETE NO ACTION
    ON UPDATE NO ACTION,
  CONSTRAINT `fk_employees_hobbies_2`
    FOREIGN KEY (`id_hobbies`)
    REFERENCES `hobbies` (`id_hobbies`)
    ON DELETE NO ACTION
    ON UPDATE NO ACTION);

如果您已经有表并希望更改它们，

ALTER TABLE employees_hobbies ADD id_employees  INT NOT NULL;
ALTER TABLE employees_hobbies ADD id_hobbies  INT NOT NULL;
ALTER TABLE employees_hobbies ADD CONSTRAINT `fk_employees_hobbies_1`
        FOREIGN KEY (`id_employees`)
        REFERENCES `employees` (`id_employees`)
        ON DELETE NO ACTION
        ON UPDATE NO ACTION;
ALTER TABLE employees_hobbies ADD CONSTRAINT `fk_employees_hobbies_2`
        FOREIGN KEY (`id_hobbies`)
        REFERENCES `hobbies` (`id_hobbies`)
        ON DELETE NO ACTION
        ON UPDATE NO ACTION);

您可以使用ADD修改现有列，而不是MODIFY。检查下面的来源。

字体： Alter Table

如何在sql plus中设置alter table add列

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