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我正在设计一个数据库模型,并且出现了一个问题:指定关系的连接表一侧是否可选的关键点是什么?生成的DDL?连接表关系中的数据库设计和可选性
例如采取以下两个不同的图:
图与在A_to_B_join侧A和A_to_B_join之间强制关系:
图与在A_to_B_join侧A和A_to_B_join之间可选关系:
我注意到他们都试图生成相同的DDL,即使他们是d ifferent!
生成的DDL:
SET @[email protected]@UNIQUE_CHECKS, UNIQUE_CHECKS=0;
SET @[email protected]@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0;
SET @[email protected]@SQL_MODE, SQL_MODE='TRADITIONAL';
CREATE SCHEMA IF NOT EXISTS `mydb` DEFAULT CHARACTER SET latin1 COLLATE latin1_swedish_ci ;
USE `mydb` ;
-- -----------------------------------------------------
-- Table `mydb`.`A`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `mydb`.`A` (
`A_ID` INT NOT NULL ,
PRIMARY KEY (`A_ID`))
ENGINE = InnoDB;
-- -----------------------------------------------------
-- Table `mydb`.`B`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `mydb`.`B` (
`B_ID` INT NOT NULL ,
PRIMARY KEY (`B_ID`))
ENGINE = InnoDB;
-- -----------------------------------------------------
-- Table `mydb`.`A_to_B_join`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `mydb`.`A_to_B_join` (
`A_ID` INT NOT NULL ,
`B_ID` INT NOT NULL ,
PRIMARY KEY (`A_ID`, `B_ID`) ,
INDEX `fk_A_to_B_join_B1` (`B_ID` ASC) ,
INDEX `fk_A_to_B_join_A` (`A_ID` ASC) ,
CONSTRAINT `fk_A_to_B_join_A`
FOREIGN KEY (`A_ID`)
REFERENCES `mydb`.`A` (`A_ID`)
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_A_to_B_join_B1`
FOREIGN KEY (`B_ID`)
REFERENCES `mydb`.`B` (`B_ID`)
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB;
SET [email protected]_SQL_MODE;
SET [email protected]_FOREIGN_KEY_CHECKS;
SET [email protected]_UNIQUE_CHECKS;
有什么意义,然后提一边是可选的还是强制的?我应该打扰吗? DDL是否与MySQL相同?
Regards,