防止在数据库中使用重复的标签

风随心旨

我想知道如何防止在数据库表中使用两个相同的标签。有人说我在表中使用两个私钥。但是，W3Schools-网站表示这是不可能的。

解决方案:


            我已经更新了NORMA模型，使其与您的图表更加匹配。我可以看到您在哪里犯了一些错误，但是其中一些错误可能是由于我的早期模型造成的。
我已更新此模型以防止重复标签。以前真的没关系。但是，既然您想要它，这里就是（对于Postgres）：
START TRANSACTION ISOLATION LEVEL SERIALIZABLE, READ WRITE;
CREATE SCHEMA so;
SET search_path TO SO,"$user",public;
CREATE DOMAIN so.HashedPassword AS
BIGINT CONSTRAINT HashedPassword_Unsigned_Chk CHECK (VALUE >= 0);
CREATE TABLE so."User"
(
USER_ID SERIAL NOT NULL,
USER_NAME CHARACTER VARYING(50) NOT NULL,
EMAIL_ADDRESS CHARACTER VARYING(256) NOT NULL,
HASHED_PASSWORD so.HashedPassword NOT NULL,
OPEN_ID CHARACTER VARYING(512),
A_MODERATOR BOOLEAN,
LOGGED_IN BOOLEAN,
HAS_BEEN_SENT_A_MODERATOR_MESSAGE BOOLEAN,
CONSTRAINT User_PK PRIMARY KEY(USER_ID)
);
CREATE TABLE so.Question
(
QUESTION_ID SERIAL NOT NULL,
TITLE CHARACTER VARYING(256) NOT NULL,
WAS_SENT_AT_TIME TIMESTAMP NOT NULL,
BODY CHARACTER VARYING NOT NULL,
USER_ID INTEGER NOT NULL,
FLAGGED_FOR_MODERATOR_REMOVAL BOOLEAN,
WAS_LAST_CHECKED_BY_MODERATOR_AT_TIME TIMESTAMP,
CONSTRAINT Question_PK PRIMARY KEY(QUESTION_ID)
);
CREATE TABLE so.Tag
(
TAG_ID SERIAL NOT NULL,
TAG_NAME CHARACTER VARYING(20) NOT NULL,
CONSTRAINT Tag_PK PRIMARY KEY(TAG_ID),
CONSTRAINT Tag_UC UNIQUE(TAG_NAME)
);
CREATE TABLE so.QuestionTaggedTag
(
QUESTION_ID INTEGER NOT NULL,
TAG_ID INTEGER NOT NULL,
CONSTRAINT QuestionTaggedTag_PK PRIMARY KEY(QUESTION_ID, TAG_ID)
);
CREATE TABLE so.Answer
(
ANSWER_ID SERIAL NOT NULL,
BODY CHARACTER VARYING NOT NULL,
USER_ID INTEGER NOT NULL,
QUESTION_ID INTEGER NOT NULL,
CONSTRAINT Answer_PK PRIMARY KEY(ANSWER_ID)
);
ALTER TABLE so.Question
ADD CONSTRAINT Question_FK FOREIGN KEY (USER_ID)
REFERENCES so."User" (USER_ID) ON DELETE RESTRICT ON UPDATE RESTRICT;
ALTER TABLE so.QuestionTaggedTag
ADD CONSTRAINT QuestionTaggedTag_FK1 FOREIGN KEY (QUESTION_ID)
REFERENCES so.Question (QUESTION_ID) ON DELETE RESTRICT ON UPDATE RESTRICT;
ALTER TABLE so.QuestionTaggedTag
ADD CONSTRAINT QuestionTaggedTag_FK2 FOREIGN KEY (TAG_ID)
REFERENCES so.Tag (TAG_ID) ON DELETE RESTRICT ON UPDATE RESTRICT;
ALTER TABLE so.Answer
ADD CONSTRAINT Answer_FK1 FOREIGN KEY (USER_ID)
REFERENCES so."User" (USER_ID) ON DELETE RESTRICT ON UPDATE RESTRICT;
ALTER TABLE so.Answer
ADD CONSTRAINT Answer_FK2 FOREIGN KEY (QUESTION_ID)
REFERENCES so.Question (QUESTION_ID) ON DELETE RESTRICT ON UPDATE RESTRICT;
COMMIT WORK;
请注意，现在有一个单独的Tag表TAG_ID作为主键。TAG_NAME是一个具有唯一性约束的单独列，可防止重复的标签。该QuestionTaggedTag表现在具有（QUESTION_ID，TAG_ID），这也是它的主键。
我希望我不会在回答这个问题上走得太远，但是当我尝试编写较小的答案时，我一直不得不解开我以前的答案，而仅仅发布它似乎更简单。

防止在数据库中使用重复的标签

风随心旨 LV1