实现“两者，或者，但不是空”数据库中的要求 [英] Implementing a "Both, Either-or, but Not Null" Requirement in a Database

问题描述

我有一个网络应用程序的要求，指出用户应该可以上传指令文档（.pdf，.doc，.txt）或提供说明文本。用户可以上传文档并提供文本，或者可以进行一对一的操作，但是它们必须执行某些操作（不可为空）。这在数据库中如何设计？这会被认为是一个完整的子类型（见下文）？

I have requirement for a web app that states that a user should be able to either upload an instruction document(s) (.pdf, .doc, .txt) or provide text for the instructions. The user can upload a document and provide text, or they can do one-or-the-other, but they have to do something (not nullable). How would this be designed in a database? Would this be considered a complete sub-type (see below)?

这是一个更大的模式的小部分，所以我刚刚发布了我觉得这个特定问题是必要的。

This is tiny part of a larger schema, so I just posted what I felt was necessary for this particular question.

推荐答案

Ypercube的回答是好的，除了这可以实际上完全通过声明完整性完成，同时保持单独的表。诀窍是将推迟的循环外部键与一点创造性的非规范化相结合：

Ypercube's answer is fine, except this can, in fact, be done purely through declarative integrity while keeping separate tables. The trick is to combine deferred circular FOREIGN KEYs with a little bit of creative denormalization:

CREATE TABLE Instruction (
    InstructionId INT PRIMARY KEY,
    TextId INT UNIQUE,
    DocumentId INT UNIQUE,
    CHECK (
        (TextId IS NOT NULL AND InstructionId = TextId)
        OR (DocumentId IS NOT NULL AND InstructionId = DocumentId)
    )
);

CREATE TABLE Text (
    InstructionId INT PRIMARY KEY,
    FOREIGN KEY (InstructionId) REFERENCES Instruction (TextId) ON DELETE CASCADE
);

CREATE TABLE Document (
    InstructionId INT PRIMARY KEY,
    FOREIGN KEY (InstructionId) REFERENCES Instruction (DocumentId) ON DELETE CASCADE
);

ALTER TABLE Instruction ADD FOREIGN KEY (TextId) REFERENCES Text DEFERRABLE INITIALLY DEFERRED;
ALTER TABLE Instruction ADD FOREIGN KEY (DocumentId) REFERENCES Document DEFERRABLE INITIALLY DEFERRED;

插入文本是这样完成的：

Inserting Text is done like this:

INSERT INTO Instruction (InstructionId, TextId) VALUES (1, 1);
INSERT INTO Text (InstructionId) VALUES (1);
COMMIT;

插入文档如下：

INSERT INTO Instruction (InstructionId, DocumentId) VALUES (2, 2);
INSERT INTO Document (InstructionId) VALUES (2);
COMMIT;

同时插入文本和文档：

INSERT INTO Instruction (InstructionId, TextId, DocumentId) VALUES (3, 3, 3);
INSERT INTO Text (InstructionId) VALUES (3);
INSERT INTO Document (InstructionId) VALUES (3);
COMMIT;

但是，尝试单独插入指令失败：

However, trying to insert Instruction alone fails on commit:

INSERT INTO Instruction (InstructionId, TextId) VALUES (4, 4);
COMMIT; -- Error (FOREIGN KEY violation).

尝试插入不匹配类型时，失败 / p>

Attempting to insert the "mismatched type" also fails on commit:

INSERT INTO Document (InstructionId) VALUES (1);
COMMIT; -- Error (FOREIGN KEY violation).

当然，尝试将错误值插入到指令失败提交前的时间）：

And of course, trying to insert bad values into Instruction fails (this time before commit):

INSERT INTO Instruction (InstructionId, TextId) VALUES (5, 6); -- Error (CHECK violation).
INSERT INTO Instruction (InstructionId) VALUES (7); -- Error (CHECK violation).

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