SQLite 长臂检查约束？

Question

我在 SQLite 中有两个表，建模一对多关系：

CREATE TABLE parent (
    id INTEGER PRIMARY KEY,
    payload TEXT
);

CREATE TABLE child (
    id INTEGER PRIMARY KEY,
    flag BOOLEAN,
    parent_id INTEGER,
    FOREIGN KEY(parent_id) REFERENCES parent (id) ON DELETE CASCADE ON UPDATE CASCADE, 
);

有没有办法在child.flag上放一个CHECK CONSTRAINT，这样对于任何[=15]，在所有child中总是只有一个True =]?

Answer 1

我的研究表明，没有办法让本地检查约束知道它的兄弟状态。我建议将检查逻辑放在应用层。

Answer 2

"Although this is solved by putting application-level logic, I still would like to see if there is any creative solution in pure database, not involving apps and triggers."

是的，可以通过 partial unique index:

实现所需的约束

CREATE UNIQUE INDEX idx ON child (flag, parent_id) WHERE flag = 1;

db<>fiddle demo

INSERT INTO parent(id, payload) VALUES(1, 'Parent1');
INSERT INTO child(id, flag, parent_id) VALUES (1, 1, 1);
INSERT INTO child(id, flag, parent_id) VALUES (2, 0, 1);
INSERT INTO child(id, flag, parent_id) VALUES (3, 0, 1);
SELECT * FROM child;

-- trying to insert second active child will cause unique index violation
INSERT INTO child(id, flag, parent_id) VALUES (4, 1, 1)
-- UNIQUE constraint failed: child.flag, child.parent_id

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