确保 SQLite 表只有一行 [英] Ensure SQLite table only has one row

问题描述

如何强制一个表只有一行?以下是我尝试过的.UPDATE 触发器可能会起作用，但是 CREATE 触发器肯定不会.对于CREATE，我想使用SET，但是SQLite 不支持SET.

How can I enforce a table to have only one row? Below is what I tried. The UPDATE trigger might work, however, the CREATE trigger definitely will not. For the CREATE, I would like to use SET, however, SET is not supported by SQLite.

CREATE TABLE IF NOT EXISTS `config` (
  `id` TINYINT NOT NULL DEFAULT 0,
  `subdomain` VARCHAR(45) NOT NULL,
  `timezone` CHAR(3) NOT NULL,
  `timeout` TINYINT NOT NULL,
  `offline` TINYINT NOT NULL,
  `hash_config` CHAR(32) NOT NULL,
  `hash_points` CHAR(32) NOT NULL,
  PRIMARY KEY (`id`));

INSERT INTO config(id,subdomain,timezone,timeout,offline,hash_config,hash_points) VALUES(0,'subdomain','UTC',5,0,'hash_config','hash_points');

CREATE TRIGGER `config_insert_zero`
BEFORE INSERT ON `config`
FOR EACH ROW
BEGIN
   -- SET NEW.id=0;
   NEW.id=OLD.id;
END;

CREATE TRIGGER `config_update_zero`
BEFORE UPDATE ON `config`
FOR EACH ROW
BEGIN
   -- SET NEW.id=0;
   NEW.id=OLD.id;
END;

推荐答案

在一般情况下，要限制表中的行数，您必须防止任何进一步的插入.在 SQLite 中，这是通过 RAISE() 完成的:

In the general case, to limit the number of rows in a table, you have to prevent any further insert. In SQLite, this is done with RAISE():

CREATE TRIGGER config_no_insert
BEFORE INSERT ON config
WHEN (SELECT COUNT(*) FROM config) >= 1   -- limit here
BEGIN
    SELECT RAISE(FAIL, 'only one row!');
END;

但是，如果限制为 1，您可以简单地将主键限制为一个固定值:

However, if the limit is one, you could instead simply constrain the primary key to a fixed value:

CREATE TABLE config (
    id INTEGER PRIMARY KEY CHECK (id = 0),
    [...]
);

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