问题描述

我有下表:

CREATE TABLE child( 
  id INTEGER PRIMARY KEY, 
  parent_id INTEGER, 
  description TEXT);

如何在 parent_id 上添加外键约束?假设启用了外键.

How do I add a foreign key constraint on parent_id? Assume foreign keys are enabled.

大多数示例都假设您正在创建表 - 我想将约束添加到现有表中.

Most examples assume you're creating the table - I'd like to add the constraint to an existing one.

推荐答案

你不能.

尽管向表中添加外键的 SQL-92 语法如下所示:

Although the SQL-92 syntax to add a foreign key to your table would be as follows:

ALTER TABLE child ADD CONSTRAINT fk_child_parent
                  FOREIGN KEY (parent_id) 
                  REFERENCES parent(id);

SQLite 不支持 ALTER TABLE 命令的 ADD CONSTRAINT 变体 (sqlite.org:SQLite 未实现的 SQL 功能).

SQLite doesn't support the ADD CONSTRAINT variant of the ALTER TABLE command (sqlite.org: SQL Features That SQLite Does Not Implement).

因此，在sqlite 3.6.1中添加外键的唯一方法是在CREATE TABLE过程中，如下所示:

Therefore, the only way to add a foreign key in sqlite 3.6.1 is during CREATE TABLE as follows:

CREATE TABLE child ( 
    id           INTEGER PRIMARY KEY, 
    parent_id    INTEGER, 
    description  TEXT,
    FOREIGN KEY (parent_id) REFERENCES parent(id)
);

不幸的是，您必须将现有数据保存到临时表中，删除旧表，使用 FK 约束创建新表，然后从临时表中复制数据.(sqlite.org - 常见问题解答:Q11)

Unfortunately you will have to save the existing data to a temporary table, drop the old table, create the new table with the FK constraint, then copy the data back in from the temporary table. (sqlite.org - FAQ: Q11)

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