> For the complete documentation index, see [llms.txt](https://docs.postgresql.tw/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.postgresql.tw/tutorial/tutorial-advanced/tutorial-fk.md).

# 3.3. Foreign Keys \#

Recall the `weather` and `cities` tables from [Chapter 2](/tutorial/tutorial-sql.md). Consider the following problem: You want to make sure that no one can insert rows in the `weather` table that do not have a matching entry in the `cities` table. This is called maintaining the *referential integrity* of your data. In simplistic database systems this would be implemented (if at all) by first looking at the `cities` table to check if a matching record exists, and then inserting or rejecting the new `weather` records. This approach has a number of problems and is very inconvenient, so PostgreSQL can do this for you.

The new declaration of the tables would look like this:

```

CREATE TABLE cities (
        name     varchar(80) primary key,
        location point
);

CREATE TABLE weather (
        city      varchar(80) references cities(name),
        temp_lo   int,
        temp_hi   int,
        prcp      real,
        date      date
);
```

Now try inserting an invalid record:

```

INSERT INTO weather VALUES ('Berkeley', 45, 53, 0.0, '1994-11-28');
```

```

ERROR:  insert or update on table "weather" violates foreign key constraint "weather_city_fkey"
DETAIL:  Key (city)=(Berkeley) is not present in table "cities".
```

The behavior of foreign keys can be finely tuned to your application. We will not go beyond this simple example in this tutorial, but just refer you to [Chapter 5](/the-sql-language/ddl.md) for more information. Making correct use of foreign keys will definitely improve the quality of your database applications, so you are strongly encouraged to learn about them.

***

原文：[PostgreSQL 18.6 Documentation](https://www.postgresql.org/docs/18/tutorial-fk.html)（英文原文，待翻譯）


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