将表列移动到新表并在 PostgreSQL 中作为外键引用

假设我们有一个包含字段的数据库表

"id", "category", "subcategory", "brand", "name", "description", etc. 

创建单独的表的好方法是什么category, subcategory and brand以及原始表中相应的列和行成为外键引用？

概述所涉及的操作：

• 获取原始表的每一列中应成为外键的所有唯一值；
• 为这些创建表
• 在原始表（或副本）中创建外键引用列

在本例中，PostgreSQL 数据库是通过 Ruby 应用程序中的 Sequel 访问的，因此可用的接口有命令行、Sequel、PGAdmin 等...

问题是：你会怎么做？

        -- Some test data
CREATE TABLE animals
        ( id SERIAL NOT NULL PRIMARY KEY
        , name varchar
        , category varchar
        , subcategory varchar
        );
INSERT INTO animals(name, category, subcategory) VALUES
 ( 'Chimpanzee' , 'mammals', 'apes' )
,( 'Urang Utang' , 'mammals', 'apes' )
,( 'Homo Sapiens' , 'mammals', 'apes' )
,( 'Mouse' , 'mammals', 'rodents' )
,( 'Rat' , 'mammals', 'rodents' )
        ;

        -- [empty] table to contain the "squeezed out" domain
CREATE TABLE categories
        ( id SERIAL NOT NULL PRIMARY KEY
        , category varchar
        , subcategory varchar
        , UNIQUE (category,subcategory)
        );

        -- The original table needs a "link" to the new table
ALTER TABLE animals
        ADD column category_id INTEGER -- NOT NULL
        REFERENCES categories(id)
        ;
        -- FK constraints are helped a lot by a supportive index.
CREATE INDEX animals_categories_fk ON animals (category_id);

        -- Chained query to:
        -- * populate the domain table
        -- * initialize the FK column in the original table
WITH ins AS (
        INSERT INTO categories(category, subcategory)
        SELECT DISTINCT a.category, a.subcategory
        FROM animals a
        RETURNING *
        )
UPDATE animals ani
SET category_id = ins.id
FROM ins
WHERE ins.category = ani.category
AND ins.subcategory = ani.subcategory
        ;

        -- Now that we have the FK pointing to the new table,
        -- we can drop the redundant columns.
ALTER TABLE animals DROP COLUMN category, DROP COLUMN subcategory;

        -- show it to the world
SELECT a.*
        , c.category, c.subcategory
FROM animals a
JOIN categories c ON c.id = a.category_id
        ;

注：片段：

WHERE ins.category = ani.category AND ins.subcategory = ani.subcategory

如果这些列包含 NULL，将会导致问题。 最好使用它们进行比较

（ins.类别，ins.子类别） 与以下没有什么不同 (ani.类别,ani.子类别)