我什么时候应该为字段创建一个单独的表？

Question

Let's say i have this SQL table:

CREATE TABLE `article` (
 `article_id` int(11) NOT NULL AUTO_INCREMENT,
 `name` varchar(255) NOT NULL,
 `family` varchar(255) NOT NULL,
 `sub_family` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
 `price` float NOT NULL,
 PRIMARY KEY (`article_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

Here every family of articles has one or multiple sub_family

should i create another table that has all possible combinations of family and sub_family and refer to it using a foreing key ?

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Data example:

Answer 1

I read your question and comments as well. I think you can create two different tables that'll for sure reduce the data redundancy and complexity.

You can create Family as a parent table: In Family table you can declare a unique key(which you can use further for foreign key referencing) for each Family and sub-Family pair for example:

Family:

ID   | Family_name | Subfamily
1       Beverage     Hot Drinks
2       Beverage     cold Drinks
3       main course  Pasta

And you can create another table for Articles which will be connected with parent via foreign key reference ID.

Article:

  ID  |  Family | Name       | Price
  1       2      Milk Shake    100.00
  2       1      Coffee        200.00

You can try something like this. I hope you find this easy to understand.

Answer 2

If you want to fix redundancy completely, please try the following table:

create table `family` (
 `family_id` int(11) NOT NULL AUTO_INCREMENT,
 `family` varchar(255) NOT NULL,
 `parent_id` int(11) NOT NULL,
 PRIMARY KEY (`family_id`),
 index idx_family_sub (`family`, `parent_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `article` (
 `article_id` int(11) NOT NULL AUTO_INCREMENT,
 `name` varchar(255) NOT NULL,
 `family_id` int(11) not null,
 `price` float NOT NULL,
 PRIMARY KEY (`article_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

The data in there 2 table may look like:

mysql> select * from family order by family_id;
+-----------+-------------+-----------+
| family_id | family      | parent_id |
+-----------+-------------+-----------+
|         1 | Main Course |         0 |
|         2 | Beverage    |         0 |
|         3 | Pasta       |         1 |
|         4 | Hot Drinks  |         2 |
|         5 | Cold Drinks |         2 |
+-----------+-------------+-----------+
5 rows in set (0.00 sec)

mysql> select * from article;
+------------+-----------+-----------+-------+
| article_id | name      | family_id | price |
+------------+-----------+-----------+-------+
|          1 | Spaghetti |         3 |   600 |
|          2 | Penne     |         3 |   500 |
|          3 | Coffee    |         4 |   100 |
|          4 | Penne     |         5 |   150 |
+------------+-----------+-----------+-------+
4 rows in set (0.00 sec)

The query to get all data like data example is:

mysql> select
    ->        a.article_id, a.name, f2.family as family, f.family as sub_family, a.price
    -> from `article2` a
    ->     join family2 f on a.family_id = f.family_id
    ->     join family2 as f2 on f.parent_id = f2.family_id;
+------------+-----------+-------------+-------------+-------+
| article_id | name      | family      | sub_family  | price |
+------------+-----------+-------------+-------------+-------+
|          1 | Spaghetti | Main Course | Pasta       |   600 |
|          2 | Penne     | Main Course | Pasta       |   500 |
|          3 | Coffee    | Beverage    | Hot Drinks  |   100 |
|          4 | Penne     | Beverage    | Cold Drinks |   150 |
+------------+-----------+-------------+-------------+-------+
4 rows in set (0.00 sec)

From my perspective, this table is used to save food menu data, and it should be very small. I think it's OK to create a dedicated table to save family and sub_family.