php 什么是数据库中的父表和子表？

What is a Parent table and a Child table in Database?

提问by BruceyBandit

I just want to know what is a parent table and what is a child table in databases. Can you please show me an example so I understand how it works please.

我只想知道什么是父表，什么是数据库中的子表。请你给我看一个例子，让我了解它是如何工作的。

Thank You

谢谢你

回答by Martin Bean

Child tables and parent tables are just normal database tables, but they're linked in a way that's described by a parent–child relationship.

子表和父表只是普通的数据库表，但它们以父子关系描述的方式链接。

It's usually used to specify where one table's value refers to the value in another table (usually a primary key of another table).

它通常用于指定一个表的值在哪里引用另一个表中的值（通常是另一个表的主键）。

For example, imagine a news article. This could be represented by a table called articlesand has fields for id, headline, body, published_dateand author. But instead of placing a name in the authorfield, you could instead put the ID value of a user in a separate table—maybe called authors—that has information on authors such as id, name, and email.

例如，想象一篇新闻文章。这可以通过代表的表中调用articles，并具有字段id，headline，body，published_date和author。但不是在放置一个名称author字段，可以代替把用户的ID值在一个单独的表，也可以叫做authors-即对作家如信息id，name和email。

Therefore, if you need to update an author's name, you only need to do so in the authors(parent) table; because the articles(child) table only contains the ID of the corresponding authorrecord.

因此，如果需要更新作者姓名，只需在authors（父）表中进行即可；因为articles（子）表只包含对应author记录的ID 。

Hope this helps you understand better.

希望这可以帮助您更好地理解。

回答by GilmoursBlackStrat

Be aware you can have relationships that appear to be parent-child but are not, for instance when lookup tables are being used. The distinction is that in a true parent-child relationship, records typically don't stand are their own very well - they are detail records for the parent and are not useful without the parent table info. A person can own multiple cars in the DMV database, but you wouldn't want records in the CARS table without a parent record in the OWNERS table - it would be nearly useless data.

请注意，您可以拥有看似父子但并非如此的关系，例如在使用查找表时。区别在于，在真正的父子关系中，记录通常不能很好地代表它们自己——它们是父的详细记录，如果没有父表信息，则没有用。一个人可以在 DMV 数据库中拥有多辆汽车，但您不希望 CARS 表中的记录没有 OWNERS 表中的父记录 - 这几乎是无用的数据。

On the other hand, if I am using a lookup table to expand a code to something more meaningful, or to constrain data entry to acceptable values, then the "child" record can still useful (can stand alone) if the lookup table is deleted. I could still have the sex information as "M" or "F" even if I no longer have the lookup table to expand that to "Male" or "Female".

另一方面，如果我使用查找表将代码扩展为更有意义的内容，或者将数据输入限制为可接受的值，那么如果查找表被删除，“子”记录仍然有用（可以独立） . 即使我不再拥有将其扩展为“男”或“女”的查找表，我仍然可以将性别信息设为“M”或“F”。

回答by pehur

Parent - The entity on the "one" (/1) side of a relation with another table

父级 - 与另一个表的关系的“一”(/1) 一侧的实体

Child - The entity on the "many" (/N/*) side of a relation with another table

Child - 与另一个表的关系的“多”(/N/*) 一侧的实体

回答by Marc B

A child table tends to be one where it has one or more foreign keys pointing at some other table(s). Note that a child table can itself be a parent to some OTHER table as well.

子表往往是一个或多个外键指向其他表的表。请注意，子表本身也可以是某些 OTHER 表的父表。

回答by KaungKhant MinTun

Those terms are used in database relationships.

这些术语用于数据库关系。

for example u have two table,

例如你有两张桌子，

1.Manifast

1.Manifast

+-------------+------------------+------+-----+---------+----------------+
| Field       | Type             | Null | Key | Default | Extra          |
+-------------+------------------+------+-----+---------+----------------+
| manifast_id | int(11) unsigned | NO   | PRI | NULL    | auto_increment |
| description | text             | NO   |     | NULL    |                |
| title       | text             | NO   |     | NULL    |                |
+-------------+------------------+------+-----+---------+----------------+

1. day_sequence

2. day_sequence

+-----------------+------------------+------+-----+---------+----------------+
| Field           | Type             | Null | Key | Default | Extra          |
+-----------------+------------------+------+-----+---------+----------------+
| day_sequence_id | int(11) unsigned | NO   | PRI | NULL    | auto_increment |
| day_number      | int(11)          | NO   |     | NULL    |                |
| day_start       | int(11)          | NO   |     | NULL    |                |
| manifast_id     | int(11)          | NO   |     | NULL    |                |
+-----------------+------------------+------+-----+---------+----------------+

+-----------------+------------------+------+-----+---------+----------------+
| Field           | Type             | Null | Key | Default | Extra          |
+-----------------+------------------+------+-----+---------+----------------+
| day_sequence_id | int(11) unsigned | NO   | PRI | NULL    | auto_increment |
| day_number      | int(11)          | NO   |     | NULL    |                |
| day_start       | int(11)          | NO   |     | NULL    |                |
| manifast_id     | int(11)          | NO   |     | NULL    |                |
+-----------------+------------------+------+-----+---------+----------------+

if u want to connect those two tables,u need to use the command with following format.

如果你想连接这两个表，你需要使用以下格式的命令。

> ALTER TABLE child_table_name ADD FOREIGN KEY (P_ID) REFERENCES
> parent_table_name (P_ID)

and so it become.

就这样变成了。

> ALTER TABLE day_sequence ADD CONSTRAINT fk_manifast FOREIGN KEY
> (manifast_Id) REFERENCES manifast(manifast_Id);

In summary, Child table is a table which has foreign key,and is connected from others table. Parent table has no foreign key and connect to other. [ Note : This ans is just for connecting two tables ]

综上所述，子表是一个有外键的表，与其他表相连。父表没有外键并连接到其他表。[注：此 ans 仅用于连接两个表]