主键有什么特别之处?

模式中表的用途是什么?表中键的作用是什么?主键有什么特别之处?围绕主键的讨论似乎忽略了一点，即主键是表的一部分，而表是模式的一部分。对表和表关系最有利的应该驱动所使用的键。

Tables (and table relationships) contain facts about information you wish to record. These facts should be self-contained, meaningful, easily understood, and non-contradictory. From a design perspective, other tables added or removed from a schema should not impact on the table in question. There must be a purpose for storing the data related only to the information itself. Understanding what is stored in a table should not require undergoing a scientific research project. No fact stored for the same purpose should be stored more than once. Keys are a whole or part of the information being recorded which is unique, and the primary key is the specially designated key that is to be the primary access point to the table (i.e. it should be chosen for data consistency and usage, not just insert performance).

ASIDE: The unfortunately side effect of most databases being designed and developed by application programmers (which I am sometimes) is that what is best for the application or application framework often drives the primary key choice for tables. This leads to integer and GUID keys (as these are simple to use for application frameworks) and monolithic table designs (as these reduce the number of application framework objects needed to represent the data in memory). These application driven database design decisions lead to significant data consistency problems when used at scale. Application frameworks designed in this manner naturally lead to table at a time designs. “Partial records” are created in tables and data filled in over time. Multi-table interaction is avoided or when used causes inconsistent data when the application functions improperly. These designs lead to data that is meaningless (or difficult to understand), data spread over tables (you have to look at other tables to make sense of the current table), and duplicated data.

It was said that primary keys should be as small as necessary. I would says that keys should be only as large as necessary. Randomly adding meaningless fields to a table should be avoided. It is even worse to make a key out of a randomly added meaningless field, especially when it destroys the join dependency from another table to the non-primary key. This is only reasonable if there are no good candidate keys in the table, but this occurrence is surely a sign of a poor schema design if used for all tables.

It was also said that primary keys should never change as updating a primary key should always be out of the question. But update is the same as delete followed by insert. By this logic, you should never delete a record from a table with one key and then add another record with a second key. Adding the surrogate primary key does not remove the fact that the other key in the table exists. Updating a non-primary key of a table can destroy the meaning of the data if other tables have a dependency on that meaning through a surrogate key (e.g. a status table with a surrogate key having the status description changed from ‘Processed’ to ‘Cancelled’ would definitely corrupt the data). What should always be out of the question is destroying data meaning.

Having said this, I am grateful for the many poorly designed databases that exist in businesses today (meaningless-surrogate-keyed-data-corrupted-1NF behemoths), because that means there is an endless amount of work for people that understand proper database design. But on the sad side, it does sometimes make me feel like Sisyphus, but I bet he had one heck of a 401k (before the crash). Stay away from blogs and websites for important database design questions. If you are designing databases, look up CJ Date. You can also reference Celko for SQL Server, but only if you hold your nose first. On the Oracle side, reference Tom Kyte.

如果有天然钥匙，通常是最好的。因此，如果datetime/char唯一地标识了行，并且这两部分对行都有意义，那就太好了。

如果只有datetime是有意义的，并且只是附加了char以使其唯一，那么您不妨使用一个identify字段。

我寻找自然主键，并尽可能地使用它们。

如果找不到自然的键，我更喜欢GUID而不是INT++，因为SQL Server使用树，总是在树的末尾添加键是不好的。

在多对多耦合的表上，我使用外键的复合主键。

因为我很幸运使用SQL Server，我可以用分析器和查询分析器研究执行计划和统计数据，并很容易地发现我的键是如何执行的。

以下是我拥有25年以上开发经验后得出的经验法则。

所有表都应该有一个单列主键auto 增量。 将它包含在任何意味着可更新的视图中 主键在应用程序上下文中不应该有任何意义。这意味着它不应该是SKU、帐号、员工id或对应用程序有意义的任何其他信息。它只是一个与实体相关联的唯一键。

主键由数据库用于优化目的，应用程序除了用于标识特定实体或与特定实体相关外，不应该使用主键。

始终使用单一值主键使得执行upsert非常简单。

Favor multiple indices on single columns over multi-column indices. For example, if you have a two column key, favor creating an index on each column over creating a two column index. If we create a multi-column key on firstname + lastname, we can't do indexed lookups on lastname without providing a firstname as well. Having indices on both columns allows the optimizer to perform indexed lookups on either or both columns regardless of how they are expressed in your WHERE clause. If your tables are massive, explore partitioning the table into segments based on the most prominent search criteria. If you have a table that has a significant number of Id fields in it, consider removing all except the primary key to a single table which has an id (PK), an org_id (FK to original table) and an id_type column. Create indices for all columns on the new table and relate it to the original table. In this manner, you can now perform indexed lookups of any number of ids using only a single index.

我总是使用自动编号或标识字段。

我曾经为一个客户工作，他使用SSN作为主键，然后由于HIPAA法规被迫更改为“MemberID”，这在更新相关表中的外键时引起了大量问题。坚持一致的标识列标准帮助我在所有项目中避免了类似的问题。

表应该一直有一个主键。如果没有，它应该是一个自动递增字段。

有时人们会省略主键，因为他们要传输大量数据，这可能会减慢(取决于数据库)进程。但是，它应该加在它之后。

一些关于链接表的评论，这是正确的，这是一个例外，但是字段应该是FK以保持完整性，并且在某些情况下，如果链接中的重复没有被授权，这些字段也可以是主键…但是要保持简单的形式，因为异常在编程中经常出现，所以应该提供主键来保持数据的完整性。