What's the best practice for primary keys in t

问题:

When designing tables, I\'ve developed a habit of having one column that is unique and that I make the primary key. This is achieved in three ways depending on requirements:

1. Identity integer column that auto increments.
2. Unique identifier (GUID)
3. A short character(x) or integer (or other relatively small numeric type) column that can serve as a row identifier column

Number 3 would be used for fairly small lookup, mostly read tables that might have a unique static length string code, or a numeric value such as a year or other number.

For the most part, all other tables will either have an auto-incrementing integer or unique identifier primary key.

The Question :-)

I have recently started working with databases that have no consistent row identifier and primary keys are currently clustered across various columns. Some examples:

• datetime/character
• datetime/integer
• datetime/varchar
• char/nvarchar/nvarchar

Is there a valid case for this? I would have always defined an identity or unique identifier column for these cases.

In addition there are many tables without primary keys at all. What are the valid reasons, if any, for this?

I\'m trying to understand why tables were designed as they were, and it appears to be a big mess to me, but maybe there were good reasons for it.

A third question to sort of help me decipher the answers: In cases where multiple columns are used to comprise the compound primary key, is there a specific advantage to this method vs. a surrogate/artificial key? I\'m thinking mostly in regards to performance, maintenance, administration, etc.?

回答1:

I follow a few rules:

1. Primary keys should be as small as necessary. Prefer a numeric type because numeric types are stored in a much more compact format than character formats. This is because most primary keys will be foreign keys in another table as well as used in multiple indexes. The smaller your key, the smaller the index, the less pages in the cache you will use.
2. Primary keys should never change. Updating a primary key should always be out of the question. This is because it is most likely to be used in multiple indexes and used as a foreign key. Updating a single primary key could cause of ripple effect of changes.
3. Do NOT use \"your problem primary key\" as your logic model primary key. For example passport number, social security number, or employee contract number as these \"primary key\" can change for real world situations.

On surrogate vs natural key, I refer to the rules above. If the natural key is small and will never change it can be used as a primary key. If the natural key is large or likely to change I use surrogate keys. If there is no primary key I still make a surrogate key because experience shows you will always add tables to your schema and wish you\'d put a primary key in place.

回答2:

Natural verses artifical keys is a kind of religious debate among the database community - see this article and others it links to. I\'m neither in favour of always having artifical keys, nor of never having them. I would decide on a case-by-case basis, for example:

• US States: I\'d go for state_code (\'TX\' for Texas etc.), rather than state_id=1 for Texas
• Employees: I\'d usually create an artifical employee_id, because it\'s hard to find anything else that works. SSN or equivalent may work, but there could be issues like a new joiner who hasn\'t supplied his/her SSN yet.
• Employee Salary History: (employee_id, start_date). I would not create an artifical employee_salary_history_id. What point would it serve (other than \"foolish consistency\")

Wherever artificial keys are used, you should always also declare unique constraints on the natural keys. For example, use state_id if you must, but then you\'d better declare a unique constraint on state_code, otherwise you are sure to eventually end up with:

state_id    state_code   state_name
137         TX           Texas
...         ...          ...
249         TX           Texas

回答3:

Just an extra comment on something that is often overlooked. Sometimes not using a surrogate key has benefits in the child tables. Let\'s say we have a design that allows you to run multiple companies within the one database (maybe it\'s a hosted solution, or whatever).

Let\'s say we have these tables and columns:

Company:
  CompanyId   (primary key)

CostCenter:
  CompanyId   (primary key, foreign key to Company)
  CostCentre  (primary key)

CostElement
  CompanyId   (primary key, foreign key to Company)
  CostElement (primary key)

Invoice:
  InvoiceId    (primary key)
  CompanyId    (primary key, in foreign key to CostCentre, in foreign key to CostElement)
  CostCentre   (in foreign key to CostCentre)
  CostElement  (in foreign key to CostElement)

In case that last bit doesn\'t make sense, Invoice.CompanyId is part of two foreign keys, one to the CostCentre table and one to the CostElement table. The primary key is (InvoiceId, CompanyId).

In this model, it\'s not possible to screw-up and reference a CostElement from one company and a CostCentre from another company. If a surrogate key was used on the CostElement and CostCentre tables, it would be.

The fewer chances to screw up, the better.

回答4:

I avoid using natural keys for one simple reason -- human error. Although natural unique identifiers are often available (SSN, VIN, Account Number, etc.), they require a human to enter them correctly. If you\'re using SSNs as a primary key, someone transposes a couple of numbers during data entry, and the error isn\'t discovered immediately, then you\'re faced with changing your primary key.

My primary keys are all handled by the database program in the background and the user is never aware of them.

回答5:

There´s no problem in making your primary key from various fields, that\'s a Natural Key.

You can use a Identity column (associated with a unique index on the candidate fields) to make a Surrogate Key.

That´s an old discussion. I prefer surrogate keys in most situations.

But there´s no excuse for the lack of a key.

RE: EDIT

Yeah, there´s a lot of controversy about that :D

I don´t see any obvious advantage on natural keys, besides the fact that they are the natural choice. You will always think in Name, SocialNumber - or something like that - instead of idPerson.

Surrogate keys are the answer to some of the problems that natural keys have (propagating changes for example).

As you get used to surrogates, it seems more clean, and manageable.

But in the end, you´ll find out that it\'s just a matter of taste - or mindset -. People \"think better\" with natural keys, and others don´t.

回答6:

Tables should have a primary key all the time. When it doesn\'t it should have been an AutoIncrement fields.

Sometime people omit primary key because they transfer a lot of data and it might slow down (depend of the database) the process. BUT, it should be added after it.

Some one comment about link table, this is right, it\'s an exception BUT fields should be FK to keep the integrity, and is some case those fields can be primary keys too if duplicate in links is not authorized... but to keep in a simple form because exception is something often in programming, primary key should be present to keep the integrity of your data.

回答7:

Besides all those good answers, I just want to share a good article I just read, The great primary-key debate.

Just to quote a few points:

The developer must apply a few rules when choosing a primary key for each table:

• The primary key must uniquely identify each record.
• A record’s primary-key value can’t be null.
• The primary key-value must exist when the record is created.
• The primary key must remain stable—you can’t change the primary-key field(s).
• The primary key must be compact and contain the fewest possible attributes.
• The primary-key value can’t be changed.

Natural keys (tend to) break the rules. Surrogate keys comply with the rules. (You better read through that article, it is worth your time!)

回答8:

What is special about the primary key?

What is the purpose of a table in a schema? What is the purpose of a key of a table? What is special about the primary key? The discussions around primary keys seem to miss the point that the primary key is part of a table, and that table is part of a schema. What is best for the table and table relationships should drive the key that is used.

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.

回答9:

A natural key, if available, is usually best. So, if datetime/char uniquely identifies the row and both parts are meaningful to the row, that\'s great.

If just the datetime is meaningful, and the char is just tacked on to make it unique, then you might as well just go with an identify field.

回答10:

Natural versus artificial keys to me is a matter of how much of the business logic you want in your database. Social Security number (SSN) is a great example.

\"Each client in my database will, and must, have an SSN.\" Bam, done, make it the primary key and be done with it. Just remember when your business rule changes you\'re burned.

I don\'t like natural keys myself, due to my experience with changing business rules. But if your sure it won\'t change, it might prevent a few critical joins.

回答11:

I suspect Steven A. Lowe\'s rolled up newspaper therapy is required for the designer of the original data structure.

As an aside, GUIDs as a primary key can be a performance hog. I wouldn\'t recommend it.

回答12:

You should use a \'composite\' or \'compound\' primary key that comprises of multiple fields.

This is a perfectly acceptable solution, go here for more info :)

回答13:

I too always use a numeric ID column. In oracle I use number(18,0) for no real reason above number(12,0) (or whatever is an int rather than a long), maybe I just don\'t want to ever worry about getting a few billion rows in the db!

I also include a created and modified column (type timestamp) for basic tracking, where it seems useful.

I don\'t mind setting up unique constraints on other combinations of columns, but I really like my id, created, modified baseline requirements.

回答14:

I look for natural primary keys and use them where I can.

If no natural keys can be found, I prefer a GUID to a INT++ because SQL Server use trees, and it is bad to always add keys to the end in trees.

On tables that are many-to-many couplings I use a compound primary key of the foreign keys.

Because I\'m lucky enough to use SQL Server I can study execution plans and statistics with the profiler and the query analyzer and find out how my keys are performing very easily.

回答15:

I always use an autonumber or identity field.

I worked for a client who had used SSN as a primary key and then because of HIPAA regulations was forced to change to a \"MemberID\" and it caused a ton of problems when updating the foreign keys in related tables. Sticking to a consistent standard of an identity column has helped me avoid a similar problem in all of my projects.

回答16:

All tables should have a primary key. Otherwise, what you have is a HEAP - this, in some situations, might be what you want (heavy insert load when the data is then replicated via a service broker to another database or table for instance).

For lookup tables with a low volume of rows, you can use a 3 CHAR code as the primary key as this takes less room than an INT, but the performance difference is negligible. Other than that, I would always use an INT unless you have a reference table that perhaps has a composite primary key made up from foreign keys from associated tables.

回答17:

If you really want to read through all of the back and forth on this age-old debate, do a search for \"natural key\" on Stack Overflow. You should get back pages of results.

回答18:

GUIDs can be used as a primary key, but you need to create the right type of GUID so that it performs well.

You need to generate COMB GUIDs. A good article about it and performance statistics is The Cost of GUIDs as Primary Keys.

Also some code on building COMB GUIDs in SQL is in Uniqueidentifier vs identity(archive).

回答19:

Here are my own rule of thumbs I have settled on after 25+ years of development experience.

• All tables should have a single column primary key that auto increments.
• Include it in any view that is meant to be updateable
• The primary key should not have any meaning in the context of your application. This means that it should not be a SKU, or an account number or an employee id or any other information that is meaningful to your application. It is merely a unique key associated with an entity.

The primary key is used by the database for optimization purposes and should not be used by your application for anything more than identifying a particular entity or relating to a particular entity.

Always having a single value primary key makes performing UPSERTs very straightforward.

Use additional indices to support multi-column keys which have meaning in your application.

回答20:

We do a lot of joins and composite primary keys have just become a performance hog. A simple int or long takes care of many problems even though you are introducing a second candidate key, but it\'s a lot easier and more understandable to join on one field versus three.

回答21:

I\'ll be up-front about my preference for natural keys - use them where possible, as they\'ll make your life of database administration a lot easier. I established a standard in our company that all tables have the following columns:

• Row ID (GUID)
• Creator (string; has a default of the current user\'s name (SUSER_SNAME() in T-SQL))
• Created (DateTime)
• Timestamp

Row ID has a unique key on it per table, and in any case is auto-generated per row (and permissions prevent anyone editing it), and is reasonably guaranteed to be unique across all tables and databases. If any ORM systems need a single ID key, this is the one to use.

Meanwhile, the actual PK is, if possible, a natural key. My internal rules are something like:

• People - use surrogate key, e.g. INT. If it\'s internal, the Active Directory user GUID is an acceptable choice
• Lookup tables (e.g. StatusCodes) - use a short CHAR code; it\'s easier to remember than INTs, and in many cases the paper forms and users will also use it for brevity (e.g. Status = \"E\" for \"Expired\", \"A\" for \"Approved\", \"NADIS\" for \"No Asbestos Detected In Sample\")
• Linking tables - combination of FKs (e.g. EventId, AttendeeId)

So ideally you end up with a natural, human-readable and memorable PK, and an ORM-friendly one-ID-per-table GUID.

Caveat: the databases I maintain tend to the 100,000s of records rather than millions or billions, so if you have experience of larger systems which contraindicates my advice, feel free to ignore me!