Do database engines utilize foreign keys transparently or a query should explicitly use them?
Based on my experience there is no explicit notion of foreign keys on a table, except that a constraint that maintains uniqueness of the key and the fact that the key (single or a group of fields) is a key which makes search efficient.
To clarify this, here is an example why it is important: I have a middleware (in particular ArcGIS for my case), for which I can control the back-end database (so I can create keys, indices, etc.) and I usually use the front (a RESTful API here). The middleware itself is a black box and to provide effective tools to take advantage of the underlying DBMS's capabilities. So what I want to understand is that if I build foreign key constraints and use queries that if implemented normally would translate into queries that would use those foreign keys, should I see performance improvements?
Is that generally the case or various engines do it differently? (I am using PostgresSQL).
Foreign keys aren't there to improve performance. They're there to enforce data integrity. They will decrease performance for inserts/updates/deletes, but they make no difference to queries.
Some DBMSs will automatically add an index to the foreign key field, which may be where the confusion is coming from. Postgres does not do this; you'll need to create the index yourself. (And yes, the database will use this index transparently.)
As far as I know Database engines needs specific queries to use foreign keys. You have to write some sort of join queries to get data from related tables.
However some Data access framework hides the complexity of accessing data from foreign keys by providing transparent way of accessing data from related tables but I am not sure that may provide much improvement in performance.
This is completely depends on the database engine.
In PostgreSQL constraints won't cause performance improvements directly, only indexes will do that.
CREATE INDEX is a PostgreSQL language extension. There are no provisions for indexes in the SQL standard.
However, adding some constraints will automatically create an index for that column(s) -- f.ex. UNIQUE & PRIMARY KEY constraints creates a btree index on the affected column(s).
The FOREIGN KEY constraint won't create indexes on the referencing column(s), but:
A foreign key must reference columns that either are a primary key or form a unique constraint. This means that the referenced columns always have an index (the one underlying the primary key or unique constraint); so checks on whether a referencing row has a match will be efficient. Since a DELETE of a row from the referenced table or an UPDATE of a referenced column will require a scan of the referencing table for rows matching the old value, it is often a good idea to index the referencing columns too. Because this is not always needed, and there are many choices available on how to index, declaration of a foreign key constraint does not automatically create an index on the referencing columns.
Related
I am creating user management database schema. I am using Postgresql as database. Following is my approach. Please suggest if there is any performance issue if I use this structure.
Requirement:
Expecting around millions of users in future.
I have to use unique user id on other systems also, may be on MongoDB, redis etc.
Approach:
I am using pseudo_encrypt() as unique user_id (BIGINT or BIGSERIAL), so that no one can guess other ids. For example: 3898573529235304961
Using user_id as foreign key in another table. I am not using primary key of user table as foreign key.
Any suggestions?
Use of unique key as foreign key everywhere in other tables, am I doing it correct?
Any performance issue during CRUD operations & with complex joins?
Use of unique key in any other database is correct way? (in case of distributed environment)
You are wading into flame war territory here over the question of natural vs surrogate primary keys. I agree with you and often use unique keys as foreign keys, and designate natural primary keys as such. On PostgreSQL this is safe (on MySQL or MS SQL it would be a bad habit though).
In PostgreSQL the only differences between primary keys and unique constraints are:
A table can have only one primary key
primary keys are not null on all columns
In practice, if you have a table defined as NOT NULL UNIQUE, it is just about the same as a single primary key.
On other dbs, often times table structure is optimized for primary key lookups which is why this is a problem, and there may be tools that don't like it but those are questions outside the realm of db design per se.
You are better to use normal serials and have real access controls than try to build things on obscurity. The obscurity controls are likely to perform worse, and be less secure than just doing things right however.
What is the difference between Primary key And unique Key constraint?
What's the use of it??
Both are used to denote candidate keys for a table.
You can only have one primary key for a table so would just need to pick one if you have multiple candidates.
Either can be used in Foreign Key constraints. In SQL Server the Primary Key columns cannot be nullable. Columns used in Unique Key constraints can be.
By default in SQL Server the Primary Key will become the clustered index if it is created on a heap but it is by no means mandatory that the PK and clustered index should be the same.
A primary key is one which is used to identify the row in question. It might also have some meaning beyond that (if there was already a piece of "real" data that could serve) or it may be purely an implementation artefact (most IDENTITY columns, and equivalent auto-incremented values on other database systems).
A unique key is a more general case, where a key cannot have repeated values. In most cases people cannot have the same social security numbers in relation to the same jurisdiction (an international case could differ). Hence if we were storing social security numbers, then we would want to model them as unique, as any case of them matching an existing number is clearly wrong. Usernames generally must be unique also, so here's another case. External identifiers (identifiers used by another system, standard or protocol) tend to also be unique, e.g. there is only one language that has a given ISO 639 code, so if we were storing ISO 639 codes we would model that as unique.
This uniqueness can also be across more than one column. For example, in most hierarchical categorisation systems (e.g. a folder structure) no item can have both the same parent item and the same name, though there could be other items with the same parent and different names, and others with the same name and different parents. This multi-column capability is also present on primary keys.
A table may also have more than one unique key. E.g. a user may have both an id number and a username, and both will need to be unique.
Any non-nullable unique key can therefore serve as a primary key. Sometimes primary keys that come from the innate data being modelled are referred to as "natural primary keys", because they are a "natural" part of the data, rather than just an implementation artefact. The decision as to which to use depends on a few things:
Likelihood of change of specification. If we modelled a social security number as unique and then had to adapt to allow for multiple jurisdictions where two or more use a similar enough numbering system to allow for collisions, we likely need just remove the uniqueness constraint (other changes may be needed). If it was our primary key, we now also need to use a new primary key, and change any table that was using that primary key as part of a relationship, and any query that joined on it.
Speed of look-up. Key efficiency can be important, as they are used in many WHERE clauses and (more often) in many JOINs. With JOINS in particular, speed of lookup can be vital. The impact will depend on implementation details, and different databases vary according to how they will handle different datatypes (I would have few qualms from a performance perspective in using a large piece of text as a primary key in Postgres where I could specify the use of hash joins, but I'd be very hesitant to do so in SQLServer [Edit: for "large" I'm thinking of perhaps the size of a username, not something the size of the entire Norse Eddas!]).
Frequency of the key being the only interesting data. For example, with a table of languages, and a table of pieces of comments in that language, very often the only reason I would want to join on the language table when dealing with the comments table is either to obtain the language code or to restrict a query to those with a particular language code. Other information about the language is likely to be much more rarely used. In this case while joining on the code is likely to be less efficient than joining on a numeric id set from an IDENTITY column, having the code as the primary key - and hence as what is stored in the foreign key column on the comments table - will remove the need for any JOIN at all, with a considerable efficiency gain. More often though I want more information from the relevant tables than that, so making the JOIN more efficient is more important.
Primary key:
Primary key is nothing but it uniquely identifies each row in a table.
Primary key does not allow duplicate values, nor NULL.
Primary key by default is a clustered index.
A table can have only one primary key.
Unique Key:
Unique key is nothing but it uniquely identifies each row in a table.
Unique key does not allow duplicate values, but it allows (at most one) NULL.
Unique key by default is a non-clustered index.
This is a fruit full link to understand the Primary Key Database Keys.
Keep in mind we have only one clustered index in a table [Talking about SQL Server 2005].
Now if we want to add another unique column then we will use Unique Key column, because
Unique Key column can be added more than one.
A primary key is just any one candidate key. In principle primary keys are not different from any other candidate key because all keys are equal in the relational model.
SQL however has two different syntax for implementing candidate keys: the PRIMARY KEY constraint and the UNIQUE constraint (on non-nullable columns of course). In practice they achieve exactly the same thing except for the essentially useless restriction that a PRIMARY KEY can only be used once per table whereas a UNIQUE constraint can be used multiple times.
So there is no fundamental "use" for the PRIMARY KEY constraint. It is redundant and could easily be ignored or dropped from the language altogether. However, many people find it convenient to single out one particular key per table as having special significance. There is a very widely observed convention that keys designated with PRIMARY KEY are used for foreign key references, although this is entirely optional.
Short version:
From the point of view of database theory, there is none. Both are simply candidate keys.
In practice, most DMBS like to have one "standard key", which can be used for e.g. deciding how to store data, and to tell tools and DB clients which is the best way to identify a record.
So distinguishing one unique key as the "primary key" is just an implementation convenience (but an important one).
Is there a benefit to having a single column primary key vs a composite primary key?
I have a table that consists of two id columns which together make up the primary key.
Are there any disadvantages to this? Is there a compelling reason for me to throw in a third column that would be unique on it's own?
Database Normalization nuts will tell you one thing.
I'm just going to offer my own opinion of what i've learned over the years..I stick an AutoIncrementing ID field to Every ($&(##$)# one of my tables. It makes life a million times easier in the long run to be able to single out with impunity a single row.
This is from a "down in the trenches" developer.
Single column keys are simple to write, simple to maintain, and simple to understand.
If you're going to have a huge number of rows - billions? - maybe saving a byte here and there will help.
But if you're not looking at extreme cases, optimizing for "simple" is often the best way to go.
If you are a coder and the database is nothing to you but a glorified object-store, then sure, by all means inject surrogate keys willy nilly. In fact go one better and just delegate all DB schema design and DB interaction to your favourite ORM and be done with it. Indeed, when I want a small or medium scale object-store, that's exactly what I do.
If you are approaching an information systems or information management problem, then it is a completely different story. When you start dealing with 10's (or more likely 100's) of millions of dirty records integrated from multiple sources, several or all of which are not under your control; at that point the seductive lure of an easy answer to the problems of 'identity' is a trap.
Yes you sometimes still introduce a surrogate key internally to allow for concise FK relationships and improved cache efficiency on covering indices; but, you gain those benefits at the cost of substantial pain at managing the natural-key/surrogate-key relationship.
In this case it will be important to make sure you don't allow the surrogate key to leak. Your public API's at the business-logic layer should use the natural-key, nothing above an document/record-cache should be aware of the existence of a surrogate key. Be aware that the cost of matching updates against the existing surrogate keys can be prohibitive, and a far larger scalability hit than the incremental cost of moving a few extra bytes per request over the internal network.
So in conclusion:
If the DB is just being used as an object-store: let the ORM worry about object identity, and there should almost certainly be a surrogate key.
If the DB is being used as a database: the introduction of a surrogate key is an engineering design decision with serious tradeoffs in both directions. The decision will need to be made on a case by case basis, with full recognition of the resulting costs to be accepted in exchange for the benefits gained either way.
Update
The 'convenience' of a surrogate key is really just the ability to punt on the question of identity. This is often necessary in a database, and reasonable in the caching layer as I allow, but beyond that it leads to brittle data designs. The problem is that identity is no something that has one correct answer. For non-trivial data-intensive systems you will routinely find yourself needing to work in terms of equivalence classes, rather than the reference identity, object-oriented programming lulls us into thinking is normal.
What it really comes down to is a realization that the whole concept of a 'primary key' is a fiction invented to help the relational model work efficiently; but, adopting a surrogate key, cements that fiction and makes the whole system brittle and inflexible. Business logic needs to be able to provide their own definitions of equality — sometimes four copies of the same file need to be considered four files, sometimes they should be considered indistinguishable from the original file; when you edit one of them, is that then a new file? the same file? The answer to both questions is of course yes, when... Working with natural keys provides this critical ability to work in terms of conceptual equivalence classes. If you let surrogate keys infect your business logic, you quickly lose this.
I have had to use multi-column primary keys in the past, and it became quite a nightmare very quickly.
If you have one table that references your first table, how does it contain that primary key? Now add another table that references only the second table but needs to find data in the first. Now another... on down the rabbit hole.
If you know that you will only have the one table, there's probably not an issue either way- use whichever represents your data better. But if you'll be using it in joins, you can lose performance pretty quickly.
Is there a benefit to having a single column primary key vs a composit[sic] primary key?
Yes. If the primary key also happens to be the clustered index, it is common that the clustered index is duplicated fully for each secondary index in the table. Therefore, having a fatter clustered index, which is what one would get with a composite, implies an increase in storage cost. Also, foreign references to this table would need to specify both fields to refer to a unique entry, which implies a further storage cost. There is also an arguably greater cost in development time because there is a slight increase in the complexity of the join.
On the other hand, depending on the distribution of the values of your two key fields, it may be the case that concurrent access to your table is greatly improved because chronologically-successive inserts could occur on different physical pages; this could be the case, for example, if your fields are time-independent (and non-monotonic like an auto-incrementer) like clientID, or something like that. This could be significant for performance in a high concurrency environment.
I have a table that consists of two id columns which together make up the primary key.
Are there any disadvantages to this? Is there a compelling reason for me
to throw in a third column that would be unique on it's own?
If the most common way in which your table is queried is to specify those three fields as restrictions, then having all three in a composite key would likely be the fastest lookup.
And there is another important point that I almost forgot. Since having a composite key means that foreign references to this table from other tables must specify all fields in the key, it also means that some queries performed on the other table that required a restriction on one or more of the parts of the composite index of this table, can be performed without requiring a join. This could be considered similar to the concept of denormalization for the sake of performance (and arguably sacrificing a little ease of maintainability).
In general I prefer to have a surrogate key becasue there are very few truly good natural keys (key problem is not uniqueness but that they change over time) and the longer the natural key, the more it affects performance when used as a PK. If you have a natural key, you should create a unique index on it and then use the surrogate key as the PK used for joining to other tables. That enforces the uniqueness of the natural key data but fixes the problems of join performance and the extra time to update all child records when the natural key changes.
There is one case where I ignore this and that is a joining table. If it is a table that is used to enforce a many to many relationship and consists only of two surrogate keys from other tables, then you really gain nothing from adding a surrogate key. Typically the individual keys are used for joins not the PK and surrogate keys almost never change. In a joining table, I just add the two colmns I need and nothing else.
In most databases I know (MySQL, PostgreSQL) the composite key will generate an index. So if you specify your key as composite the DB should provide you an efficient way to lookup tuples from the DB using that key. I think it is the case for all DBs. I think you do not have to bother about performance there.
Don't use multi-column keys. They get very difficult to maintain, especially if the components of the key are not human-understandable.
Use an internally generated key instead.
Imagine you have a composite primary key (field1 and field2 for example) instead of just one autoincremental identifier. Clients' requirements are very changeable and after some development the client says that field2 is not compulsory and it can be nullable, it won't be possible to continue as the primary key of the table. Imagine this table is one of the most importants in your model. Then all the foreign keys should be changed if field 2 cannot be in the composite primary key. It's a nightmare changing the primary key all over the model.
As well if there is a lot of foreign keys I think is not a very good Idea to add several keys to each table just to make the link.
I'm not sure there's enough information for us to make your call for you. Here are a few observations that might be helpful though.
is the primary key a clustered index? Is the table referenced by other tables through a foreign key? If yes, then you may benefit from a single-column key, because that key will appear in those other tables. This is how you would save space.
If the table is not referenced by other tables, then you would be using extra space in your table without much additional benefit. And, if this table only contains the two columns now, then you would increase the table size by 50%.
If you use an extra column for the primary key, do not forget your natural key (the two-column key). Create a unique constraint on the composite key. You still want to maintain the integrity of the real data.
The decision should always be based on requirements and the intended meaning of the data. A table with only a single attribute key clearly enforces a different kind of constraint and implies that your table has a very different meaning to the same table with a multi attribute key. On the other hand adding an additional unique column would also be a waste of resources and add meaningless complexity if you don't actually need to use it anywhere.
One caveat to the auto-incrementing column is that it can give a false impression of uniqueness. Sure, your identity column is always unique, but that's just a meaningless value you've attached to the table. Unless you also have a unique constraint attached to the set of columns that represent the actual semantic primary key of the table, you have no guarantee of meaningful uniqueness.
what is the benefit of having an additional identity column in each table of a database? What are the drawbacks?
Update:
Now i want to expand the case and introduce replication. What for is this surogate key (identity comlumn) aside of the rowguid we get with the replication. Behalf of K. Brian Kelley's objection one should set the clustered index on this rowguid (and forget the identity-column). WHat do you think?
Short version: Surrogate or synthetic key (what you probably mean by "identity column") versus natural key is a very old debate.
Pros of surrogate keys:
makes you independent of changes in the natural key (think new requirements / changing domain model), which would otherwise cascade through your data model
sometimes, there is no natural key (e.g. persons in an address book)
is often faster, because it is shorter (single int column, instead of e.g. several varchars)
is more convenient in joins etc. because it's only a single column
Cons:
you still need a unique index for every candidate natural key (so one more index needed)
it is foreign to the domain, and may require an additional join to get the real data
Generally, the agreement is that surrogate keys are usually a good idea, except in simple cases like join tables.
For all the details, see Wikipedia, which has a good article on the topic.
Often the use of surrogate keys, of which IDENTITY columns are the most common, are done for performance reasons. You should still identify the natural keys (the columns which make the row unique based on the data).
Typically surrogate keys are integer values. That makes them easy to link together using joins with other tables (and restrict accordingly using foreign keys). Also, when talking about SQL Server, all nonclustered indexes depend on the clustered index. So if the clustered index is based on the natural key and is large in size as a result, then all the nonclustered indexes are going to be large as well, because they will refer back to the clustered index. Therefore, a lot of folks build the primary key around that integer-based surrogate key. I know I'm simplifying it a bit, but that's a key reason for the use of surrogate keys.
The drawback is that the surrogate key is effectively meaningless. If someone were to change the value of the key, you could break a relationship if the foreign key constraints are not present or are disabled. In the case of making a change which alters the alternate key, you're actually changing the data itself. So you would expect such a break if the entities are built properly and you would be changing the data in related tables as well.
I'm currently designing a brand new database. In school, we always learned to put a primary key in each table.
I read a lot of articles/discussions/newsgroups posts saying that it's better to use unique constraint (aka unique index for some db) instead of PK.
What's your point of view?
A Primary Key is really just a candidate key that does not allow for NULL. As such, in SQL terms - it's no different than any other unique key.
However, for our non-theoretical RDBMS's, you should have a Primary Key - I've never heard it argued otherwise. If that Primary Key is a surrogate key, then you should also have unique constraints on the natural key(s).
The important bit to walk away with is that you should have unique constraints on all the candidate (whether natural or surrogate) keys. You should then pick the one that is easiest to reference in a Foreign Key to be your Primary Key*.
You should also have a clustered index*. this could be your Primary Key, or a natural key - but it's not required to be either. You should pick your clustered index based on query usage of the table. When in doubt, the Primary Key is not a bad first choice.
Though it's technically only required to refer to a unique key in a foreign key relationship, it's accepted standard practice to greatly favor the primary key. In fact, I wouldn't be surprised if some RDBMS only allow primary key references.
Edit: It's been pointed out that Oracle's term of "clustered table" and "clustered index" are different than Sql Server. The equivalent of what I'm speaking of in Oracle-ese is an Index Ordered Table and it is recommended for OLTP tables - which, I think, would be the main focus of SO questions. I assume if you're responsible for a large OLAP data warehouse, you should already have your own opinions on database design and optimization.
Can you provide references to these articles?
I see no reason to change the tried and true methods. After all, Primary Keys are a fundamental design feature of relational databases.
Using UNIQUE to serve the same purpose sounds really hackish to me. What is their rationale?
Edit: My attention just got drawn back to this old answer. Perhaps the discussion that you read regarding PK vs. UNIQUE dealt with people making something a PK for the sole purpose of enforcing uniqueness on it. The answer to this is, If it IS a key, then make it key, otherwise make it UNIQUE.
A primary key is just a candidate key (unique constraint) singled out for special treatment (automatic creation of indexes, etc).
I expect that the folks who argue against them see no reason to treat one key differently than another. That's where I stand.
[Edit] Apparently I can't comment even on my own answer without 50 points.
#chris: I don't think there's any harm. "Primary Key" is really just syntactic sugar. I use them all the time, but I certainly don't think they're required. A unique key is required, yes, but not necessarily a Primary Key.
It would be very rare denormalization that would make you want to have a table without a primary key. Primary keys have unique constraints automatically just by their nature as the PK.
A unique constraint would be used when you want to guarantee uniqueness in a column in ADDITION to the primary key.
The rule of always have a PK is a good one.
http://msdn.microsoft.com/en-us/library/ms191166.aspx
You should always have a primary key.
However I suspect your question is just worded bit misleading, and you actually mean to ask if the primary key should always be an automatically generated number (also known as surrogate key), or some unique field which is actual meaningful data (also known as natural key), like SSN for people, ISBN for books and so on.
This question is an age old religious war in the DB field.
My take is that natural keys are preferable if they indeed are unique and never change. However, you should be careful, even something seemingly stable like a persons SSN may change under certain circumstances.
Unless the table is a temporary table to stage the data while you work on it, you always want to put a primary key on the table and here's why:
1 - a unique constraint can allow nulls but a primary key never allows nulls. If you run a query with a join on columns with null values you eliminate those rows from the resulting data set because null is not equal to null. This is how even big companies can make accounting errors and have to restate their profits. Their queries didn't show certain rows that should have been included in the total because there were null values in some of the columns of their unique index. Shoulda used a primary key.
2 - a unique index will automatically be placed on the primary key, so you don't have to create one.
3 - most database engines will automatically put a clustered index on the primary key, making queries faster because the rows are stored contiguously in the data blocks. (This can be altered to place the clustered index on a different index if that would speed up the queries.) If a table doesn't have a clustered index, the rows won't be stored contiguously in the data blocks, making the queries slower because the read/write head has to travel all over the disk to pick up the data.
4 - many front end development environments require a primary key in order to update the table or make deletions.
Primary keys should be used in situations where you will be establishing relationships from this table to other tables that will reference this value. However, depending on the nature of the table and the data that you're thinking of applying the unique constraint to, you may be able to use that particular field as a natural primary key rather than having to establish a surrogate key. Of course, surrogate vs natural keys are a whole other discussion. :)
Unique keys can be used if there will be no relationship established between this table and other tables. For example, a table that contains a list of valid email addresses that will be compared against before inserting a new user record or some such. Or unique keys can be used when you have values in a table that has a primary key but must also be absolutely unique. For example, if you have a users table that has a user name. You wouldn't want to use the user name as the primary key, but it must also be unique in order for it to be used for log in purposes.
We need to make a distinction here between logical constructs and physical constructs, and similarly between theory and practice.
To begin with: from a theoretical perspective, if you don't have a primary key, you don't have a table. It's just that simple. So, your question isn't whether your table should have a primary key (of course it should) but how you label it within your RDBMS.
At the physical level, most RDBMSs implement the Primary Key constraint as a Unique Index. If your chosen RDBMS is one of these, there's probably not much practical difference, between designating a column as a Primary Key and simply putting a unique constraint on the column. However: one of these options captures your intent, and the other doesn't. So, the decision is a no-brainer.
Furthermore, some RDBMSs make additional features available if Primary Keys are properly labelled, such as diagramming, and semi-automated foreign-key-constraint support.
Anyone who tells you to use Unique Constraints instead of Primary Keys as a general rule should provide a pretty damned good reason.
the thing is that a primary key can be one or more columns which uniquely identify a single record of a table, where a Unique Constraint is just a constraint on a field which allows only a single instance of any given data element in a table.
PERSONALLY, I use either GUID or auto-incrementing BIGINTS (Identity Insert for SQL SERVER) for unique keys utilized for cross referencing amongst my tables. Then I'll use other data to allow the user to select specific records.
For example, I'll have a list of employees, and have a GUID attached to every record that I use behind the scenes, but when the user selects an employee, they're selecting them based off of the following fields: LastName + FirstName + EmployeeNumber.
My primary key in this scenario is LastName + FirstName + EmployeeNumber while unique key is the associated GUID.
posts saying that it's better to use unique constraint (aka unique index for some db) instead of PK
i guess that the only point here is the same old discussion "natural vs surrogate keys", because unique indexes and pk´s are the same thing.
translating:
posts saying that it's better to use natural key instead of surrogate key
I usually use both PK and UNIQUE KEY. Because even if you don't denote PK in your schema, one is always generated for you internally. It's true both for SQL Server 2005 and MySQL 5.
But I don't use the PK column in my SQLs. It is for management purposes like DELETEing some erroneous rows, finding out gaps between PK values if it's set to AUTO INCREMENT. And, it makes sense to have a PK as numbers, not a set of columns or char arrays.
I've written a lot on this subject: if you read anything of mine be clear that I was probably referring specifically to Jet a.k.a. MS Access.
In Jet, the tables are physically ordered on the PRIMARY KEY using a non-maintained clustered index (is clustered on compact). If the table has no PK but does have candidate keys defined using UNIQUE constraints on NOT NULL columns then the engine will pick one for the clustered index (if your table has no clustered index then it is called a heap, arguably not a table at all!) How does the engine pick a candidate key? Can it pick one which includes nullable columns? I really don't know. The point is that in Jet the only explicit way of specifying the clustered index to the engine is to use PRIMARY KEY. There are of course other uses for the PK in Jet e.g. it will be used as the key if one is omitted from a FOREIGN KEY declaration in SQL DDL but again why not be explicit.
The trouble with Jet is that most people who create tables are unaware of or unconcerned about clustered indexes. In fact, most users (I wager) put an autoincrement Autonumber column on every table and define the PRIMARY KEY solely on this column while failing to put any unique constraints on the natural key and candidate keys (whether an autoincrement column can actually be regarded as a key without exposing it to end users is another discussion in itself). I won't go into detail about clustered indexes here but suffice to say that IMO a sole autoincrement column is rarely to ideal choice.
Whatever you SQL engine, the choice of PRIMARY KEY is arbitrary and engine specific. Usually the engine will apply special meaning to the PK, therefore you should find out what it is and use it to your advantage. I encourage people to use NOT NULL UNIQUE constraints in the hope they will give greater consideration to all candidate keys, especially when they have chosen to use 'autonumber' columns which (should) have no meaning in the data model. But I'd rather folk choose one well considered key and used PRIMARY KEY rather than putting it on the autoincrement column out of habit.
Should all tables have a PK? I say yes because doing otherwise means at the very least you are missing out on a slight advantage the engine affords the PK and at worst you have no data integrity.
BTW Chris OC makes a good point here about temporal tables, which require sequenced primary keys (lowercase) which cannot be implemented via simple PRIMARY KEY constraints (SQL key words in uppercase).
PRIMARY KEY
1. Null
It doesn’t allow Null values. Because of this we refer PRIMARY KEY =
UNIQUE KEY + Not Null CONSTRAINT.
2. INDEX
By default it adds a clustered index.
3. LIMIT
A table can have only one PRIMARY KEY Column[s].
UNIQUE KEY
1. Null
Allows Null value. But only one Null value.
2. INDEX
By default it adds a UNIQUE non-clustered index.
3. LIMIT
A table can have more than one UNIQUE Key Column[s].
If you plan on using LINQ-to-SQL, your tables will require Primary Keys if you plan on performing updates, and they will require a timestamp column if you plan on working in a disconnected environment (such as passing an object through a WCF service application).
If you like .NET, PK's and FK's are your friends.
I submit that you may need both. Primary keys by nature need to be unique and not nullable. They are often surrogate keys as integers create faster joins than character fileds and especially than multiple field character joins. However, as these are often autogenerated, they do not guarantee uniqueness of the data record excluding the id itself. If your table has a natural key that should be unique, you should have a unique index on it to prevent data entry of duplicates. This is a basic data integrity requirement.
Edited to add: It is also a real problem that real world data often does not have a natural key that truly guarantees uniqueness in a normalized table structure, especially if the database is people centered. Names, even name, address and phone number combined (think father and son in the same medical practice) are not necessarily unique.
I was thinking of this problem my self. If you are using unique, you will hurt the 2. NF. According to this every non-pk-attribute has to be depending on the PK. The pair of attributes in this unique constraint are to be considered as part of the PK.
sorry for replying to this 7 years later but didn't want to start a new discussion.