Just a quick database design question: Do you ALWAYS use an ID field in EVERY table, or just most of them? Clearly most of your tables will benefit, but are there ever tables that you might not want to use an ID field?
For example, I want to add the ability to add tags to objects in another table (foo). So I've got a table FooTag with a varchar field to hold the tag, and a fooID field to refer to the row in foo. Do I really need to create a clustered index around an essentially arbitrary ID field? Wouldn't it be more efficient to use fooID and my text field as the clustered index, since I will almost always be searching by fooID anyway? Plus using my text in the clustered index would keep the data sorted, making sorting easier when I have to query my data. The downside is that inserts would take longer, but wouldn't that be offset by the gains during selection, which would happen far more often?
What are your thoughts on ID fields? Bendable rule, or unbreakable law?
edit: I am aware that the example provided is not normalized. If tagging is to be a major part of the project, with multiple tables being tagged, and other 'extras', a two-table solution would be a clear answer. However in this simplest case, would normalization be worthwhile? It would save some space, but require an extra join when running queries
As in much of programming: rule, not law.
Proof by exception: Some two-column tables exist only to form relationships between other more meaningful tables.
If you are making tables that bridge between two or more other tables and the only fields you need are the dual PK/FK's, then I don't know why you would need ID column in there as well.
ID columns generally can be very helpful, but that doesn't mean you should go peppering them in at every occasion.
As others have said, it's a general, rather than absolute, rule and there are plenty of exceptions (tables with composite keys for example).
There are some occasional but useful occasions where you might want to create an artificial ID in a table that already has a (usually composite) unique identifier. For example, in one system I've created a table to store part numbers; although the part numbers are unique, they may actually change - we add an arbitrary integer PartID. Not so common, but it's a typical real-world example.
In general what you really want is to be able if at all possible to have some kind of way to uniquely identify a record. It could be an id field or it could be a unique index (which does not have to be on just one field). Anytime I thought I could get away without creating a way to uniquely identify a record, I have been proven wrong. All tables do not have a natural key though and if they do not, you really need to have an id file of some kind. If you have a natural key, you could use that instead, but I find that even then I need an id field in most cases to prevent having to do too much updating when the natural key changes (it always seems to change). Plus having worked with literally hundreds of databases concerning many many differnt topics, I can tell you that a true natural key is rare. As others have nmentioned there is no need for an id field in a table that is simply there to join two tables that havea many to many relationship, but even this should have a unique index.
If you need to retrieve records from that table with unique id then yes. If you will retrieve them by some other composite key made up of foreign keys then no. The last thing you need is fields, data, and indexes that you do not use.
A clustered index does not need to be on primary key or a surrogate (identity column) either.
Your design, however, is not normalized. Typically for tagging, I use two tables, a table of tags (with a surrogate key) and a table of links from the tags to the subject table(s) using the surrogate key in the tag table and theprimary key in the subject table. This allows your tags to apply to different entities (photos, articles, employees, locations, products, whatever). It allows you to enforce foreign key relationships to multiple tables, and also allows you to invent tag hierarchies and other things about the tag table.
As far as the indexes on this design, it will be dictated by the usage patterns.
In general developers love having an ID field on all tables except for 'linking' tables because it makes development much easier, and I am no exception to this. DBA's on the other hand see no problem with making natural primary keys made up of 3 or 4 columns. It can be a butting of heads to try and get a good database design.
Related
So say I have database table tours (PK tour_id) holding region independent information and tours_regional_details (PK tour_id, region_id) holding region specific information.
Let's say I want to populate select control with entities from tours_regional_details table (my real scenarios are bit different, just imagine this for the sake of simplicity).
So, how would you tackle this? My guts says concatenate PKs into delimited strings, like "pk1|pk2" or "pk1,pk2" and use that as value of select control. While it works, feels dirty and possibly needs additional validation steps before splitting the string again, which again feels dirty.
I don't want to start a composite vs single pk holy war, but may this be a bad database design decision on my part? I always believed identifying relationships and composite keys are there for a reason, but I feel tempted to alter my tables and just stuff them with auto incremental IDs and unique constraints. I'm just not sure what kind of a fresh hell will that introduce.
I am a little bit flabbergasted that I encounter this for the first time now after so many years.
EDIT: Yes, there is a table regions (PK region_id) but is mostly irrelevant for the topic. While in some scenarios two select boxes would make sense, let's say here they don't, let's say I want only one select box and want to select from:
Dummy tour (Region 1)
Dummy tour (Region 2)
Another dummy tour (region 3)
...
Composite primary keys aren't bad database design. In an ideal world, our programming languages and UI libraries would support tuples and relations as first-class values, so you'd be able to assign a pair of values as the value of an option in your dropdown control. However, since they generally only support scalar variables, we're stuck trying to encode or reduce our identifiers.
You can certainly add surrogate keys / autoincrement columns (and unique constraints on the natural keys where available) to every table. It's a very common pattern, most databases I've seen have at least some tables set up like this. You may be able to keep existing composite foreign keys as is, or you may want/need to change them to reference the surrogate primary keys instead.
The risk with using surrogate keys for foreign keys is that your access paths in the database become fixed. For example, let's assume tours_regional_details had a primary key tours_regional_detail_id that's referenced by a foreign key in another table. Queries against this other table would always need to join with tours_regional_details to obtain the tour_id or region_id. Natural keys allow more flexible access paths since identifiers are reused throughout the database. This becomes significant in deep hierarchies of dependent concepts. These are exactly the scenarios where opponents of composite keys complain about the "explosion" of keys, and I can at least agree that it becomes cumbersome to remember and type out joins on numerous columns when writing queries.
You could duplicate the natural key columns into the referencing tables, but storing redundant information requires additional effort to maintain consistency. I often see this done for performance or convenience reasons where surrogate keys were used as foreign keys, since it allows querying a table without having to do all the joins to dereference the surrogate identifiers. In these cases, it might've been better to reference the natural key instead.
If I'm allowed to return to my ideal world, perhaps DBMSs could allow naming and storing joins.
In practice, surrogate keys help balance the complexity we have to deal with. Use them, but don't worship them.
I have the following partial database design:
All the tables are dependent on each other so the table bvd_docflow_subdocuments is dependent on the table bdd_docflow_subsets
and the table bvd_docflow_subdocuments is dependent on bvd_docflow_subsets. So I thought I could me smart and use foreign keys on every table (and ON DELETE CASCADE). However the FK are being drilldown how further I go in to the tables.
The problem is the table bvd_docflow_documents has no point having a reference to the 1docflow_documentset_id` PK / FK. Is there a way (and maybe my design is crappy) that only the table standing above it has an FK relationship between the tables and not all the tables above it.
Edit:
More explanation:
In the bvd_docflow_subsets table information is stored about objects to create documents. There is an relation between that table and bvd_docflow_subdocuments table (This table stores master data about all the documents for an subset. (docflow_subset_id is in both tables). This is the link between those to tables.
Going further down we also got the table bvd_docflow_documents this table contains the actual document data. The link between bvd_docflow_documents and bvd_docflow_subdocuments is bvd_docflow_subdocument_id.
On every table I got an foreign key defined so when data is removed on a table all the data linked to that data is also removed.
However when we look to the bvd_docflow_documents table it has all the foreign keys from the other tables (docflow_subset_id and docflow_documentset_id) and there is the problem. The only foreign key needed for that bvd_docflow_documents table is docflow_subdocument_id and no other.
Edit 2
I have changed my design further and removed information that I don't need after initial import of the data.
See the following link for the (total) databse design:
https://sqldbm.com/Project/SQLServer/Share/_AUedvNutCEV2DGLJleUWA
The tables subsets, subdocuments and documents have a many to many relationship so I thought a table in between those 3 documents_subdocuments is the way to go were I define all the different keys for those tables.
I am not used to the database design first and then build it. But, for everything there is a first time, and I try to do make a database that is using standards and is using the power of SQL Server the correct way.
I'll address the bottom-most table and ignore the rest for the most part.
But first some comments. Your schema is simply a model of a system. To provide feedback, one must understand this "system" and how it actually works to evaluate your model. In addition, it is important to understand your entities and your reasons for choosing them and modelling them in the specified manner. Without that understanding all of this guessing based on experience.
And another comment. Slapping an identity column into every table is just lazy modelling IMO. Others will disagree, but you need to also enforce all natural keys. Do you have natural keys? It is rare not to have any. Enforce those that do exist.
And one last comment. Stop the ridiculous pattern of prepending the column names with the table names. And you should really think long and hard about using very long table names. Given what you have, I sense you need a schema for your docflow stuff.
For the documents table, your current PK makes no sense. Again, you've slapped an identity column into the table. By itself, this column is a key for the table. The inclusion of any other columns does not make the key any more "unique" - that inclusion is logical nonsense. Following your pattern, you would designate the identity column as the primary key. But ...
According to your image, the documents table is related to one and only one subdocument. You added a foreign key to that table - which matches the image. You also added additional columns and foreign keys to the "higher" tables. So now a document "points" to a specific subdocument. It also points to a specific subset - which may have no relationship to the subdocument. The same thought applies to the other FK. I have a doubt that this is logically correct. So why do these columns (and related FKs) exist? Perhaps this is the result of premature optimization - which everyone knows is the root of all evil coding. Again, it is impossible to know if this is "right" or even "useful" for your model.
To answer your question "... is there a way", the answer is obviously yes. You remove the columns of which you complain. You added them - Why? Is this perhaps a problem with the tool you are using?
And some last comments. There is nothing special about "varchar(50)". Perhaps this is a place holder that will be updated later. It may also be another sign of laziness. And generally speaking, columns with names like "type" and "code" tend to be foreign keys to "lookup" tables - because people like to add, modify, or remove these sorts categorization values over time. I'm also concerned about the column name overlap among the tables. "Location" exists in multiple tables, as do action_code and action_id. And a column named "id" (action_id) suggests a lookup to another table - is it? Should it be? Is there a relationship between action_id and action_code? From a distance it is impossible to answer any of these questions.
But designing a database is more art than science. Sometimes you just need to create something, populate it with some sample data, and then determine if it works for your needs. Everyone will get something wrong in the first try. That is expected; that is how you learn. The most difficult part is actually completing your first attempt.
I was working on an Access database which loved auto-numbered identifiers. Every table used them except one, which used a key made up of the first name, last name and birthdate of a person. Anyways, people started running into a lot of problems with duplicates, as tables representing relationships could hold the same relationship twice or more. I decided to get around this by implementing composite keys for the relationship tables and I haven't had a problem with duplicates since.
So I was wondering what's the deal with the bad rep of composite keys in the Access world? I guess it's slightly more difficult to write a query, but at least you don't have to put in place tons of checks every time data is entered or even edited in the front end. Are they incredibly super inefficient or something?
A composite key works fine for a single table, but when you start to create relations between tables it can get a bit much.
Consider two tables Person and Event, and a many-to-many relations between them called Appointment.
If you have a composite key in the Person table made up of the first name, last name and birth date, and a compossite key in the Event table made up of place and name, you will get five fields in the Appointment table to identify the relation.
A condition to bind the relation will be quite long:
select Person,*, Event.*
from Person, Event, Appointment
where
Person.FirstName = Appointment.PersonFirstName and
Person.LastName = Appointment.PersonLastName and
Person.BirthDate = Appointment.PersonBirthDate and
Event.Place = Appointment.EventPlace and
Event.Name = Appointment.EventName`.
If you on the other hand have auto-numbered keys for the Person and Event tables, you only need two fields in the Appointment table to identify the relation, and the condition is a lot smaller:
select Person,*, Event.*
from Person, Event, Appointment
where
Person.Id = Appointment.PersonId and Event.Id = Appointment.EventId
If you only use pure self-written SQL to access your data, they are OK.
However, some ORMs, adapters etc. require having a single PK field to identify a record.
Also note that a composite primary key is almost invariably a natural key (there is hardly a point in creating a surrogate composite key, you can as well use a single-field one).
The most common usage of a composite primary key is a many-to-many link table.
When using the natural keys, you should ensure they are inherently unique and immutable, that is an entity is always identified by the same value of the key, once been reflected by the model, and only one entity can be identified by any value.
This it not so in your case.
First, a person can change their name and even the birthdate
Second, I can easily imagine two John Smiths born at the same day.
The former means that if a person changes their name, you will have to update it in each and every table that refers to persons; the latter means that the second John Smith will not be able to make it into your database.
For the case like yours, I would really consider adding a surrogate identifier to your model.
Unfortunately one reason for those negative opinions is probably ignorance. Too many people don't understand the concept of Candidate Keys properly. There are people who seem to think that every table needs only one key, that one key is sufficient for data integrity and that choosing that one key is all that matters.
I have often speculated that it would be a good thing to deprecate and phase out the use of the term "primary key" altogether. Doing that would focus database designers minds on the real issue: that a table should have as many keys as are necessary to ensure the correctness of the data and that some of those keys will probably be composite. Abolishing the primary key concept would do away with all those fatuous debates about what the primary key ought to be or not be.
If your RDBMS supports them and if you use them correctly (and consistently), unique keys on the composite PK should be sufficient to avoid duplicates. In SQL Server at least, you can also create FKs against a unique key instead of the PK, which can be useful.
The advantage of a single "id" column (or surrogate key) is that it can improve performance by making for a narrower key. Since this key may be carried to indexes on that table (as a pointer back to the physical row from the index row) and other tables as a FK column that can decrease space and improve performance. A lot of it depends on the specific architecture of your RDBMS though. I'm not familiar enough with Access to comment on that unfortunately.
As Quassnoi points out, some ORMs (and other third party applications, ETL solutions, etc.) don't have the capability to handle composite keys. Other than some ORMs though, most recent third party apps worth anything will support composite keys though. ORMs have been a little slower in adopting that in general though.
My personal preference for composite keys is that although a unique index can solve the problem of duplicates, I've yet to see a development shop that actually fully used them. Most developers get lazy about it. They throw on an auto-incrementing ID and move on. Then, six months down the road they pay me a lot of money to fix their duplicate data issues.
Another issue, is that auto-incrementing IDs aren't generally portable. Sure, you can move them around between systems, but since they have no actual basis in the real world it's impossible to determine one given everything else about an entity. This becomes a big deal in ETL.
PKs are a pretty important thing in the data modeling world and they generally deserve more thought then, "add an auto-incrementing ID" if you want your data to be consistent and clean.
Surrogate keys are also useful, but I prefer to use them when I have a known performance issue that I'm trying to deal with. Otherwise it's the classic problem of wasting time trying to solve a problem that you might not even have.
One last note... on cross-reference tables (or joining tables as some call them) it's a little silly (in my opinion) to add a surrogate key unless required by an ORM.
Composite Keys are not just composite primary keys, but composite foreign keys as well. What do I mean by that? I mean that each table that refers back to the original table needs a column for each column in the composite key.
Here's a simple example, using a generic student/class arrangement.
Person
FirstName
LastName
Address
Class
ClassName
InstructorFirstName
InstructorLastName
InstructorAddress
MeetingTime
StudentClass - a many to many join table
StudentFirstName
StudentLastName
StudentAddress
ClassName
InstructorFirstName
InstructorLastName
InstructorAddress
MeetingTime
You just went from having a 2-column many-to-many table using surrogate keys to having an 8-column many-to-many table using composite keys, because they have 3 and 5 column foreign keys. You can't really get rid of any of these fields, because then the records wouldn't be unique, since both students and instructors can have duplicate names. Heck, if you have two people from the same address with the same name, you're still in serious trouble.
Most of the answers given here don't seem to me to be given by people who work with Access on a regular basis, so I'll chime in from that perspective (though I'll be repeating what some of the others have said, just with some Access-specific comments).
I use surrogate a key only when there is no single-column candidate key. This means I have tables with surrogate PKs and with single-column natural PKs, but no composite keys (except in joins, where they are the composite of two FKs, surrogate or natural doesn't matter).
Jet/ACE clusters on the PK, and only on the PK. This has potential drawbacks and potential benefits (if you consider a random Autonumber as PK, for instance).
In my experience, the non-Null requirement for a composite PK makes most natural keys impossible without using potentially problematic default values. It likewise wrecks your unique index in Jet/ACE, so in an Access app (before 2010), you end up enforcing uniqueness in your application. Starting with A2010, table-level data macros (which work like triggers) can conceivably be used to move that logic into the database engine.
Composite keys can help you avoid joins, because they repeat data that with surrogate keys you'd have to get from the source table via a join. While joins can be expensive, it's mostly outer joins that are a performance drain, and it's only with non-required FKs that you'd get the full benefit of avoiding outer joins. But that much data repetition has always bothered me a lot, since it seems to go against everything we've ever been taught about normalization!
As I mentioned above, the only composite keys in my apps are in N:N join tables. I would never add a surrogate key to a join table except in the relatively rare case in which the join table is itself a parent to a related tables (e.g., Person/Company N:N record might have related JobTitles, i.e., multiple jobs within the same company). Rather than store the composite key in the child table, you'd store the surrogate key. I'd likely not make the surrogate key the PK, though -- I'd keep the composite PK on the pair of FK values. I would just add an Autonumber with a unique index for joining to the child table(s).
I'll add more as I think of it.
It complicates queries and maintenance. If you are really interested in this subject I'd recommend looking over the number of posts that already cover this. This will give you better info than any one response here.
https://stackoverflow.com/search?q=composite+primary+key
In the first place composite keys are bad for performance in joins. Further they are much worse for updating records as you have to update all the child records as well. Finally very few composite keys are actually really good keys. To be a good key it should be unique and not be subject to change. The example you gave as a composite key you used fails both tests. It is not unique (there are people with the same name born on the same day) and names change frequently causing much unnecessary updating of all the child tables.
As far as table with autogenrated keys casuing duplicates, that is mostly due to several factors:
the rest of the data in the table
can't be identified in any way as
unique
a design failure of forgetting to
create a unique index on the possible
composite key
Poor design of the user interface
which doesn't attempt to find
matching records or which allows data
entry when a pull down might be more
appropriate.
None of those are the fault of the surrogate key, they just indicate incompetent developers.
I think some coders see the complexity but want to avoid it, and most coders don't even think to look for the complexity at all.
Let's consider a common example of a table that had more than one candidate key: a Payroll table with columns employee_number, salary_amount, start_date and end_date.
The four candidate keys are as follows:
UNIQUE (employee_number, start_date); -- simple constraint
UNIQUE (employee_number, end_date); -- simple constraint
UNIQUE (employee_number, start_date, end_date); -- simple constraint
CHECK (
NOT EXISTS (
SELECT Calendar.day_date
FROM Calendar, Payroll AS P1
WHERE P1.start_date <= Calendar.day_date
AND Calendar.day_date < P1.end_date
GROUP
BY P1.employee_number, Calendar.day_date
)
); -- sequenced key i.e. no over-lapping periods for the same employee
Only one of those keys are required to be enforced i.e. the sequenced key. However, most coders wouldn't think to add such a key, let alone know how to code it in the first place. In fact, I would wager that most Access coders would add an incrementing autonumber column to the table, make the autonumber column the PRIMARY KEY, fail to add constraints for any of the candidate keys and will have convinced themselves that their table has a key!
I have two tables, products and categories which have a many to many relationship, so I'm adding a products_categories table which will contain category_id and product_id.
Should I add another (auto-incrementing) index column or use the two existing ones as primary key?
That depends.
Are you seeing your data more as set of objects (and relational database is just
a storage medium) or as set of facts represented and analyzed natively
by relational algebra.
Some ORMs/Frameworks/Tools don't have good support for multicolumn primary keys.
If you happen to use one of them, you'll need additional id column.
If it's just a many-to-many relationship with no additional data associated with it,
it's better to avoid additional id column and have both columns as a primary key.
If you start adding some additional information to this association, then it may reach a point when it becomes
something more then many-to-many relationship of two entities.
It becomes an entity in it's own right and it'd be more convenient if it had it's own id
independent to entities it connects.
You don't need to add an extra, auto-incrementing index column, but I (perhaps contrary to most others) still recommend that you do. First, it is easier in the application program to refer to a row using a single number, for example when you delete a row. Second, it sometimes turns out to be useful to be able to know the order in which the rows were added.
No, it's not necessary at all, given that these two columns are already executing the function of a primary key.
This third column whould just add more space to your table.
But... You could use it maybe to see the order in which your records where added to your table. That's the only function I can see to this column.
You don't need to add an auto-incrementing index column. Standard practice is to use just the two existing columns as your primary key for M:M association tables like you describe.
I would make the primary key category_id and product_id. Add an auto increment only if the order will ever be relevent in later uses.
There's a conceptual question - is products_categories an entity or is simply a table that represents a relationship between two entities? If it's an entity then, even if there are no additional attributes, I'd advocate for a separate ID column for the entity. If it's a relationship, if there are additional attributes (say, begin_date, end_date or something like that), I'd advocate to have a multi-column primary key.
We are having a rather long discussion in our company about whether or not to put an autoincrement key on EVERY table in our database.
I can understand putting one on tables that would have a FK reference to, but I kind-of dislike putting such keys on each and every one of our tables, even though the keys would never be used.
Please help with pros and cons for putting autoincrement keys on every table apart from taking extra space and slowing everything a little bit (we have some tables with hundreds of millions of records).
Thanks
I'm assuming that almost all tables will have a primary key - and it's just a question of whether that key consists of one or more natural keys or a single auto-incrementing surrogate key. If you aren't using primary keys then you will generally get a lot of advantages of using them on almost all tables.
So, here are some pros & cons of surrogate keys. First off, the pros:
Most importantly: they allow the natural keys to change. Trivial example, a table of persons should have a primary key of person_id rather than last_name, first_name.
Read performance - very small indexes are faster to scan. However, this is only helpful if you're actually constraining your query by the surrogate key. So, good for lookup tables, not so good for primary tables.
Simplicity - if named appropriately, it makes the database easy to learn & use.
Capacity - if you're designing something like a data warehouse fact table - surrogate keys on your dimensions allow you to keep a very narrow fact table - which results in huge capacity improvements.
And cons:
They don't prevent duplicates of the natural values. So, you'll still usually want a unique constraint (index) on the logical key.
Write performance. With an extra index you're going to slow down inserts, updates and deletes that much more.
Simplicity - for small tables of data that almost never changes they are unnecessary. For example, if you need a list of countries you can use the ISO list of countries. It includes meaningful abbreviations. This is better than a surrogate key because it's both small and useful.
In general, surrogate keys are useful, just keep in mind the cons and don't hesitate to use natural keys when appropriate.
You need primary keys on these tables. You just don't know it yet.
If you use small keys like this for Clustered Indexes, then there's quite significant advantages.
Like:
Inserts will always go at the end of pages.
Non-Clustered Indexes (which need a reference to the CIX key(s)) won't have long row addresses to consider.
And more... Kimberly Tripp's stuff is the best resource for this. Google her...
Also - if you have nothing else ensuring uniqueness, you have a hook into each row that you wouldn't otherwise have. You should still put unique indexes on fields that should be unique, and use FKs onto appropriate fields.
But... please consider the overhead of creating such things on existing tables. It could be quite scary. You can put unique indexes on tables without needing to create extra fields. Those unique indexes can then be used for FKs.
I'm not a fan of auto-increment primary keys on every table. The ideas that these give you fast joins and fast row inserts are really not true. My company calls this meatloaf thinking after the story about the woman who always cut the ends off her meatloaf just because her mother always did it. Her mother only did it because the pan was too short--the tradition keeps going even though the reason no longer exists.
When the driving table in a join has an auto-increment key, the joined table frequently shouldn't because it must have the FK to the driving table. It's the same column type, but not auto-increment. You can use the FK as the PK or part of a composite PK.
Adding an auto-increment key to a table with a naturally unique key will not always speed things up--how can it? You are adding more work by maintaining an extra index. If you never use the auto-increment key, this is completely wasted effort.
It's very difficult to predict optimizer performance--and impossible to predict future performance. On some databases, compressed or clustered indexes will decrease the costs of naturally unique PKs. On some parallel databases, auto-increment keys are negotiated between nodes and that increases the cost of auto-increment. You can only find out by profiling, and it really sucks to have to change Company Policy just to change how you create a table.
Having autoincrementing primary keys may make it easier for you to switch ORM layers in the future, and doesn't cost much (assuming you retain your logical unique keys).
You add surrogate auto increment primary keys as part of the implementation after logical design to respect the physical, on-disk architecture of the db engine.
That is, they have physcial properties (narrow, numeric, strictly monotonically increasing) that suit use as clustered keys, in joins etc.
Example: If you're modelling your data, then "product SKU" is your key. "product ID" is added afterwards, (with a unique constraint on "product SKU") when writing your "CREATE TABLE" statements because you know SQL Server.
This is the main reason.
The other reason a brain dead ORM that can't work without one...
Many tables are better off with a compound PK, composed of two or more FKs. These tables correspond to relationships in the Entity-Relationship (ER) model. The ER model is useful for conceptualizing a schema and understanding the requirements, but it should not be confused with a database design.
The tables that represent entities from an ER model should have a smiple PK. You use a surrogate PK when none of the natural keys can be trusted. The decision about whether a key can be trusted or not is not a technical decision. It depends on the data you are going to be given, and what you are expected to do with it.
If you use a surrogate key that's autoincremented, you now have to make sure that duplicate references to the same entity don't creep into your databases. These duplicates would show up as two or more rows with a distinct PK (because it's been autoincremented), but otherwise duplicates of each other.
If you let duplicates into your database, eventually your use of the data is going to be a mess.
The simplest approach is to always use surrogate keys that are either auto-incremented by the db or via an orm. And on every table. This is because they are the generally fasted method for joins and also they make learning the database extremely simple, i.e. none of this whats my key for a table nonsense as they all use the same kind of key. Yes they can be slower but in truth the most important part of design is something that wont break over time. This is proven for surrogate keys. Remember, maintenance of the system happens a lot longer than development. Plan for a system that can be maintained. Also, with current hardware the potential performance loss is really negligable.
Consider this:
A record is deleted in one table that has a relationship with another table. The corresponding record in the second table cannot be deleted for auditing reasons. This record becomes orphaned from the first table. If a new record is inserted into the first table, and a sequential primary key is used, this record is now linked to the orphan. Obviously, this is bad. By using an auto incremented PK, an id that has never been used before is always guaranteed. This means that orphans remain orphans, which is correct.
I would never use natural keys as a PK. A numeric PK, like an auto increment is the ideal choice the majority of the time, because it can be indexed efficiently. Auto increments are guaranteed to be unique, even when records are deleted, creating trusted data relationships.