问题
If I don't need a primary key should I not add one to the database?
回答1:
A primary key uniquely identifies a row in your table.
The fact it's indexed and/or clustered is a physical implementation issue and unrelated to the logical design.
You need one for the table to make sense.
回答2:
You do need a primary key. You just don't know that yet.
回答3:
If you don't need a primary key then don't use one. I usually have the need for primary keys, so I usually use them. If you have related tables you probably want primary and foreign keys.
回答4:
Yes, but only in the same sense that it's okay not to use a seatbelt if you're not planning to be in an accident. That is, it's a small price to pay for a big benefit when you need it, and even if you think you don't need it odds are you will in the future. The difference is you're a lot more likely to need a primary key than to get in a car accident.
You should also know that some database systems create a primary key for you if you don't, so you're not saving that much in terms of what's going on in the engine.
回答5:
No, unless you can find an example of, "This database would work so much better if table_x didn't have a primary key."
You can make an arguement to never use a primary key, if performance, data integrity, and normalization are not required. Security and backup/restore capabilities may not be needed, but eventually, you put on your big-boy pants and join the real world of database implementation.
回答6:
Yes, a table should ALWAYS have a primary key... unless you don't need to uniquely identify the records in it. (I like to make absolute statements and immediately contradict them)
When would you not need to uniquely identify the records in a table? Almost never. I have done this before though for things like audit log tables. Data that won't be updated or deleted, and wont be constrained in any way. Essentially structured logging.
回答7:
A primary key will always help with query performance. So if you ever need to query using the "key" to a "foreign key", or used as lookup then yes, craete a foreign key.
回答8:
I don't know. I have used a couple tables where there is just a single row and a single column. Will always only be a single row and a single column. There is no foreign key relationships.
Why would I put a primary key on that?
回答9:
A primary key is mainly formally defined to aid referencial Integrity, however if the table is very small, or is unlikely to contain unique data then it's an un-necessary overhead. Defining indexes on the table can normally be used to imply a primary key without formally declaring one. However you should consider that defining the Primary key can be useful for Developers and Schema generation or SQL Dev tools, as having the meta data helps understanding, and some tools rely on this to correctly define the Primary/foreign key relationships in the model.
回答10:
Well...
Each table in a relational DB needs a primary key. As already noted, a primary key is data that identies a record uniquely...
You might get away with not having an "ID" field, if you have a N-M table that joins 2 different tables, but you can uniquely identifiy the record by the values from both columns you join. (Composite primary key)
Having a table without an primary key is against the first normal form, and has nothing to do in a relational DB
回答11:
You should always have a primary key, even if it's just on ID. Maybe NoSQL is what you're after instead (just asking)?
回答12:
That depends very much on how sure you can be that you don't need one. If you have just the slightest bit of doubt, add one - you'll thank yourself later. An indicator being if the data you store could be related to other data in your DB at one point.
One use case I can think of is a logging kind-of table, in which you simply dump one entry after the other (to properly process them later). You probably won't need a primary key there, if you're storing enough data to filter out the relevant messages (like a date). Of course, it's questionable to use a RDBMS for this.
来源：https://stackoverflow.com/questions/1771237/is-it-ok-not-to-use-a-primary-key-when-i-dont-need-one