问题
What are primary keys used aside from identifying a unique column in a table? Couldn't this be done by simply using an autoincrement constraint on a column? I understand that PK and FK are used to relate different tables, but can't this be done by just using join?
Basically what is the database doing to improve performance when joining using primary keys?
回答1:
Mostly for referential integrity with foreign keys,, When you have a PK it will also create an index behind the scenes and this way you don't need table scans when looking up values
回答2:
RDBMS providers are usually optimized to work with tables that have primary keys. Most store statistics which helps optimize query plans. These statistics are very important to performance especially on larger tables and they are not going to work the same without primary keys, and you end up getting unpredictable query response times.
Most database best practices books suggest creating all tables with a primary key with no exceptions, it would be wise to follow this practice. Not many things say junior software dev more than one who builds a database without referential integrity!
回答3:
Some PKs are simply an auto-incremented column. Also, you typically join USING the PK and FK. There has to be some relationship to do a join. Additionally, most DBMS automatically index PKs by default, which improves join performance as well as querying for a particular record based on ID.
回答4:
You can join without a primary key within a query, however, you must have a primary key defined to enforce data integrity constraints, at least with SQL Server. (Foreign Keys, etc..)
Also, here is an interesting read for you on Primary Keys.
回答5:
In Microsoft Access, if you have a linked table to, say, SQL Server, the source table must have a primary key in order for the linked table to be writeable. At least, that was the case with Access 2000 and SQL Server 6.5. It may be different with later versions.
回答6:
Keys are about data integrity as well as identification. The uniqueness of a key is guaranteed by having a constraint in the database to keep out "bad" data that would otherwise violate the key. The fact that data integrity rules are guaranteed in that way is precisely what makes a key usable as an identifier. That goes for any key. One key per table by convention is called a "primary" key but that doesn't make other alternate keys any less important.
In practice we need to be able to enforce uniqueness rules against all types of data (not just numbers) to satisfy the demands of data quality and usability.
来源：https://stackoverflow.com/questions/2186460/why-use-primary-keys