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Does a relational table need a primary key?
Should you create a database table without a primary key? No. Every table should have some column (or set of columns) that uniquely identifies one and only one row. It makes it much easier to maintain the data.
Does a many-to-many table have a primary key?
A many-to-many relationship occurs when multiple records in a table are associated with multiple records in another table. Each record in a join table includes a match field that contains the value of the primary keys of the two tables it joins. (In the join table, these match fields are foreign keys.)
Why is primary key necessary in a table?
Each database table needs a primary key because it ensures row-level accessibility. If you choose an appropriate primary key, you can specify a primary key value, which lets you query each table row individually and modify each row without altering other rows in the same table.
When do you need a primary key for a table?
Long answer: You need your table to be joinable on something. If you want your table to be clustered, you need some kind of a primary key. If your table design does not need a primary key, rethink your design: most probably, you are missing something.
When to create composite primary key for many to many table?
When you create a many-to-many table, should you create a composite primary key on the two foreign key columns, or create a auto-increment surrogate “ID” primary key, and just put indexes on your two FK columns (and maybe a unique constraint)? What are the implications on performance for inserting new records/re-indexing in each case?
Why do you put a sequential primary key on a link table?
As you stated/quoted, the index for the link table will either become fragmented, or the DBMS will work harder to insert records if the link table does not have its own sequential primary key. This is the reason most people put a sequentially incrementing primary key on link tables.
When to use the primary key table constraint?
When the primary key has more than one column, you must use the PRIMARY KEY constraint as a table constraint. In this syntax, you separate columns in the column_list by commas (,). The PRIMARY KEY table constraint can be used when the primary key has one column: