Can a database table be without a primary key?

Question

Can anyone tell me if a table in a relational database (such as MySQL / SQL SERVER) can be without a primary key?

For example, I could have table day_temperature

Answer 1

Like always it depends.

Table does not have to have primary key. Much more important is to have correct indexes. On database engine depends how primary key affects indexes (i.e. creates unique index for primary key column/columns).

However, in your case (and 99% other cases too), I would add a new auto increment unique column like temp_id and make it surrogate primary key.

It makes much easier maintaining this table -- for example finding and removing records (i.e. duplicated records) -- and believe me -- for every table comes time to fix things :(.

Answer 2

I've got a better example of a table that doesn't need a primary key - a joiner table. Say I have a table with something called "capabilities", and another table with something called "groups", and I want a joiner table that tells me all the capabilities that all the groups might have, so it's basicallly

create table capability_group
(  capability_id varchar(32),
    group_id     varchar(32));

There is no reason to have a primary key on that, because you never address a single row - you either want all the capabilities for a given group, or all the groups for a given capabilty. It would be better to have a unique constraint on (capabilty_id,group_id), and separate indexes on both fields.

Answer 3

Technically, you can declare such a table.

But in your case, the time should be made the PRIMARY KEY, since it's probably wrong to have different temperatures for the same time and probably useless to have same more than once.

Logically, each table should have a PRIMARY KEY so that you could distinguish two records.

If you don't have a candidate key in you data, just create a surrogate one (AUTO_INCREMENT, SERIAL or whatever your database offers).

The only excuse for not having a PRIMARY KEY is a log or similar table which is a subject to heavy DML and having an index on it will impact performance beyond the level of tolerance.

Answer 4

I would include a surrogate/auto-increment key, especially if there is any possibility of duplicate time/temperature readings. You would have no other way to uniquely identify a duplicate row.

Answer 5

Even if you do not add a primary key to an InnoDB table in MySQL, MySQL adds a hidden clustered index to that table. If you do not define a primary key, MySQL locates the first UNIQUE index where all the key columns are NOT NULL and InnoDB uses it as the clustered index.

If the table has no primary key or suitable UNIQUE index, InnoDB internally generates a clustered index GEN_CLUST_INDEX on a synthetic column containing row ID values.

https://dev.mysql.com/doc/refman/8.0/en/innodb-index-types.html