表格中的两个主非复合键

Question

I am working on a project and I realized I am unsure about how to use multiple primary keys. I have a table named "User_Details" that has the details of Customer ID, email address and password. From my understanding, I can use both Customer ID and email address as the primary key. In this case do I use only one as Primary Key or both? If I use both, do they become composite primary keys?

(PS. I have other tables, where the foreign key is the customer ID)

Answer 1

You can only have one primary key, but you could definitely have other unique fields.

Usually using an integer / id as primary key is preferred over a string key, and an id is presumably auto assigned, where as email could change - which would be a problem for foreign key relations.

Since you already use customer Id as a foreign key in other tables, I would suggest you continue to do that.

Answer 2

You can only have one primary key, but you can have multiple columns in your primary key, alternatively you can also have Unique Indexes on your table, which will work a bit like a primary key in that they will enforce unique values, and will speed up querying of those values.

Easiest way tho is a Composite Primary Key which is a primary key made from two or more columns. For example:

CREATE TABLE userdata (
userid INT,
userdataid INT,
info char(200),
primary key (userid, userdataid),
);

Here is more info: Link

Answer 3

You can have a Composite Primary Key which is a primary key made from two or more columns. For example:

CREATE TABLE userdata (
  userid INT,
  userdataid INT,
  info char(200),
  primary key (userid, userdataid),
);

A table can have multiple candidate keys. Each candidate key is a column or set of columns that are UNIQUE, taken together, and also NOT NULL. Thus, specifying values for all the columns of any candidate key is enough to determine that there is one row that meets the criteria, or no rows at all.

Candidate keys are a fundamental concept in the relational data model.

It's common practice, if multiple keys are present in one table, to designate one of the candidate keys as the primary key. It's also common practice to cause any foreign keys to the table to reference the primary key, rather than any other candidate key.

I recommend these practices, but there is nothing in the relational model that requires selecting a primary key among the candidate keys.