Can I have multiple primary keys in a MySQL table?

Each table in a database can have only one primary key, and it is not possible to have multiple primary keys.

The primary key is used to ensure the uniqueness and integrity of the data, and at the same time, the search speed can be increased through the primary key search table.

The so-called multiple primary keys of a table are called joint primary keys.

Note: Joint primary key: Multiple fields are used together as the primary key of a table.

Create a federated primary key:

1. Select multiple columns in the GUI at the same time, and click Set as the primary key.

2. The SQL statement sets multiple columns as the primary key:

Method 1: Write out the table when you create it

create table table (field name 1 int not null, field name 2 nvarchar(13) not null primary key (field name 1, field name 2), field name 3.........

The field name is n.........

Method 2: Change the table after it is created

alter table name with nocheck add

constraint [pk table name] primary key nonclustered

Field Name 1], Field Name 2].

No, there can only be one primary key.

However, if you want to use multiple 'unique indexes', you can have more than one.

There can only be one primary key.

Multiple fields can be used as joint primary keys.

A table cannot read two primary keys. But it is possible to combine two fields into a single primary key, which is why sometimes there are two fields in the table that have a primary key flag, and that's because they are combined to form a primary key.

Composition of the SQL language:

1. An SQL database is a collection of tables, which are defined by one or more SQL schemas.

2. An SQL table is composed of a set of rows, a row is a sequence (set) of columns, and each column corresponds to a data item with a row.

3. A table is either a basic table or a view. A base table is a table that is actually stored in a database, while a view is the definition of a table that consists of several base tables or other views.

There is only one primary key, but you can set multiple fields as primary keys, that is, joint primary keys.

You can set the foreign key yourself, that is, there can be more than one, and you can set all fields except the primary key to be foreign keys.

Extended information: 1. Each table in the database can only have one primary key, and it is impossible to have multiple primary keys.

2. The so-called multiple primary keys of a table are called joint primary keys.

Note: Joint primary key: Multiple fields are used together as the primary key of a table.

3. The function of the primary key of the primary key is to ensure the uniqueness and integrity of the data, and at the same time, the search speed can be increased through the primary key search table.

There is only one primary key in a table, but there are two kinds of primary keys, single and compound, and if you want to set a composite primary key, you must first establish a constraint, and there can be more than one foreign key.

I've never seen my foreign key associated with my primary key, I tried it with postgre and mysql, and the situation you said is okay.

So the question is, what's the point???

Here, the primary key is the primary key, and the key is not the primary key, but a normal index. It's okay to add another one.

MySQL key and index are a bit similar to index constraints, and the actual meaning of the key (primary key) combined with other keywords is different.

A database table can have only one primary key, and two primary keys are not allowed. However, two fields are allowed to be combined to set as the primary key, which is called a joint primary key.

Here's how to create a primary key:

alter table

Add Constraint PK Primary Key (Student ID);

Here's how to create a federated primary key:

alter table

Add Constraint PK Primary Key (Student ID, Course Number);

This is called a joint primary key.

When the uniqueness of one field cannot be determined, other fields are needed to form uniqueness. That is, if there are more than one field used to compose uniqueness, it is a joint primary key.

You can add a few more. As long as it matches your needs.

It's okay if all fields are primary keys.

The number of primary keys is set based on the needs of the business.

There is no fixed number, one is enough to meet the needs, and different business tables can be set according to business needs.

It is recommended that you look at the next, two, and three paradigms.