MYSQL - What is a primary key?

Question

I'm in the process of learning Mysql, and I'm creating databases. So, after looking at several websites, the definition for a primary key is:

The PRIMARY KEY constraint uniquely identifies each record in a database table.

and is used like this:

    CREATE TABLE Persons
(
    P_Id int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Address varchar(255),
    City varchar(255),
    PRIMARY KEY (P_Id)          //primary key is on this line
)

However, I still don't know what it's used for and why we need it. So my question is.

Can someone explain to me what a primary key is (in basic english) and why we need one and what is it used for?

Thank-you.

The only difficult part is... which other question is this a duplicate of? — p.campbell, Mar 04 '12 at 00:30
possible duplicate of [What's the difference between Primary Key, Unique Key and Index in MySQL?](http://stackoverflow.com/questions/3844899/whats-the-difference-between-primary-key-unique-key-and-index-in-mysql) — p.campbell, Mar 04 '12 at 00:33
Possible duplicate of [Difference between Key, Primary Key, Unique Key and Index in MySQL](https://stackoverflow.com/questions/3844899/difference-between-key-primary-key-unique-key-and-index-in-mysql) — philipxy, Jun 07 '19 at 04:04

score 6 · Answer 1 · edited Jul 17 '18 at 02:55

6

A primary key is a column that is defined as uniquely identifying each row in a table.

Also, by defining a column as PRIMARY KEY, it may be referenced as a foreign key in other tables when defining referential integrity constraints.

edited Jul 17 '18 at 02:55

Wolverine

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answered Mar 04 '12 at 00:31

Bohemian

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+1 You may also find http://stackoverflow.com/questions/8038641/please-confirm-my-use-of-primary-key-and-unique-index/8038647#8038647 useful. – Michael Durrant Mar 04 '12 at 00:34

score 3 · Answer 2 · 2021-02-27T11:57:03.780

3

A primary key is a unique identifier for the row. It is normally automatically assigned by the database management system (if you specify auto-increment for that value). So if you have a database of people in an organisation, their primary key may be their employee number. Every time you add an employee, they receive a unique employee number that is usually the previous employees number + 1.

Without primary keys you could not distinguish between two employees called John Smith (without other information) Hope that is clear enough.

edited Feb 27 '21 at 11:57

answered Mar 04 '12 at 00:33

1

I'd say 'sometimes assigned by the dbms' rather than 'normally', but otherwise agree. – Richard A Mar 04 '12 at 00:35
Okay, fair call, I was being picky. Not meaning to start a fight. – Richard A Mar 04 '12 at 00:50
@RichardA I would imagine "normal" is the norm these days (just as is not having a [real] DBA involved at any level...) – Mar 04 '12 at 00:53

score 1 · Answer 3 · answered Feb 27 '21 at 12:06

1

In simple words,

Primary Key = NOT NULL + UNIQUE

that means whichever column is assigned primary key, that column will neither take NULL values nor accept the duplicate values.

answered Feb 27 '21 at 12:06

Abhishek Kumar Saw

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MYSQL - What is a primary key?

3 Answers3

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