MySQL - Meaning of "PRIMARY KEY", "UNIQUE KEY" and "KEY" when used together while creating a table

Question

Can anyone explain about the purpose of PRIMARY KEY, UNIQUE KEY and KEY, if it is put together in a single CREATE TABLE statement in MySQL?

CREATE TABLE IF NOT EXISTS `tmp` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `uid` varchar(255) NOT NULL,
  `name` varchar(255) NOT NULL,
  `tag` int(1) NOT NULL DEFAULT '0',
  `description` varchar(255),
  PRIMARY KEY (`id`),
  UNIQUE KEY `uid` (`uid`),
  KEY `name` (`name`),
  KEY `tag` (`tag`)
) ENGINE=InnoDB AUTO_INCREMENT=1 ;

How do I convert this query to MSSQL?

Bayonne

related: mySQL's KEY keyword? Basically KEY is synonymous with INDEX. – Crossly

Answer 1

A key is just a normal index. A way over simplification is to think of it like a card catalog at a library. It points MySQL in the right direction.

A unique key is also used for improved searching speed, but it has the constraint that there can be no duplicated items (there are no two x and y where x is not y and x == y).

The manual explains it as follows:

A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. This constraint does not apply to NULL values except for the BDB storage engine. For other engines, a UNIQUE index permits multiple NULL values for columns that can contain NULL. If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within the prefix.

A primary key is a 'special' unique key. It basically is a unique key, except that it's used to identify something.

The manual explains how indexes are used in general: here.

In MSSQL, the concepts are similar. There are indexes, unique constraints and primary keys.

Untested, but I believe the MSSQL equivalent is:

CREATE TABLE tmp (
  id int NOT NULL PRIMARY KEY IDENTITY,
  uid varchar(255) NOT NULL CONSTRAINT uid_unique UNIQUE,
  name varchar(255) NOT NULL,
  tag int NOT NULL DEFAULT 0,
  description varchar(255),
);

CREATE INDEX idx_name ON tmp (name);
CREATE INDEX idx_tag ON tmp (tag);

Edit: the code above is tested to be correct; however, I suspect that there's a much better syntax for doing it. Been a while since I've used SQL server, and apparently I've forgotten quite a bit :).

Waterloo

Great explanation! The code is working well too. Thank you very much! – Bayonne

Thank you for pointing me to the correct place in the manual! – Hued

Great! This link will refer to the typing for MySQL and SQL in the Primary Key thing. w3schools.com/sql/sql_primarykey.asp – Caddoan

Answer 2

Just to add to the other answers, the documentation gives this explanation:

• KEY is normally a synonym for INDEX. The key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition. This was implemented for compatibility with other database systems.

• A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. For all engines, a UNIQUE index permits multiple NULL values for columns that can contain NULL.

• A PRIMARY KEY is a unique index where all key columns must be defined as NOT NULL. If they are not explicitly declared as NOT NULL, MySQL declares them so implicitly (and silently). A table can have only one PRIMARY KEY. The name of a PRIMARY KEY is always PRIMARY, which thus cannot be used as the name for any other kind of index.

Crossly

Compatibility with other database systems? What other DBMS uses the word "KEY" for something that isn't a key at all (an index)? I'd always assumed that was just a bit of MySQL strangeness - and that the authors of MySQL don't actually know what a key is :) – Belkisbelknap

Does each subsequent definition contain all properties of the previous definition. Eg: Is a "UNIQUE KEY" also a "KEY"? – Cortez

Answer 3

MySQL unique and primary keys serve to identify rows. There can be only one Primary key in a table but one or more unique keys. Key is just index.

for more details you can check http://www.geeksww.com/tutorials/database_management_systems/mysql/tips_and_tricks/mysql_primary_key_vs_unique_key_constraints.php

to convert mysql to mssql try this and see http://gathadams.com/2008/02/07/convert-mysql-to-ms-sql-server/

Selachian

@Paul Bellora , your explanation is more clear than mine. Great. – Selachian

Well I just quoted the docs, and I didn't see the second part about converting. I think Corbin's answer sums it up well. – Crossly