Question

Keys in Mysql (PRI and MUL)

Asked by: Simon336697

Hi guys Im hoping you can help me understand what the MUL key actually means for a field in a MySQL table.

Field                   Type                        Key
------------------------------------------------------------------
System                int(10)                    PRI
SystemName       varchar(11)           MUL
BrandCode          char(1)                  MUL

In the above, does this mean:

SystemName and BrandCode are multiple keys?

Can you have the following rows?

SystemName                   BrandCode
test001                            a
test001                            b

or does both systemname and brandcode have to be unique?

Im just not quite understanding what the multiple keys really means.

Thank you :.)

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Asked On
2008-01-23 at 22:12:22ID23106917
Tags

mul

,

mysql

,

pri

,

key

Topic

MySQL Server

Participating Experts
1
Points
500
Comments
8

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Answers

 

by: hernst42Posted on 2008-01-23 at 23:17:42ID: 20731312

multiple means the index is based on the two columns.
> Can you have the following rows?
Yes

> does both systemname and brandcode have to be unique?
Depends on the definition of the index. The information you provided does not show if it's unique or not

 

by: Simon336697Posted on 2008-01-24 at 00:00:43ID: 20731510

Hi hernst42!

This is the table structure................................................................................................

create table system
`System` int(8)  NOT NULL auto_increment,
  `SystemName` varchar(9) NOT NULL default '',
  `BrandCode` char(3) NOT NULL default '',
  `SiteID` varchar(2) NOT NULL default '',
  PRIMARY KEY  (`System`),
  KEY `SystemName` (`SystemName`),
  KEY `BrandCode` (`BrandCode`)
)

======================================================================
show indexes from systems;

Table      Non_unique      Key_name      Seq_in_index   Column_name
systems      0      PRIMARY         1         System
systems      1      SystemName         1         SystemName
systems      1      BrandCode         1                   BrandCode
systems      1      BrandCode_2  1         BrandCode
systems      1      BrandCode_2  2         SiteID

Does this stop you from having the same systemname entered more than once?

 

by: hernst42Posted on 2008-01-24 at 00:16:38ID: 20731554

No, in this case BrandCode_2 must show 0 for column Non_unique
use
CREAT INDEX BrandCode_2 on system (BrandCode, SystemName)

 

by: Simon336697Posted on 2008-01-24 at 00:29:51ID: 20731598

Thanks hernst42,

What im trying to do is to do the following hernst.....

insert into systems (systemName, systemType, siteCode)
VALUES ('system01','P', 'too')

The thing is, if i run this several times, it creates new rows..and doesnt stop me from adding a new row where the systemName is the same. Id like a way to only have 1 row where the systemName is unique

 

by: hernst42Posted on 2008-01-24 at 00:36:31ID: 20731618

So there should only be entry for SystemName ? In this case make the column systemName Unique

CREAT UNIQUE INDEX systemName_idx  on system (SystemName)

 

by: Simon336697Posted on 2008-01-24 at 00:44:56ID: 20731645

I cant thank you enough hernst :>)

Mate can you include the unique index when you create the table?

eg.
use vmdb;
CREATE TABLE systems (
  systemId  int(10) unsigned NOT NULL auto_increment,
  systemName varchar(40) NOT NULL default '' UNIQUE,
  systemType char(1) NOT NULL default '',
  siteCode char(3) NOT NULL default '',
  PRIMARY KEY (`systemId`)
)

Would the above do the same things as what you did hernst?
eg. with
CREAT UNIQUE INDEX systemName_idx  on system (SystemName)

 

by: Simon336697Posted on 2008-01-24 at 00:49:09ID: 20731656

I really appreciate your help on this hernst.
I think it does as i test it and thank you so very much for working with me on this :>)

 

by: Simon336697Posted on 2008-01-24 at 00:49:43ID: 20731657

hernst,
I hope you dont mind, but as I learn this stuff, i will be posting a lot of questions as i work through building a really simple database :>)

20120131-EE-VQP-002

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