Databases
StackOverflow2013
Postsdbo
POWhat does a Cardinality of 0 indicate in this mySQL table and index?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POWhat does a Cardinality of 0 indicate in this mySQL table and index?
    primarykey
    data
    text
    <p>I am trying to learn mySQL database, and have started taking a look at the database behind wordpress. I am using the <a href="http://codex.wordpress.org/Database_Description" rel="nofollow noreferrer">Wordpress Data Base Descrition</a> to review the structure and I came upon the following:</p> <pre><code>|Table: wp_commentmeta| --------------------------------------------------------------------------------- |Field |Type |Null |Key |Default | Extra |-----------|--------------------|------|----|--------|-------------------------- |meta_id |bigint(20) unsigned | |PRI |NULL |auto_increment |comment_id |bigint(20) unsigned | |IND |0 |FK-&gt;wp_comments.comment_id |meta_key |varchar(255) |YES |IND |NULL | |meta_value |longtext |YES | |NULL | |Indexes| --------------------------------------------------------- |Keyname |Type |Cardinality |Field |------------|---------|------------|-------------------- |PRIMARY |PRIMARY |0 |meta_ID |comment_id |INDEX |none |comment_id |meta_key |INDEX |none |meta_key </code></pre> <p>My question is, <em>what does a Cardinality value of 0 indicate in the table above</em>. I understand the explanation of cardinality as explained in <a href="https://stackoverflow.com/questions/755569/why-does-the-cardinality-of-an-index-in-mysql-remain-unchanged-when-i-add-a-new/755599#755599">this SO answer</a> in that it is the number of unique values within a particular set of indexed values, but I dont get what setting a value of 0 does in this particular case.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USCommunity
    1. USHari Seldon
    plurals
    1. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POWhat does a Cardinality of 0 indicate in this mySQL table and index?
      1. USHong
      1. VTFavorite
    1. This table or related slice is empty.
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload