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Data

There are multiple data types in WordPress, but the basic data types stored in the database are:
Post
Comments
Terms
User
Blogs
Network
Links
Options
Site Options
Some of these data types can have additional data attached to them in the form of meta data, some of them have types and statuses.
There are also roles and capabilities, which are not considered content and apply exclusively to users.
Meta
Meta data is data with a key/name and a value, attached to another piece of data. Some people will know them as custom fields. Others will know them as user meta.
Post Meta ( or Custom fields ) are normally shown in the post edit screen, but if the post meta's key/name begins with an underscore, that field is hidden. This way features such as featured images can be built with their own user interfaces.
Post types
Posts, pages, attachments, and menus are all different kinds of posts. You can also register your own post types, and these are referred to as custom post types. Remember to flush permalinks (manually: Dashboard > Settings > Permalinks > Save Changes or programmatically via flush_rewrite_rules) if you modify or add a post type. If you do not do this, some links will generate 404 errors.
*Warning: Developers sometimes attempt to work around the rewrite rules by flushing rewrite rules on the init action using the flush_rewrite_rules, but this is a mistake. It can lead to unexpected behaviours, and has a large negative performance impact. Rewrite rules are expensive to build.
Default Post Types
The default post types are as follows:
Post (post)
Page (page)
Attachment (attachment)
Revision (revision)
Navigation menu (nav_menu_item)
Menus
Menu items are stored as nav_menu_item posts, however, the menu itself is a term in a custom taxonomy that contains nav_menu_item posts.
Revisions
A post of type revision, revisions are historical copies of posts, and are tied to their original post via the post_parent field. To get a posts revisions, grab all its children of type revision. If a database is growing very large or a particular post/page is frequently/automatically edited, you can limit the number of revisions stored.
Uploaded Files & Images
When you upload a file, WordPress does not reference the image or file using its URL, it uses an attachment. Attachments are posts of type attachment, and are referred to by their post ID. For example, when you set a featured image on a post, it stores your chosen image's post ID in that post's meta (_thumbnail_id).
If a file is uploaded whilst editing a post (rather than in the Media Library itself), it's post_parent field is set to that of the post.
WordPress generates and saves resized versions of the original at the time of upload for better performance. They can be cropped and manipulated independently and the dimensions and filenames are stored in the attachment postmeta.
The default image sizes are configured in Dashboard > Settings > Media. If you need more, you can use add_image_size and also control cropping. You can request a specific size of image using the attachment ID and the image size name (e.g. 'medium', 'large'.)
The Regenerate Thumbnails plugin is useful if you change sizes at a later date.
Comments
Comments have their own table, and are attached to a post. Comments are not a type of post however, but they are capable of storing meta data. This is rarely used by developers but allows for interesting things.
Terms and Taxonomies
A taxonomy is a way of categorising or organising things. Items are organised using terms in that taxonomy.
For example, yellow is a term in the colour taxonomy. Big and small are both terms in the size taxonomy.
The Tags and categories that come with WordPress are both taxonomies. Individual tags and categories are called terms.
You can register your own taxonomies, but remember to flush permalinks (see Post Types above) if you make changes.
Taxonomy terms are tied to Object IDs, where an object ID can be any kind of data. This includes posts, users, or comments. These IDs are normally post IDs, but this is purely convention. There is nothing preventing a user or a comment taxonomy. A user taxonomy would be useful for grouping users into locations or job roles.
Options
Options are stored as key value pairs in their own table. Some options have an autoload flag set and are loaded on every page load to reduce the number of queries.
Transients
Transients are stored as options and are used to cache things temporarily
Object Cache
By default WordPress will use in memory caching that does not persist between page loads. There are plugins available that extend this to use APC or Memcache amongst others.
Data Overview
Here's a table showing the full spectrum of data types in WordPress that are stored in the database:
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Post
Comments
Term
User
Blogs
Network
Links
Options
Site Options
Description
Content, e.g. articles
Commentary on a post
A type of objects, for classifying
Users and Authors
A website
A collection of websites
Links to websites
key value pairs
Network-wide key value pairs
Supported
Yes
Yes
Yes
Yes
Yes
Multisite
Deprecated
Yes
Yes
Meta
Custom fields
Comment meta
Planned
User Meta
Options
Site Options
No
No
No
Meta Access
get_post_meta
get_comment_meta
Planned
get_user_meta
get_option
get_site_option
n/a
n/a
n/a
Type
Post type
Comment type
Taxonomy
Roles and Capabilities
No
subdomain or subdir
link_category taxonomy
n/a
n/a
Type registration
register_post_type
defined on use
register_taxonomy
add_role add_cap
n/a
wp-config.php DEFINE
n/a
n/a
n/a
Taxonomy UI?
Yes
No
Yes
No
No
No
only link_category
n/a
n/a
Default Types
post page attachment nav_menu nav_menu_item
none pingback trackback
cat tag
admin editor author contributor subscriber
n/a
SUBDOMAIN_INSTALL true/false
n/a
n/a
n/a
Query Class
WP_Query
WP_Comment_Query
get_terms
WP_User_Query
wp_get_sites
wp_get_sites
get_links
get_option
get_site_option
Has Archives
Yes
No
Per blog
Yes
No
No
No
No
No
Has Widget
Recent Posts
Recent Comments
Categories and Tags
No
No
No
Bookmarks
No
No
Data Availability
Per blog
Per blog
Per blog
Per install
Per network
Per install
Per blog
Per blog
Per network
Set current
setup_postdata
n/a
n/a
n/a
switch_to_blog
no native function
n/a
n/a
n/a
Database Table
wp_posts
wp_comments
wp_terms wp_term_relationships wp_term_taxonomy
wp_users
wp_blogs
wp_site
wp_links
wp_options
wp_site_options
Meta Table
wp_postmeta
wp_commentmeta
Planned
wp_usermeta
wp_options
wp_sitemeta
n/a
n/a
n/a

Constants of wp-config.php

Core

Last modified 3yr ago

	Post	Comments	Term	User	Blogs	Network	Links	Options	Site Options
Description	Content, e.g. articles	Commentary on a post	A type of objects, for classifying	Users and Authors	A website	A collection of websites	Links to websites	key value pairs	Network-wide key value pairs
Supported	Yes	Yes	Yes	Yes	Yes	Multisite	Deprecated	Yes	Yes
Meta	Custom fields	Comment meta	Planned	User Meta	Options	Site Options	No	No	No
Meta Access	`get_post_meta`	`get_comment_meta`	Planned	get_user_meta	get_option	get_site_option	n/a	n/a	n/a
Type	Post type	Comment type	Taxonomy	Roles and Capabilities	No	subdomain or subdir	link_category taxonomy	n/a	n/a
Type registration	`register_post_type`	defined on use	`register_taxonomy`	`add_role add_cap`	n/a	wp-config.php DEFINE	n/a	n/a	n/a
Taxonomy UI?	Yes	No	Yes	No	No	No	only link_category	n/a	n/a
Default Types	post page attachment nav_menu nav_menu_item	none pingback trackback	cat tag	admin editor author contributor subscriber	n/a	SUBDOMAIN_INSTALL true/false	n/a	n/a	n/a
Query Class	`WP_Query`	`WP_Comment_Query`	`get_terms`	`WP_User_Query`	`wp_get_sites`	`wp_get_sites`	`get_links`	`get_option`	`get_site_option`
Has Archives	Yes	No	Per blog	Yes	No	No	No	No	No
Has Widget	Recent Posts	Recent Comments	Categories and Tags	No	No	No	Bookmarks	No	No
Data Availability	Per blog	Per blog	Per blog	Per install	Per network	Per install	Per blog	Per blog	Per network
Set current	`setup_postdata`	n/a	n/a	n/a	switch_to_blog	no native function	n/a	n/a	n/a
Database Table	`wp_posts`	`wp_comments`	`wp_terms` `wp_term_relationships` `wp_term_taxonomy`	`wp_users`	`wp_blogs`	`wp_site`	`wp_links`	`wp_options`	`wp_site_options`
Meta Table	`wp_postmeta`	`wp_commentmeta`	Planned	`wp_usermeta`	`wp_options`	`wp_sitemeta`	n/a	n/a	n/a