Manual:Coding conventions/Database

MySQL

Use UPPERCASE for MySQL keywords, and lowercase for things like types. Do not specify the lengths of numeric types, but do for varchar() and varbinary() types. Use varbinary(14) for all timestamps, and parse them to the standard format using $dbw->timestamp( $ts ); do not use the timestamp field type.

Make sure to include the /*_*/ comment immediately before any table name; this will be replaced with the wiki's database prefix if necessary, and ommitting it will cause breakage. Similarly, include the /*$wgDBTableOptions*/ comment after any table declaration, and /*i*/ immediately before any index names.

Create indices as separate statements, do not include them in the table creation query; the separate syntax is clearer and makes it easier to see the difference between unique and non-unique indices. Don't create indices with ALTER TABLE ... ADD INDEX ..., always use CREATE INDEX ... ON ... instead.

--
-- Track page-to-page hyperlinks within the wiki.
--
CREATE TABLE /*_*/pagelinks (
  -- Key to the page_id of the page containing the link.
  pl_from int unsigned NOT NULL default 0,
 
  -- Key to page_namespace/page_title of the target page.
  -- The target page may or may not exist, and due to renames
  -- and deletions may refer to different page records as time
  -- goes by.
  pl_namespace int NOT NULL default 0,
  pl_title varchar(255) binary NOT NULL default ''
) /*$wgDBTableOptions*/;
 
CREATE UNIQUE INDEX /*i*/pl_from ON /*_*/pagelinks (pl_from,pl_namespace,pl_title);
CREATE UNIQUE INDEX /*i*/pl_namespace ON /*_*/pagelinks (pl_namespace,pl_title,pl_from);

Table naming

• Table names should be singular nouns: user, page, revision, etc. There are some historical exceptions: pagelinks, categorylinks…
• Column names are given a prefix derived from the table name: the name itself if it's short, or an abbreviation:
  • page → page_id, page_namespace, page_title…
  • categorylinks → cl_from, cl_namespace…

Changing the schema

See Development policy#Database_patches, especially https://www.mediawiki.org/w/index.php?title=Development_policy&diff=prev&oldid=537762 .

When updating the schema for an extension, it is advisable to both update the original schema file and create a patch file for those updating from a previous version.