Manual:$wgDBsqluser

Database settings: $wgDBsqluser
The SQL user used for queries.
Introduced in version:	Before 1.1.0
Removed in version:	1.5.0
Allowed values:	(string)
Default value:	'sqluser'
Other settings: Alphabetical | By function

Details

The SQL user used for read-only queries in the (no longer existing) Special:Asksql.

Warning:

THIS IS VERY INSECURE. If you want to allow semipublic read-only SQL access for your sysops, you should define a MySQL user with limited privileges. Review the MySQL documentation at http://dev.mysql.com/doc/refman/5.7/en/grant.html on how permissions can be granted in the database subsystem.

The MySQL user shouldn't have too many rights over the database, restrict it to SELECT only on "cur" table for example.