Is it mandatory for each user to have entries in usermeta table as well or it's optional? Recently, I came across a wordpress database which has thousands of users in user table however only a few of them have entries in usermeta table.
I am wondering in what scenario this could have happened as it doesn't look like those entries were manually removed from database.
install a fresh wordpress setup on your local machine and then have a look at wp_usersmeta table . It contains meta(additional) information for each user.You (admin) can also make data entry specific to any user by this function update_user_meta() in usermeta table. and retrive the same using get_user_meta() funciton.
All fields for all users are not necessary. meta keys can be user specific also.
defaults meta key entries for admin in wordpress 3.6 --
first_name
last_name
nickname
description
rich_editing
comment_shortcuts
admin_color
use_ssl
show_admin_bar_front
wp_capabilities
wp_user_level
dismissed_wp_pointers
show_welcome_panel
wp_dashboard_quick_press_last_post_id
managenav-menuscolumnshidden
metaboxhidden_nav-menus
nav_menu_recently_edited
closedpostboxes_page
metaboxhidden_page
metaboxhidden_page
wp_user-settings wp_user-settings-time
