What are the different tables in ServiceNow, and what does 'table extends table' mean?

The Intent Behind This Question

Table inheritance is one of ServiceNow's most important concepts. It affects how data is stored, how scripts execute, and how security is applied. Interviewers ask this to check if you understand why a Business Rule on 'task' fires on incidents too.

Tables in ServiceNow

Every piece of data in ServiceNow lives in a table. Tables are like database tables — they have columns (fields) and rows (records). But ServiceNow tables support inheritance, which means one table can extend another and inherit all its fields.

The Task Table Hierarchy

// task (base table)
// ├── Fields: number, state, priority, assigned_to, short_description
// │
// ├── incident (extends task)
// │   └── Adds: category, subcategory, caller_id, resolve_time
// │
// ├── change_request (extends task)
// │   └── Adds: risk, type, planned_start_date, cab_required
// │
// ├── problem (extends task)
// │   └── Adds: known_error, root_cause, workaround
// │
// ├── sc_task (extends task)
// │   └── Adds: request_item, catalog fields
// │
// └── sc_req_item (extends task)
//     └── Adds: cat_item, stage, price
//     NOTE: sc_req_item extends task directly, NOT sc_task

What 'Extends' Means in Practice

• Field inheritance: The incident table has all fields from task (number, state, priority...) PLUS its own fields (category, caller_id). You don't need to recreate common fields.
• Script inheritance: A Business Rule on the task table fires for ALL child tables — incidents, changes, problems, etc. This is powerful but dangerous if unintended.
• ACL inheritance: A security rule on task applies to all extending tables unless overridden at the child level.
• Physical storage: ServiceNow uses single-table inheritance — child records are physically stored in the parent table's database. A sys_class_name field acts as a discriminator to identify the record type.

// Important: Querying 'task' returns EVERYTHING
var gr = new GlideRecord('task');
gr.addQuery('assigned_to', userId);
gr.query();
// Returns incidents + changes + problems + catalog tasks + ...
// Could be hundreds of thousands of records!

// If you only want incidents, query the child table:
var gr = new GlideRecord('incident');
gr.addQuery('assigned_to', userId);
gr.query();
// Returns only incidents

Other Important Tables (Non-Task)

• sys_user — User records
• sys_user_group — Group records
• cmdb_ci — Configuration Items (extends to cmdb_ci_server, cmdb_ci_database, etc.)
• kb_knowledge — Knowledge articles
• sys_choice — Dropdown/choice list values