Data Schema, Tables, and Fields
ServiceNow stores data in tables, each with a dictionary of fields. Tables can extend other tables (inheritance): the child inherits all parent fields and adds its own. The task table is the root of most work records (incident, problem, change extend task). Candidates must know sys_id (unique GUID), reference fields (store a sys_id pointing to another table), choice fields (store a value with a display label), and glide_record vs glide_list types.
Understand table hierarchy: a query on a parent table returns child records (e.g., querying task returns incidents). This is a frequent trap - candidates assume querying task only returns task records. Also know calculated fields, formulas, default values, and mandatory flags. Indexes affect query performance; audit can be enabled per table to track changes.
Traps: confusing a reference field's stored value (sys_id) with its display value; assuming deleting a parent record cascades (it depends on configuration); and thinking a field's max length can be changed after data exists without risk. Another trap: assuming choice values are stored as labels - they store the value, not the label.
Worked example: To find all work items, query the task table - it returns incidents, problems, changes, and catalog tasks because they extend task. If you only want incidents, query the incident table directly. Remember that a reference field stores the sys_id, so filtering on a reference uses the sys_id or the display value depending on the operator.