Which statement correctly reflects a core constraint of 1NF?

Which statement correctly reflects a core constraint of 1NF?

• A. Tables must contain only one row.
• B. All data must be unique.
• C. Each column must hold only atomic (indivisible) values.
• D. Data types must be identical across a column.

Answer: (C)

The central idea being tested is that each field must contain a single, indivisible value. In first normal form, every cell in every column holds one atomic value, so there are no lists, arrays, or multi-valued attributes in a single cell. If a column ever stores multiple items in one cell (like "555-1234, 555-5678"), that breaks 1NF and you’d split it so each value is stored separately, often by creating additional rows or separating the data into related tables. This atomicity is what makes queries, comparisons, and indexing reliable, because every field represents exactly one value to work with. The other statements don’t capture this constraint: a table can have many rows; duplicates across rows are allowed in general relational design; and while columns have data types, the defining feature of 1NF is the indivisible value requirement, not the notion of uniform types across a column.

The central idea being tested is that each field must contain a single, indivisible value. In first normal form, every cell in every column holds one atomic value, so there are no lists, arrays, or multi-valued attributes in a single cell. If a column ever stores multiple items in one cell (like "555-1234, 555-5678"), that breaks 1NF and you’d split it so each value is stored separately, often by creating additional rows or separating the data into related tables. This atomicity is what makes queries, comparisons, and indexing reliable, because every field represents exactly one value to work with. The other statements don’t capture this constraint: a table can have many rows; duplicates across rows are allowed in general relational design; and while columns have data types, the defining feature of 1NF is the indivisible value requirement, not the notion of uniform types across a column.