Database Management System (DBMS) / Relational Model

Properties of Relation/Table in Relational Model

Properties of a Relation

📌 Relation Names Must Be Unique

  • Every table in a database needs its own distinct name.

  • You can't have two separate tables both called Student in the same schema.

📌 Attribute Names Must Be Unique Within a Table

  • No two columns in the same table can share a name.

  • A Student table can't have two different columns both labeled Name.

📌 Tuples (Rows) Cannot Repeat

  • Every row in a table has to be distinct — no exact duplicates allowed.

  • This is typically enforced through a primary key.

✅ Example:

✔️ (101, "Alice", 20)
✔️ (102, "Bob", 21)
❌ (101, "Alice", 20)  ← Exact duplicate of the first row — not allowed

📌 Row Order Is Irrelevant

  • A table is a set of rows, not a list — there's no inherent "first" or "last."

  • Rearranging rows doesn't change what the relation represents.

📌 Column Order Is Irrelevant

  • Where a column sits doesn't affect the table's meaning.

  • (ID, Name, Age) describes the same relation as (Name, Age, ID).

📌 Every Value Must Be Atomic

  • Each cell can hold exactly one indivisible value — never a list or set.

  • This requirement is what defines First Normal Form (1NF).

❌ Not allowed: Subjects = {"Math", "Science"} ✅ Correct: Subject = "Math"

📌 A Column's Values Share One Domain

  • Every entry in a column must come from the same data type or value set.

  • An Age column should only ever hold integers like 18 or 21 — never text like "twenty".

📌 NULLs Are Permitted

  • A relation can store NULL when a value is unknown or doesn't apply.

  • That said, leaning on NULLs too heavily undermines data integrity — use them sparingly.