In standard SQL, which constraint type is used directly on a column or table to limit allowed values (for example, enforce a range, a list, or a comparison rule)?

Introduction / Context:
To keep data valid at the source, SQL provides declarative constraints that the database enforces automatically. Choosing the right constraint ensures data integrity without application-side duplication.

Given Data / Assumptions:

• We want to restrict a column's allowed values (e.g., 1–100, set membership, or column relationships).
• The constraint should be enforced on every INSERT and UPDATE.
• We prefer a portable, standard mechanism.

Concept / Approach:
The CHECK constraint evaluates a Boolean expression per row. If the expression evaluates to true, the row is allowed; otherwise the statement fails. CHECK can reference one or more columns of the same row and supports ranges, sets, and simple predicates.

Step-by-Step Solution:

Verification / Alternative check:
Attempt to insert an invalid value; the database rejects it with a CHECK violation, proving the constraint works.

Why Other Options Are Wrong:
LIMIT / VALUE / FILTER constraint: Not standard SQL constraint names.
None of the above: Incorrect because CHECK is the standard mechanism.

Common Pitfalls:
Writing overly complex CHECK expressions can hurt readability; keep them clear and document business rules.

Final Answer:
CHECK constraint