Assertion (A) : We can retrieve records from more than one table in MYSQL. Reason (R) : Foreign key is used to

Question 16 of 29

Q. Assertion (A) : We can retrieve records from more than one table in MYSQL. Reason (R) : Foreign key is used to establish a relationship between two tables. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A). (B) Both Assertion (A) and Reason (R) are true and Reason (R) is not the correct explanation for Assertion (A). (C) Assertion (A) is true but, Reason (R) is false. (D) Assertion (A) is false but, Reason (R) is true.

Odisha ChseCBSE Class XII Board 2025MCQ· 1mImportance★★★★★

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Concept understanding — Foreign Key Nullability

Imagine you are the registrar at a college. Every student belongs to a department — say, Physics or Economics. You keep a list of students, and next to each student’s name you write their department code. That department code is a foreign key: it points to the official list of departments. Now, a question arises: what happens if a student has not yet been assigned to any department? Do you leave that slot blank, or do you force them to pick one?

That blankness is the heart of foreign key nullability.

Foreign key nullability simply means: can a foreign key column contain a NULL value? In database terms, NULL stands for “unknown” or “not applicable.” It is not the same as zero or an empty string — it is the absence of a value. When you design a table, you decide whether the foreign key column is allowed to hold NULL or not.

If the foreign key allows NULL, then a record in the child table can exist without being linked to any parent record. In our college example, a student can be enrolled without a department assignment — the department code is left blank.
If the foreign key does not allow NULL (it is “NOT NULL”), then every record in the child table must point to a valid, existing parent record. Every student must belong to a department, no exceptions.

Why does this matter? Because it directly controls the referential integrity of your data — the rule that every foreign key value must match a primary key value in the parent table. A NULL is not a violation of that rule, because NULL simply means “no reference.” But a non-null value that does not match any parent key is a violation.

Important

A NULL foreign key does not break referential integrity. The rule only applies to non-null values. This is a common point of confusion in exams: a foreign key can be NULL unless you explicitly forbid it.

Here are the practical consequences:

When NULL is allowed: You can insert a child record before you know its parent. For example, a new employee can be added to the payroll before their manager is assigned. This gives flexibility, but it also means some records may be “orphaned” in the sense that they have no parent link. …

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Both statements are true: MySQL can retrieve records from multiple tables, and foreign keys establish relationships between tables — but foreign keys are not required to query multiple tables, so the reason doesn't explain the assertion.

The question tests whether you understand the distinction between capability (what MySQL can do) and mechanism (what enables or requires that capability).

Assertion (A) claims MySQL can retrieve records from more than one table. This is absolutely true. MySQL supports several ways to combine data from multiple tables:

JOIN operations (INNER, LEFT, RIGHT, CROSS) that merge rows based on conditions
UNION that stacks result sets vertically
Subqueries that nest one query inside another
Even simple comma-separated table lists in the FROM clause (implicit Cartesian product)

None of these require a foreign key to exist. You can join any two tables on any column, or even on a constant condition. The database engine doesn't check for foreign key constraints before allowing a multi-table query.

Reason (R) states that foreign keys establish relationships between tables. This is also true. A foreign key constraint enforces referential integrity: it ensures that a value in one table's column must exist in another table's primary key. This logically represents a relationship (e.g., every order must belong to an existing customer).

The critical question: does (R) explain (A)?

No. Foreign keys are a data integrity tool, not a query prerequisite. They help maintain consistent relationships and can guide query optimizers, but MySQL happily retrieves data from multiple tables whether or not foreign keys are defined. Many databases in production use JOINs extensively without declaring foreign key constraints (relying instead on application logic or performance considerations). …

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