GATE CSE · Dbms

Sql Practice Questions

Master topic for SQL. Includes SQL - Aggregate Functions & NULLs, SQL - Nested Subqueries.

85 questions · 0 PYQs · 5 AI practice · GATE CSE 2027

Q81MediumMCQ

Consider a relation $\texttt{EMPLOYEE(EID, NAME, SALARY, DID)}$ . Some employees have NULL salary. The query $\texttt{SELECT AVG(SALARY) FROM EMPLOYEE}$ will:

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📖 Explanation

$\texttt{AVG}$ , like all SQL aggregate functions except $\texttt{COUNT(*)}$ , ignores NULL values. The average is computed only over rows where $\texttt{SALARY}$ is not NULL. For example, if salaries are $\{10000, \text{NULL}, 30000\}$ , $\texttt{AVG(SALARY)} = (10000 + 30000) / 2 = 20000$ , not $13333$ .

Q82MediumMCQ

Consider the table $\texttt{T(A, B)}$ with tuples: $(1, 2)$ , $(1, 3)$ , $(2, 2)$ , $(2, 4)$ , $(3, \text{NULL})$ . The query is: $\texttt{SELECT A FROM T GROUP BY A HAVING MIN(B) > 1;}$ What is the result?

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📖 Explanation

Groups: $A=1$ : $B \in \{2, 3\}$ , $\texttt{MIN(B)} = 2 > 1$ ✓. $A=2$ : $B \in \{2, 4\}$ , $\texttt{MIN(B)} = 2 > 1$ ✓. $A=3$ : $B \in \{\text{NULL}\}$ , $\texttt{MIN(B)} = \text{NULL}$ . The comparison $\text{NULL} > 1$ evaluates to UNKNOWN (not TRUE), so the $\texttt{HAVING}$ condition fails and $A=3$ is excluded. Result: $\{1, 2\}$ .

Q83

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