SQL Queries— Concepts, Formulas & Shortcuts
- Logical execution order: FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY → LIMIT.
- WHERE filters individual rows before grouping; HAVING filters groups after aggregation.
- INNER JOIN keeps only matching rows; LEFT JOIN keeps all left rows with NULLs for missing right matches.
- DELETE is DML, row-by-row, can be rolled back and fires triggers. TRUNCATE is DDL, deallocates pages and cannot be rolled back in most engines.
- NULL is unknown: NULL = NULL is not true — use IS NULL. COUNT(column) skips NULLs but COUNT(*) does not.
- UNION removes duplicates and sorts; UNION ALL keeps everything and is faster.
SQL Queries Practice Questions with Answers
Attempt each question first, then open the explanation. All 8 questions below are free to read and require no signup.
Q1.Which SQL clause filters rows AFTER aggregation?
Easy- AWHERE
- BHAVING
- CGROUP BY
- DORDER BY
SQL Queries question 1 of 8+Show Answer & Explanation
Answer: B. HAVING
Explanation: HAVING applies to grouped results; WHERE is evaluated before grouping and cannot use aggregate functions.
Q2.Which JOIN returns all rows from the left table and matching rows from the right?
Easy- AINNER JOIN
- BLEFT JOIN
- CRIGHT JOIN
- DCROSS JOIN
SQL Queries question 2 of 8+Show Answer & Explanation
Answer: B. LEFT JOIN
Explanation: A LEFT (OUTER) JOIN preserves every left-table row, filling unmatched right-table columns with NULL.
Q3.Which command removes all rows from a table but cannot normally be rolled back?
Moderate- ADELETE
- BTRUNCATE
- CDROP
- DREMOVE
SQL Queries question 3 of 8+Show Answer & Explanation
Answer: B. TRUNCATE
Explanation: TRUNCATE is a DDL operation that deallocates data pages; it is far faster than DELETE but is not row-by-row logged.
Q4.What does SELECT COUNT(*) FROM table return when the table is empty?
Moderate- ANULL
- B0
- CAn error
- DNo rows
SQL Queries question 4 of 8+Show Answer & Explanation
Answer: B. 0
Explanation: COUNT(*) is an aggregate over the whole table and returns a single row containing 0.
Q5.Which operator tests for a value within a list?
Easy- ABETWEEN
- BIN
- CLIKE
- DEXISTS
SQL Queries question 5 of 8+Show Answer & Explanation
Answer: B. IN
Explanation: IN checks membership of a value list, e.g. WHERE city IN ('Pune', 'Mumbai').
Q6.To compare a column against NULL, you must use:
Moderate- A= NULL
- BIS NULL
- C== NULL
- DLIKE NULL
SQL Queries question 6 of 8+Show Answer & Explanation
Answer: B. IS NULL
Explanation: NULL means unknown, so equality comparisons return unknown rather than true. IS NULL is the only correct test.
Q7.Which set operator removes duplicate rows from the combined result?
Moderate- AUNION ALL
- BUNION
- CINTERSECT ALL
- DJOIN
SQL Queries question 7 of 8+Show Answer & Explanation
Answer: B. UNION
Explanation: UNION de-duplicates (and therefore sorts); UNION ALL simply concatenates and is faster when duplicates are acceptable.
Q8.A PRIMARY KEY constraint enforces that a column is:
Easy- AUnique only
- BNot null only
- CBoth unique and not null
- DIndexed but nullable
SQL Queries question 8 of 8+Show Answer & Explanation
Answer: C. Both unique and not null
Explanation: A primary key must uniquely identify each row, so it is both unique and non-nullable, and it creates an index automatically.
SQL Queries — Frequently Asked Questions
What is the difference between WHERE and HAVING?+
WHERE filters rows before grouping and cannot reference aggregate functions. HAVING filters after GROUP BY and is where you put conditions like COUNT(*) > 5.
What is the difference between DELETE, TRUNCATE and DROP?+
DELETE removes selected rows and can be rolled back; TRUNCATE removes all rows quickly by deallocating pages and resets identity; DROP removes the entire table structure along with its data.
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