Strong SQL interview answers explain both what a query returns and why. These questions cover SELECT structure, filtering, grouping, joins, set operations, subqueries, common table expressions, ordering, and practical query problems. Examples use broadly shared SQL syntax where possible; check row-limiting and other dialect-specific syntax against your target database.
What SQL questions are asked in interviews?
Interviewers commonly test whether you can read query structure, reason about rows and groups, combine data correctly, and make results predictable. Be ready to describe your approach before writing a query, state assumptions about ties or duplicates, and name the database dialect when syntax may vary.
What is the general shape of a SELECT query?
A SELECT statement returns expressions from rows supplied by its table expressions. A typical query is written in this order:
SELECT department_id, COUNT(*) AS employee_count
FROM employees
WHERE salary > 0
GROUP BY department_id
HAVING COUNT(*) > 1
ORDER BY employee_count DESC;FROMidentifies the input tables or other table expressions.WHEREfilters input rows.GROUP BYforms groups for aggregate calculations.HAVINGfilters those groups.SELECTspecifies the output expressions.ORDER BYrequests a sort; a row-limiting clause can restrict the returned rows.
PostgreSQL 17 describes a logical processing sequence that helps explain these roles: inputs are established, rows filtered, groups formed and filtered, output expressions computed, and results ordered and limited. This is an explanatory model, not a promise about the database’s physical execution plan. See the PostgreSQL 17 SELECT documentation.
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What does GROUP BY do?
GROUP BY divides input rows into groups sharing the same value or values for the grouping expressions. Aggregate functions such as COUNT, SUM, and AVG can then produce a result per group. For example, this returns a total per customer:
SELECT customer_id, SUM(amount) AS total_spend
FROM orders
GROUP BY customer_id;Selected nonaggregate expressions must comply with the grouping rules of the database. In a typical grouped query, an output expression is either aggregated or represented by the grouping key.
What is the difference between WHERE and HAVING?
WHERE filters individual rows before grouping; HAVING filters groups after aggregate values are available. Put an aggregate condition such as “keep customers whose total exceeds 1,000” in HAVING:
SELECT customer_id, SUM(amount) AS total_spend
FROM orders
WHERE order_date >= DATE '2026-01-01'
GROUP BY customer_id
HAVING SUM(amount) > 1000;Here, WHERE first limits the input orders by date, and HAVING then keeps only customer groups whose summed amount passes the threshold. Microsoft’s SELECT examples show WHERE, GROUP BY, and HAVING used together.
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How do INNER JOIN and LEFT JOIN differ?
An INNER JOIN returns row combinations that satisfy its join condition. A LEFT JOIN preserves every row from its left input and supplies nulls for right-side columns when there is no match. Use a join when the result needs columns from related table inputs.
SELECT c.customer_id, o.order_id
FROM customers AS c
LEFT JOIN orders AS o
ON o.customer_id = c.customer_id;This keeps customers even if they have no order. Predicate placement matters: a condition on the right table in ON can restrict which rows match while preserving unmatched left rows; moving that condition to WHERE can filter out null-extended rows and change the result. Explain which behavior the question requires. Joins are part of the table-expression portion of a query; consult the target engine’s documentation for its detailed semantics, including the PostgreSQL SELECT reference or Microsoft SELECT documentation.
What is the difference between UNION and UNION ALL?
Both set operators combine the results of compatible queries; their duplicate handling differs. The inputs need corresponding columns that can be combined by the database.
| Operator | Result behavior | Use it when |
|---|---|---|
UNION |
Combines rows and removes duplicate result rows. | You want a distinct combined result. |
UNION ALL |
Combines rows while retaining duplicates. | Repeated rows are meaningful or should remain in the output. |
SELECT email FROM customers
UNION ALL
SELECT email FROM prospects;Use UNION instead if duplicate result rows should be removed. PostgreSQL documents set operators in its SELECT reference; Microsoft illustrates the behavior in its SELECT examples. Set operators act on result sets, unlike joins, which combine related table inputs within a query.
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What is a common table expression (CTE)?
A common table expression is a named query introduced with WITH, available to the statement that follows it. It can make a multi-stage query easier to read without changing its intended result:
WITH customer_totals AS (
SELECT customer_id, SUM(amount) AS total_spend
FROM orders
GROUP BY customer_id
)
SELECT customer_id, total_spend
FROM customer_totals
WHERE total_spend > 1000;A CTE is a way to organize a query, not a blanket performance optimization. PostgreSQL 17 documents cases where a multiply referenced WITH query is computed once unless NOT MATERIALIZED is specified; do not assume the same planning behavior across engines. See the PostgreSQL 17 SELECT documentation.
Why should you use ORDER BY?
Without ORDER BY, SQL does not promise a stable row order. If an interviewer asks for the latest orders or a top-N result, state the sort rule explicitly. Include a tie-breaker when the primary sort key can match across rows:
SELECT order_id, customer_id, order_date
FROM orders
ORDER BY order_date DESC, order_id DESC;The second key makes ordering more specific when dates are equal. PostgreSQL states that absent ORDER BY, rows can be returned in whatever order the system finds fastest to produce; an order observed in one run is not a guarantee. See its SELECT documentation.
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How do you find the highest-paid employee in each department?
First decide what should happen when employees tie for the highest salary. To return exactly one employee per department, use ROW_NUMBER() with a stable employee identifier as a tie-breaker, then keep row 1. The following illustrates the pattern; confirm window-function and row-limiting syntax for the target database.
WITH ranked_employees AS (
SELECT employee_id, department_id, salary,
ROW_NUMBER() OVER (
PARTITION BY department_id
ORDER BY salary DESC, employee_id
) AS rn
FROM employees
)
SELECT employee_id, department_id, salary
FROM ranked_employees
WHERE rn = 1;Because the sort includes employee_id, this chooses one row deterministically among equal salaries, rather than returning every employee tied for first. If the requirement is to return all top-paid ties, use a ranking approach that preserves ties instead, and verify its syntax for the chosen dialect.
How do you find duplicate values?
Define what “duplicate” means before writing the query. To find email addresses used more than once:
SELECT email, COUNT(*) AS occurrences
FROM users
GROUP BY email
HAVING COUNT(*) > 1;Grouping by email finds repeated email values; grouping by every column instead tests for repeated full rows. The HAVING condition filters groups by their count. Microsoft’s SELECT examples include aggregate filtering with HAVING.
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What is the difference between a join and a subquery?
A join relates table inputs in the query’s table-expression portion. A subquery is a query nested inside another query, where it may provide a scalar value, a set of rows, or an existence test. Some tasks can be expressed either way; choose the form that makes the intended relationship and result clearest, then consider the target engine.
For example, a subquery can test whether a customer has any orders:
SELECT c.customer_id
FROM customers AS c
WHERE EXISTS (
SELECT 1
FROM orders AS o
WHERE o.customer_id = c.customer_id
);Microsoft’s SELECT examples demonstrate joins, subqueries, and correlated subqueries.
How do I prepare for a SQL interview?
Practise explaining the result and the reasoning, not just memorizing query shapes. Use small sample tables to check edge cases such as missing matches, duplicate values, tied scores, and groups with only one row.
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- Choose the query structure. Decide whether the task needs related table inputs, a nested query, a CTE, grouping, or a set operator.
- Place conditions at the right stage. Filter input rows with
WHERE; filter aggregate groups withHAVING. - Make ordering and ties explicit. Add
ORDER BYfor ordered output and a tie-breaker when a single deterministic row is required. - Name the SQL dialect. PostgreSQL and SQL Server share many SELECT fundamentals but differ in details. PostgreSQL documents
LIMIT/FETCHforms, while SQL Server documentsTOP; verify syntax and behavior for the database and version you will use. See the PostgreSQL 17 reference and SQL Server SELECT reference.
For practice, solve a customer-total problem with a threshold, find duplicate emails, combine two compatible result sets both with and without deduplication, and return the most recent order per customer with an explicit tie-breaker. For each, say what happens at the boundaries before you present the query.
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