easyNULL and IS NULL
Why does this query return no rows even though the column contains NULLs?
SELECT * FROM t WHERE val = NULL;SQL · PRACTICE
SQL interview questions.
SQL is the query language every relational database speaks. Interviews test whether you can reason about set semantics, not just write SELECTs.
587questions
105easy
236medium
246hard
Join semantics with NULLs, GROUP BY versus window functions, index selectivity, isolation levels, and reading a query plan.
Where it shows up: Backend, data engineering, analytics, and any role that touches a production database.
Sample SQL questions
Twenty real SQL questions from the library, code snippet included. Nothing here is paraphrased for search engines. It is the same text a signed-in user sees.
mediumEXISTS vs IN
Why might IN with a subquery and EXISTS with the same correlation produce identical results yet the optimizer treats them differently?
-- A:
SELECT * FROM customers c
WHERE c.id IN (SELECT o.customer_id FROM orders o);
-- B:
SELECT * FROM customers c
WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.id);easyaggregate functions
Given this table, what does the query return?
-- scores: 10, NULL, 30
SELECT AVG(score) FROM scores;easyaggregate functions
Given the salary values 1000, 2000, NULL, what do COUNT(salary) and COUNT(*) return for this table of 3 rows?
SELECT COUNT(salary), COUNT(*) FROM staff;mediumcorrelated subqueries
You want each employee whose salary is the maximum within their own department. Which correlated form is correct?
-- Goal: keep only rows that are the dept max
-- Option A
SELECT e.* FROM emp e
WHERE e.salary = (SELECT MAX(x.salary) FROM emp x WHERE x.dept_id = e.dept_id);
-- Option B
SELECT e.* FROM emp e
WHERE e.salary >= ALL (SELECT x.salary FROM emp x);
-- Option C
SELECT e.* FROM emp e
WHERE e.salary = (SELECT MAX(x.salary) FROM emp x);
-- Option D
SELECT e.* FROM emp e
WHERE EXISTS (SELECT 1 FROM emp x WHERE x.salary > e.salary);mediumwindow functions ROW_NUMBER RANK
You need the top earner per department, but when there is a tie you want EVERY tied top earner returned, not just one. Which window function in the filter achieves this?
SELECT * FROM (
SELECT e.*, <FN>() OVER (PARTITION BY dept ORDER BY salary DESC) AS rnk
FROM emp e
) s WHERE rnk = 1;easyLIKE pattern matching
Given names 'Anna', 'anna', and 'ANNA' in PostgreSQL, how many match this query?
SELECT name FROM users WHERE name LIKE 'a%';easyIN and BETWEEN
What happens when the bounds of BETWEEN are reversed?
SELECT * FROM t WHERE val BETWEEN 20 AND 10;easyINSERT UPDATE DELETE
After this UPDATE runs, what is the value of `price` for a product that had `price = 100`?
UPDATE products
SET price = price * 1.1
WHERE category = 'books';easyINNER JOIN
What happens when you INNER JOIN two tables on a column that is NULL in some rows of both tables?
SELECT a.id
FROM a
INNER JOIN b ON a.code = b.code;
-- both a.code and b.code are NULL in some rowseasyCREATE TABLE
What does the SERIAL type do in this PostgreSQL table definition?
CREATE TABLE t (
id SERIAL PRIMARY KEY,
name TEXT
);easyLEFT and RIGHT JOIN
A LEFT JOIN has a condition in the WHERE clause referencing the right table. What is the effect?
SELECT c.name, o.total
FROM customers c
LEFT JOIN orders o ON c.id = o.customer_id
WHERE o.total > 100;easyaliases AS
In PostgreSQL, what does this query order results by?
SELECT name, price * 2 AS doubled
FROM products
ORDER BY doubled DESC;easysimple subqueries
What does this query return when used as a value-producing subquery?
SELECT name
FROM products
WHERE price > (SELECT AVG(price) FROM products);easyCREATE TABLE
What does adding double quotes around a table name do in this PostgreSQL statement?
CREATE TABLE "Orders" (id int);easyaliases AS
What is the result of this query when run on PostgreSQL?
SELECT 'A' AS grade, count(*)
FROM students;easySELECT and WHERE
Given the rows below, how many rows does the query return?
-- price column values: 10, 20, NULL, 30
SELECT * FROM items WHERE price BETWEEN 10 AND 30;easySELECT and WHERE
How many rows does this query return if the products table has 10 rows but only 3 have a non-NULL category?
SELECT *
FROM products
WHERE category IN ('books', 'toys', NULL);easySELECT and WHERE
A table `users` has rows where some `country` values are NULL. What does this query return?
SELECT name FROM users WHERE country != 'US';easyGROUP BY and HAVING
What is wrong with this query?
SELECT department, name, COUNT(*)
FROM employees
GROUP BY department;Sign in to answer
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SQL topics covered
50 distinct SQL topics are tagged across the library. These are the real topic labels stored on the questions, not a hand-written list.
- window functions ROW_NUMBER RANK (30)
- nested-loop join (24)
- isolation levels and anomalies (24)
- composite and partial indexes (23)
- table bloat and VACUUM (22)
- the query planner optimizer (22)
- UNION vs UNION ALL (17)
- correlated subqueries (16)
- JSONB querying (16)
- aggregate functions (15)
- CTEs WITH (15)
- self-joins (15)
- pivot conditional aggregation (15)
- hash join (15)
- transactions BEGIN COMMIT (14)
- reading EXPLAIN ANALYZE (14)
- statistics and cardinality estimation (14)
- recursive CTEs (14)
- EXISTS vs IN (13)
- covering indexes (13)
- MVCC and row versions (13)
- merge join (13)
- row vs table locking (13)
- INNER JOIN (12)
- B-tree index internals (12)
- index-only scans (12)
- upserts ON CONFLICT MERGE (12)
- SELECT and WHERE (11)
- GROUP BY and HAVING (11)
- views and materialized views (11)
- query plan cache prepared plans (11)
- LEFT and RIGHT JOIN (10)
- ORDER BY and LIMIT (10)
- stored procedures functions (10)
- INSERT UPDATE DELETE (9)
- triggers (9)
- the write-ahead log WAL (9)
- dirty phantom non-repeatable reads (9)
- DISTINCT (6)
- IN and BETWEEN (6)
- generated columns (6)
- aliases AS (3)
- CREATE TABLE (3)
- the write-ahead log (WAL) (3)
- dirty / phantom / non-repeatable reads (3)
- NULL and IS NULL (2)
- LIKE pattern matching (2)
- primary and foreign keys (2)
- CASE WHEN (2)
- simple subqueries (1)
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