Top SQL Interview Questions & Answers

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115 pages · 289 questions

What Is SQL and What Is It Used For?
3 questions 1 easy 1 med 1 hardFree
SQL vs MySQL: What Is the Difference?
2 questions 1 easy 1 medFree
What Are DDL, DML, DCL and TCL?
3 questions 1 easy 1 med 1 hardFree
How Do SQL Command Types Fit Together?
2 questions 1 easy 1 medFree
Primary Key vs Unique Key
3 questions 1 easy 1 med 1 hardFree
What Is an RDBMS?
2 questions 1 easy 1 medFree
How Do SQL Dialects Differ?
3 questions 1 easy 1 med 1 hardFree
DELETE vs TRUNCATE vs DROP
2 questions 1 easy 1 medFree
WHERE vs HAVING in SQL
3 questions 1 easy 1 med 1 hardFree
How Does NULL Behave in SQL?
2 questions 1 easy 1 medFree
How Does the SELECT Statement Work?
3 questions 1 easy 1 med 1 hardFree
How Does the WHERE Clause Filter Rows?
2 questions 1 easy 1 medFree
Combining Conditions with AND, OR, NOT
3 questions 1 easy 1 med 1 hardFree
IN, BETWEEN and LIKE in SQL
2 questions 1 easy 1 medFree
What Does DISTINCT Deduplicate?
3 questions 1 easy 1 med 1 hardFree
Column and Table Aliases in SQL
2 questions 1 easy 1 medFree
Why Does NULL = NULL Fail?
3 questions 1 easy 1 med 1 hardFree
EXISTS vs IN in SQL
2 questions 1 easy 1 medFree
How Does the CASE Expression Work?
3 questions 1 easy 1 med 1 hardFree
COALESCE and NULLIF in Practice
2 questions 1 easy 1 medFree
How Does ORDER BY Sort Results?
3 questions 1 easy 1 med 1 hardFree
LIMIT and OFFSET for Paging
2 questions 1 easy 1 medFree
TOP vs LIMIT Across Dialects
3 questions 1 easy 1 med 1 hardFree
What Does GROUP BY Produce?
2 questions 1 easy 1 medFree
How Does HAVING Filter Groups?
3 questions 1 easy 1 med 1 hardFree
In What Order Does SQL Evaluate a Query?
2 questions 1 med 1 hardFree
Which String Functions Come Up Most?
3 questions 1 easy 1 med 1 hardFree
Which Date Functions Matter in Interviews?
2 questions 1 easy 1 medFree
ROUND, CEIL and FLOOR in Queries
3 questions 1 easy 1 med 1 hardFree
UNION vs UNION ALL
2 questions 1 med 1 hardFree
COUNT, SUM, AVG, MIN and MAX
3 questions 1 easy 1 med 1 hardFree
Which Columns Must GROUP BY Include?
2 questions 1 easy 1 medFree
HAVING vs WHERE: the Real Difference
3 questions 1 easy 1 med 1 hardFree
COUNT(*) vs COUNT(column)
2 questions 1 med 1 hardFree
Grouping by Multiple Columns
3 questions 1 easy 1 med 1 hardFree
What Do ROLLUP and CUBE Add?
2 questions 1 easy 1 medFree
Practice: Revenue and Traffic Queries
3 questions 1 easy 1 med 1 hardFree
Filtering on Aggregate Results
2 questions 1 med 1 hardFree
How Do You Pivot Data in SQL?
3 questions 1 easy 1 med 1 hardFree
How Do Aggregates Treat NULL and Empty Sets?
2 questions 1 easy 1 medFree
How Does INNER JOIN Match Rows?
3 questions 1 easy 1 med 1 hardFree
How Does LEFT JOIN Keep Unmatched Rows?
2 questions 1 med 1 hardFree
RIGHT JOIN vs FULL OUTER JOIN
3 questions 1 easy 1 med 1 hardFree
What Is a Self Join and When Is It Useful?
2 questions 1 easy 1 medFree
What Does CROSS JOIN Produce?
3 questions 1 easy 1 med 1 hardFree
Joining Three or More Tables
2 questions 1 med 1 hardFree
JOIN vs Subquery: Which and When?
3 questions 1 easy 1 med 1 hardFree
How Do You Find Non-Matching Rows?
2 questions 1 easy 1 medFree
What Makes Joins Slow?
3 questions 1 easy 1 med 1 hardFree
NATURAL JOIN and USING Clauses
2 questions 1 med 1 hardFree
How Do Subqueries Work in SQL?
3 questions 1 easy 1 med 1 hardFree
What Is a Correlated Subquery?
2 questions 1 easy 1 medFree
Scalar vs Table Subqueries
3 questions 1 easy 1 med 1 hardFree
What Is a Common Table Expression?
2 questions 1 med 1 hardFree
How Do Recursive CTEs Walk Hierarchies?
3 questions 1 easy 1 med 1 hardFree
CTE vs Subquery: Readability and Plans
2 questions 1 easy 1 medFree
EXISTS vs IN: Which Is Faster?
3 questions 1 easy 1 med 1 hardFree
What Are Derived Tables?
2 questions 1 med 1 hardFree
Subqueries Inside SELECT Lists
3 questions 1 easy 1 med 1 hardFree
Practice: Layered Query Problems
2 questions 1 easy 1 medFree
How Does INSERT Work in SQL?
3 questions 1 easy 1 med 1 hardFree
Inserting Multiple Rows Efficiently
2 questions 1 med 1 hardFree
How Does UPDATE Change Rows Safely?
3 questions 1 easy 1 med 1 hardFree
How Does DELETE Remove Rows?
2 questions 1 easy 1 medFree
How Do Upserts Work (MERGE and Friends)?
3 questions 1 easy 1 med 1 hardFree
What Is a Transaction in SQL?
2 questions 1 med 1 hardFree
COMMIT and ROLLBACK in Practice
3 questions 1 easy 1 med 1 hardFree
Updating Across Tables
2 questions 1 easy 1 medFree
How Do You Avoid Accidental Mass Updates?
3 questions 1 easy 1 med 1 hardFree
Loading Data in Bulk
2 questions 1 med 1 hardFree
How Do You Design a CREATE TABLE?
3 questions 1 easy 1 med 1 hardFree
Choosing Data Types Wisely
2 questions 1 easy 1 medFree
What Does a Primary Key Guarantee?
3 questions 1 easy 1 med 1 hardFree
How Do Foreign Keys Protect Relationships?
2 questions 1 med 1 hardFree
When Do You Need a Composite Key?
3 questions 1 easy 1 med 1 hardFree
AUTO_INCREMENT vs IDENTITY vs Sequences
2 questions 1 easy 1 medFree
What Is Normalization For?
3 questions 1 easy 1 med 1 hardFree
1NF, 2NF and 3NF with Examples
2 questions 1 med 1 hardFree
When Does Denormalization Win?
3 questions 1 easy 1 med 1 hardFree
From ER Diagram to Schema
2 questions 1 easy 1 medFree
NOT NULL and DEFAULT Constraints
3 questions 1 easy 1 med 1 hardFree
What Do CHECK Constraints Enforce?
2 questions 1 med 1 hardFree
UNIQUE Constraints in Practice
3 questions 1 easy 1 med 1 hardFree
What Does ON DELETE CASCADE Do?
2 questions 1 easy 1 medFree
How Is Referential Integrity Maintained?
3 questions 1 easy 1 med 1 hardFree
How Does ALTER TABLE Change Schema?
2 questions 1 med 1 hardFree
Renaming Columns and Tables
3 questions 1 easy 1 med 1 hardFree
Which Constraints Create Indexes?
2 questions 1 easy 1 medFree
How Do Schema Migrations Stay Safe?
3 questions 1 easy 1 med 1 hardFree
What Is a View in SQL?
3 questions 1 easy 1 med 1 hardFree
When Can You Update Through a View?
2 questions 1 med 1 hardFree
What Does a Materialized View Buy You?
3 questions 1 easy 1 med 1 hardFree
What Is an Index in SQL?
2 questions 1 easy 1 medFree
Clustered vs Non-Clustered Indexes
3 questions 1 easy 1 med 1 hardFree
Composite Indexes and Column Order
2 questions 1 med 1 hardFree
What Is a Covering Index?
3 questions 1 easy 1 med 1 hardFree
What Do Indexes Cost on Writes?
2 questions 1 easy 1 medFree
How Do You Tune a Slow Query?
3 questions 1 easy 1 med 1 hardFree
What Are Window Functions?
3 questions 1 easy 1 med 1 hardFree
ROW_NUMBER vs RANK vs DENSE_RANK
3 questions 1 easy 1 med 1 hardFree
How Do LEAD and LAG Compare Rows?
2 questions 1 med 1 hardFree
What Does PARTITION BY Change?
3 questions 1 easy 1 med 1 hardFree
Computing Running Totals
2 questions 1 easy 1 medFree
NTILE and PERCENT_RANK in Practice
3 questions 1 easy 1 med 1 hardFree
Window Functions vs GROUP BY
2 questions 1 med 1 hardFree
Reshaping Data with Window Logic
3 questions 1 easy 1 med 1 hardFree
Which Window Mistakes Break Queries?
2 questions 1 easy 1 medFree
How Do You Prepare for a SQL Interview?
3 questions 1 easy 1 med 1 hardFree
Which Queries Get Asked Every Time?
2 questions 1 med 1 hardFree
How Do You Find Duplicates in a Table?
3 questions 1 easy 1 med 1 hardFree
How Do You Find the Second Highest Salary?
2 questions 1 easy 1 medFree
How Do You Find the Nth Highest Value?
3 questions 1 easy 1 med 1 hardFree
Join Drills from Real Interviews
2 questions 1 med 1 hardFree
Case-Study Questions and How to Attack Them
3 questions 1 easy 1 med 1 hardFree
How Do You Discuss SQL Projects?
2 questions 2 medFree

What Is SQL and What Is It Used For?

SQL vs MySQL: What Is the Difference?

What Are DDL, DML, DCL and TCL?

How Do SQL Command Types Fit Together?

Primary Key vs Unique Key

What Is an RDBMS?

How Do SQL Dialects Differ?

DELETE vs TRUNCATE vs DROP

WHERE vs HAVING in SQL

How Does NULL Behave in SQL?

How Does the SELECT Statement Work?

How Does the WHERE Clause Filter Rows?

Combining Conditions with AND, OR, NOT

IN, BETWEEN and LIKE in SQL

What Does DISTINCT Deduplicate?

Column and Table Aliases in SQL

Why Does NULL = NULL Fail?

EXISTS vs IN in SQL

How Does the CASE Expression Work?

COALESCE and NULLIF in Practice

How Does ORDER BY Sort Results?

LIMIT and OFFSET for Paging

TOP vs LIMIT Across Dialects

What Does GROUP BY Produce?

How Does HAVING Filter Groups?

In What Order Does SQL Evaluate a Query?

Which String Functions Come Up Most?

Which Date Functions Matter in Interviews?

ROUND, CEIL and FLOOR in Queries

UNION vs UNION ALL

COUNT, SUM, AVG, MIN and MAX

Which Columns Must GROUP BY Include?

HAVING vs WHERE: the Real Difference

COUNT(*) vs COUNT(column)

Grouping by Multiple Columns

What Do ROLLUP and CUBE Add?

Practice: Revenue and Traffic Queries

Filtering on Aggregate Results

How Do You Pivot Data in SQL?

How Do Aggregates Treat NULL and Empty Sets?

How Does INNER JOIN Match Rows?

How Does LEFT JOIN Keep Unmatched Rows?

RIGHT JOIN vs FULL OUTER JOIN

What Is a Self Join and When Is It Useful?

What Does CROSS JOIN Produce?

Joining Three or More Tables

JOIN vs Subquery: Which and When?

How Do You Find Non-Matching Rows?

What Makes Joins Slow?

NATURAL JOIN and USING Clauses

How Do Subqueries Work in SQL?

What Is a Correlated Subquery?

Scalar vs Table Subqueries

What Is a Common Table Expression?

How Do Recursive CTEs Walk Hierarchies?

CTE vs Subquery: Readability and Plans

EXISTS vs IN: Which Is Faster?

What Are Derived Tables?

Subqueries Inside SELECT Lists

Practice: Layered Query Problems

How Does INSERT Work in SQL?

Inserting Multiple Rows Efficiently

How Does UPDATE Change Rows Safely?

How Does DELETE Remove Rows?

How Do Upserts Work (MERGE and Friends)?

What Is a Transaction in SQL?

COMMIT and ROLLBACK in Practice

Updating Across Tables

How Do You Avoid Accidental Mass Updates?

Loading Data in Bulk

How Do You Design a CREATE TABLE?

Choosing Data Types Wisely

What Does a Primary Key Guarantee?

How Do Foreign Keys Protect Relationships?

When Do You Need a Composite Key?

AUTO_INCREMENT vs IDENTITY vs Sequences

What Is Normalization For?

1NF, 2NF and 3NF with Examples

When Does Denormalization Win?

From ER Diagram to Schema