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SQL

SQL — Zero to Advanced

From your first SELECT to window functions and query optimisation — 62 modules, no prerequisites

Self-paced April 2026
🎓Complete beginners — zero SQL knowledge needed
📊Analysts who want to go beyond basic queries
💼Anyone preparing for SQL interview questions
🔄Developers switching into data roles
62
Modules
13
Sections
268+
Topics covered
15h
Total content
100%
Free forever
What makes this different
▶
Live SQL Playground
Run real queries in your browser — no install, no account. Uses DuckDB-WASM with the FreshMart database preloaded.
Open →
◎
Try It Challenges
Every module ends with a practice question. Reveal the answer and explanation when you're ready.
Open →
⊞
Visual JOIN Diagrams
Row-matching visualizations that make joins click — switch between INNER, LEFT, RIGHT, and FULL OUTER live.
Open →
≡
SQL Cheat Sheet
All 62 modules' syntax on one printable page. Bookmark it for interviews.
Open →
Your learning dataset

The FreshMart Database

Every module, every query, every example uses FreshMart — a fictional Indian grocery chain with 10 stores across Bangalore, Hyderabad, Mumbai, Delhi, Chennai, and Ahmedabad.

customers20 rows
customer_idINTEGER
first_nameVARCHAR
last_nameVARCHAR
emailVARCHAR
phoneVARCHAR
+5 more columns
orders30 rows
order_idINTEGER
customer_idINTEGER
store_idVARCHAR
order_dateDATE
delivery_dateDATE
+3 more columns
order_items118 rows
item_idINTEGER
order_idINTEGER
product_idINTEGER
quantityINTEGER
unit_priceDECIMAL
+2 more columns
products25 rows
product_idINTEGER
product_nameVARCHAR
categoryVARCHAR
sub_categoryVARCHAR
brandVARCHAR
+4 more columns
stores10 rows
store_idVARCHAR
store_nameVARCHAR
cityVARCHAR
stateVARCHAR
manager_nameVARCHAR
+2 more columns
employees15 rows
employee_idINTEGER
first_nameVARCHAR
last_nameVARCHAR
roleVARCHAR
departmentVARCHAR
+4 more columns
// Curriculum

62 Modules. Zero to Advanced.

Follow in order. Each module builds on the last. Module 01 assumes you know nothing — Module 62 ends with real projects and 50 interview questions.

1
Section 1 — Databases & Setup
MODULE 01✓ LIVE

What is a Database?

Why Excel fails at scale — and what databases do instead

Excel vs databaseWhy databases existReal-world scaleFreshMart intro
8 min
read time
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MODULE 02✓ LIVE

How Databases Work

Tables, rows, columns, keys — the building blocks explained simply

Tables & rowsColumnsPrimary keysForeign keys
10 min
read time
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MODULE 03✓ LIVE

Types of Databases

Relational vs NoSQL — which one to use and when

RelationalNoSQLDocument storesWhen to use each
8 min
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MODULE 04✓ LIVE

Setting Up Your Environment

Install MySQL or PostgreSQL, or use the browser playground

MySQL setupPostgreSQLBrowser playgroundFirst connection
12 min
read time
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2
Section 2 — Reading Data — SELECT
MODULE 05✓ LIVE

Your First Query — SELECT & FROM

The two most important words in SQL

SELECTFROMReading columnsBasic syntax
10 min
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MODULE 06✓ LIVE

Filtering Rows — WHERE Clause

Get only the rows you actually need

WHEREEquality filterComparison operatorsFiltering rows
12 min
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MODULE 07✓ LIVE

Multiple Conditions — AND, OR, NOT

Combine filters to answer complex questions

ANDORNOTCombining conditions
10 min
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MODULE 08✓ LIVE

Sorting Results — ORDER BY

Control the order your rows come back in

ORDER BYASCDESCMulti-column sort
8 min
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MODULE 09✓ LIVE

Limiting Results — LIMIT / TOP / FETCH

Only get the rows you need — critical for performance

LIMITTOPFETCH NEXTPagination basics
8 min
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MODULE 10✓ LIVE

Removing Duplicates — DISTINCT

See only unique values in your results

DISTINCTUnique valuesDeduplicationCOUNT DISTINCT
7 min
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MODULE 11✓ LIVE

Working with NULL Values

NULL is not zero, not empty — it is the absence of data

NULLIS NULLIS NOT NULLCOALESCENULLIF
12 min
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MODULE 12✓ LIVE

Column Calculations — Arithmetic

Do math directly inside your SQL query

ArithmeticExpressionsPrice calculationsDerived columns
8 min
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MODULE 13✓ LIVE

Renaming Columns — AS (Aliases)

Give columns and tables readable names

ASColumn aliasTable aliasReadability
7 min
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MODULE 14✓ LIVE

Pattern Matching — LIKE & Wildcards

Find rows that match a pattern, not an exact value

LIKE% wildcard_ wildcardPattern search
10 min
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3
Section 3 — Filtering & Logic
MODULE 15✓ LIVE

IN and BETWEEN Operators

Shorthand for multiple OR conditions and range checks

INNOT INBETWEENRange filters
8 min
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MODULE 16✓ LIVE

CASE WHEN — Conditional Logic

The SQL equivalent of an if-else statement

CASE WHENTHENELSEConditional columns
12 min
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MODULE 17✓ LIVE

Complex WHERE — Combining Conditions

Mastering brackets, precedence, and complex filters

BracketsPrecedenceAND vs ORComplex logic
10 min
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4
Section 4 — Writing & Changing Data
MODULE 18✓ LIVE

Data Types in SQL

INT, VARCHAR, DATE, BOOLEAN, DECIMAL — choosing the right type

INTVARCHARDATEBOOLEANDECIMALChoosing types
12 min
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MODULE 19✓ LIVE

Creating Tables — CREATE TABLE

Design and build your first database table

CREATE TABLEColumn definitionsData typesTable structure
14 min
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MODULE 20✓ LIVE

Adding Data — INSERT INTO

Put data into your tables, one row or many at once

INSERT INTOVALUESBulk insertMulti-row insert
10 min
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MODULE 21✓ LIVE

Updating Data — UPDATE

Change existing rows safely — the WHERE matters more than ever

UPDATESETWHERE on UPDATESafe updates
10 min
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MODULE 22✓ LIVE

Deleting Data — DELETE

Remove rows — and why you should always double-check first

DELETEWHERE on DELETESoft deleteDangerous without WHERE
10 min
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5
Section 5 — Database Design
MODULE 23✓ LIVE

Constraints — Rules for Your Data

PRIMARY KEY, FOREIGN KEY, NOT NULL, UNIQUE, DEFAULT, CHECK

PRIMARY KEYFOREIGN KEYNOT NULLUNIQUEDEFAULTCHECK
16 min
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MODULE 24✓ LIVE

Changing Structure — ALTER TABLE

Add, remove, or rename columns in an existing table

ALTER TABLEADD COLUMNDROP COLUMNRENAME
10 min
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MODULE 25✓ LIVE

DROP & TRUNCATE — Removing Tables

The difference between deleting data and deleting structure

DROP TABLETRUNCATEDELETE vs TRUNCATEIrreversible ops
8 min
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MODULE 26✓ LIVE

Normalization & ER Diagrams

1NF, 2NF, 3NF — designing tables that make sense

1NF2NF3NFER diagramsGood table design
20 min
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6
Section 6 — Aggregation
MODULE 27✓ LIVE

Aggregate Functions

COUNT, SUM, AVG, MIN, MAX — summarize your data

COUNTSUMAVGMINMAXSummarizing data
12 min
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MODULE 28✓ LIVE

Grouping Data — GROUP BY

The most powerful tool for data analysis in SQL

GROUP BYGrouping rowsAggregate per groupAnalysis patterns
14 min
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MODULE 29✓ LIVE

Filtering Groups — HAVING

Filter after grouping — not the same as WHERE

HAVINGFilter groupsHAVING vs WHEREPost-aggregation filter
10 min
read time
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7
Section 7 — Joins
MODULE 30✓ LIVE

Introduction to Joins

Why data lives in multiple tables and how to combine it

Why joins existRelational modelJoin conceptON clause
12 min
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MODULE 31✓ LIVE

INNER JOIN — Only Matching Rows

The most common join — rows that exist in both tables

INNER JOINMatching rowsON conditionMost used join
14 min
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MODULE 32✓ LIVE

LEFT JOIN & RIGHT JOIN

Keep all rows from one side, even without a match

LEFT JOINRIGHT JOINNULL in joinsUnmatched rows
14 min
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MODULE 33✓ LIVE

FULL OUTER JOIN

All rows from both tables — matched or not

FULL OUTER JOINAll rowsBoth sidesNULL filling
10 min
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MODULE 34✓ LIVE

SELF JOIN — A Table Joining Itself

The trick behind org charts and manager hierarchies

SELF JOINHierarchyManager-employeeRecursive relation
12 min
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MODULE 35✓ LIVE

CROSS JOIN — Every Combination

Every row paired with every other row — use with care

CROSS JOINCartesian productAll combinationsUse cases
8 min
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8
Section 8 — Subqueries & Set Operations
MODULE 36✓ LIVE

Subqueries — Queries Inside Queries

Use the result of one query inside another

SubqueryNested querySubquery in WHERESubquery in SELECT
14 min
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MODULE 37✓ LIVE

Correlated Subqueries

Subqueries that reference the outer query — row by row

CorrelatedOuter query referenceRow-by-rowPerformance impact
14 min
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MODULE 38✓ LIVE

EXISTS & NOT EXISTS

Check if any matching rows exist — faster than IN for large sets

EXISTSNOT EXISTSExistence checkvs IN operator
12 min
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MODULE 39✓ LIVE

UNION, INTERSECT, EXCEPT

Combine or compare results from multiple queries

UNIONUNION ALLINTERSECTEXCEPTSet operations
12 min
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MODULE 40✓ LIVE

Derived Tables — Subquery in FROM

Treat a query result as a temporary table

Derived tableSubquery in FROMInline viewTemporary result
10 min
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9
Section 9 — SQL Functions
MODULE 41✓ LIVE

String Functions

UPPER, LOWER, CONCAT, LENGTH, TRIM, REPLACE, SUBSTRING

UPPERLOWERCONCATTRIMSUBSTRINGREPLACE
14 min
read time
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MODULE 42✓ LIVE

Date & Time Functions

NOW, DATEDIFF, DATE_FORMAT — work with dates like a pro

NOWDATEDIFFDATE_FORMATDATEPARTDate arithmetic
14 min
read time
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MODULE 43✓ LIVE

Math Functions

ROUND, FLOOR, CEIL, ABS, MOD — math inside SQL

ROUNDFLOORCEILINGABSMODMath in queries
10 min
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MODULE 44✓ LIVE

Type Conversion — CAST & CONVERT

Change a column's data type on the fly

CASTCONVERTType conversionImplicit vs explicit
10 min
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10
Section 10 — Advanced SQL Objects
MODULE 45✓ LIVE

Views — Virtual Tables

Save a query as a named view — query it like a table

CREATE VIEWVirtual tableUpdatable viewsView use cases
12 min
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MODULE 46✓ LIVE

Indexes — Making Queries Fast

The single most impactful performance tool in SQL

CREATE INDEXB-tree indexComposite indexWhen to index
14 min
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MODULE 47✓ LIVE

Transactions — COMMIT & ROLLBACK

Group queries into all-or-nothing operations

BEGINCOMMITROLLBACKSAVEPOINTAtomicity
12 min
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MODULE 48✓ LIVE

ACID Properties

The 4 rules every reliable database follows

AtomicityConsistencyIsolationDurabilityWhy ACID matters
14 min
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MODULE 49✓ LIVE

Stored Procedures

Save and reuse complex SQL logic on the server

CREATE PROCEDUREParametersCALLReusable logic
14 min
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MODULE 50✓ LIVE

User-Defined Functions

Build your own SQL functions for repeated logic

CREATE FUNCTIONScalar UDFTable-valued UDFRETURNS
12 min
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MODULE 51✓ LIVE

Triggers — Auto-Run on Events

SQL that runs automatically when data changes

CREATE TRIGGERBEFORE / AFTERINSERT / UPDATE / DELETEAudit logs
14 min
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11
Section 11 — Window Functions & CTEs
MODULE 52✓ LIVE

Window Functions — OVER & PARTITION BY

Aggregate without collapsing rows — the most powerful SQL feature

OVERPARTITION BYWindow frameNo GROUP BY collapse
16 min
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MODULE 53✓ LIVE

Ranking — ROW_NUMBER, RANK, DENSE_RANK

Rank rows within groups — critical for interview questions

ROW_NUMBERRANKDENSE_RANKNTILERanking patterns
14 min
read time
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MODULE 54✓ LIVE

LAG, LEAD, FIRST_VALUE, LAST_VALUE

Access other rows' values from the current row

LAGLEADFIRST_VALUELAST_VALUERow comparison
14 min
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MODULE 55✓ LIVE

CTEs — WITH Clause

Write readable, layered queries with Common Table Expressions

WITHCTENamed subqueryMultiple CTEsReadability
14 min
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MODULE 56✓ LIVE

Recursive CTEs — Hierarchies & Trees

Query parent-child relationships with recursive queries

RECURSIVEHierarchyTree traversalUNION ALL in CTE
16 min
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12
Section 12 — Performance & Optimization
MODULE 57✓ LIVE

EXPLAIN & EXPLAIN ANALYZE

Read the query execution plan — know why a query is slow

EXPLAINQuery planSeq scan vs index scanExecution cost
14 min
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MODULE 58✓ LIVE

Index Strategies

Composite indexes, covering indexes, when NOT to index

Composite indexCovering indexIndex selectivityWhen to avoid
14 min
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MODULE 59✓ LIVE

Query Best Practices

Write clean, fast, readable SQL your team will thank you for

SELECT * avoidSARGabilityAvoid functions on indexed colsClean SQL
14 min
read time
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13
Section 13 — Real-World & Interview
MODULE 60✓ LIVE

SQL for Data Analysis

Real case studies using Swiggy, Zepto, HDFC, and FreshMart data

Swiggy case studyZepto dataHDFC analysisFreshMart full query
30 min
read time
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MODULE 61✓ LIVE

Top 50 SQL Interview Questions

Every question with full explanation, query, and common traps

Window fn questionsJoin questionsCTE questionsTricky NULLs50 answers
60 min
read time
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MODULE 62✓ LIVE

3 Real SQL Projects

Build a full database, write analysis queries, present results

FreshMart projectE-commerce DBAnalytics dashboardEnd to end
90 min
read time
Start →
// Ready to start?

Start with Module 01. No setup required.

Every module runs entirely in your browser. Write real SQL against the FreshMart database from your very first lesson.

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