SQL LENGTH() — সম্পূর্ণ ২ ঘন্টার লাইভ ক্লাস নোট

01

কেন LENGTH()?

১৫ মিনিট

Online shop registration: Name, Email, Username, Address, Phone।

Rules: Username max 20 · Password min 8 · Product/Employee codes fixed length

জিজ্ঞেস

Rahim / Alexander / InternationalBusinessCompany — চোখে আন্দাজ করা যায়, কিন্তু database কীভাবে length বের করবে?

"Rahim" → LENGTH() → 5 bytes

LENGTH() দিয়ে MySQL-এ string কত bytes জায়গা নিচ্ছে জানা যায়।

02

Real-life Use Cases

১০ মিনিট

Username → LENGTH() → too short / OK / too long Product Code → LENGTH() → correct format? Employee Code EMP2026001 → LENGTH() → how many bytes? Customer Name → LENGTH() → name size Raw Data → LENGTH() → unexpectedly long values

03

LENGTH() কী?

১০ মিনিট

LENGTH(string);

LENGTH() → function → takes a value → returns length in BYTES "Hello" → LENGTH() → 5

MySQL LENGTH() = bytes — সবসময় “visible character” নয়।

04

প্রথম Simple Examples

১০ মিনিট

SELECT LENGTH('Hello'); -- 5 SELECT LENGTH('SQL'); -- 3 SELECT LENGTH('Data Science'); -- 12

Data + Space + Science → space-ও byte count-এ পড়ে

05

Manual Counting

১০ মিনিট

Hello

H e l l o → 1+1+1+1+1 = 5 bytes

SQL

S Q L → 3 bytes

Hello World

Hello(5) + space(1) + World(5) = 11

06

Table Columns

১৫ মিনিট

CREATE TABLE employees ( employee_id INT, employee_name VARCHAR(100) ); INSERT INTO employees VALUES (101,'Rahim'), (102,'Karim Hossain'), (103,'Jannat Ahmed'), (104,'Alexander'); SELECT employee_name, LENGTH(employee_name) AS name_length FROM employees;

employee_name	name_length
Rahim	5
Karim Hossain	13
Jannat Ahmed	12
Alexander	9

AS name_length শুধু result column-এর নাম — ফোকাস LENGTH।

07

Space Matters

৫ মিনিট

SELECT LENGTH('RahimKhan'); -- 9 SELECT LENGTH('Rahim Khan'); -- 10

RahimKhan = 9 · Rahim Khan = 10 (space = 1 byte)

Space-ও string-এর অংশ।

08

Empty vs Space

১০ মিনিট

SELECT LENGTH('A'), LENGTH('AB'), LENGTH('ABC'); SELECT LENGTH('12345'), LENGTH('Hello World'); SELECT LENGTH(''); -- 0 SELECT LENGTH(' '); -- 1

'' = empty (0) · ' ' = one space (1) — এক নয়।

09

সবচেয়ে গুরুত্বপূর্ণ: Bytes

১৫ মিনিট

LENGTH() = সবসময় “কত visible character” নয় → MySQL-এ bytes।

ASCII: A B C → usually 1 byte each → LENGTH('ABC') = 3 Unicode / Bangla: SELECT LENGTH('বাংলা'); → visible chars থেকে বেশি হতে পারে (UTF-8 multi-byte)

English ASCII → usually 1 byte Some Unicode → multiple bytes

LENGTH() দিয়ে visible count ও byte count এক ধরে নেওয়া যাবে না।

10

LENGTH vs Visible Characters

১০ মিনিট

LENGTH() → Bytes

যদি business-এ “কতগুলো character” চাই — MySQL-এ আলাদা function আছে; সেটি অন্য String Function module-এ। এখানে শেখাব না।

11

Product Data

১০ মিনিট

product_id	product_name	product_code
1	iPhone	IPHONE15
2	Laptop	DELL15
3	Keyboard	KEY123

SELECT product_name, LENGTH(product_name) AS product_name_length FROM products; SELECT product_code, LENGTH(product_code) AS code_length FROM products;

Expected code length ঠিক আছে কিনা বুঝতে LENGTH useful।

12

Employee Codes

১০ মিনিট

employee_id	employee_name	employee_code
101	Rahim Khan	EMP001
102	Karim Hossain	EMP002
103	Jannat Ahmed	EMP003

SELECT employee_name, LENGTH(employee_name) AS name_length FROM employees; SELECT employee_code, LENGTH(employee_code) AS code_length FROM employees;

13

Data Quality Checking

১০ মিনিট

usernames: rahim · karim123 · abc · verylongusername123456789 · test Rule example: max 15 bytes (simplified)

SELECT username, LENGTH(username) AS username_length FROM users;

কোন username সবচেয়ে বড়?

LENGTH result দেখে identify করো।

14

Field Rules (conceptual)

১০ মিনিট

Input → LENGTH() → Measure size → Business rule (min 8 / max 20)

Password security implement নয় — শুধু size measure।

15

LENGTH with Numbers

৫ মিনিট

SELECT LENGTH(12345); SELECT LENGTH('12345');

MySQL string function-এ number → string convert হতে পারে। Magnitude নয় — string form-এর length।

16

LENGTH with Decimals

৫ মিনিট

SELECT LENGTH('100.50'); SELECT LENGTH(100.50);

LENGTH মূলত string-oriented — numeric input সাবধানে।

17

Predict the Output (১৫)

১০ মিনিট

Predict: LENGTH('Hello')

5

Predict: LENGTH('SQL')

3

Predict: LENGTH('Hello World')

11

Predict: LENGTH('')

0

Predict: LENGTH(' ')

1

Predict: LENGTH('123456')

6

Predict: LENGTH('Data')

4

Predict: LENGTH('Data Science')

12

Predict: LENGTH('RahimKhan')

9

Predict: LENGTH('Rahim Khan')

10

Predict: LENGTH('A')

1

Predict: LENGTH('ABC')

3

Predict: LENGTH(' ')

2 (two spaces)

Predict: LENGTH('employee_name') literal

13 — measures the word, not a column

Predict: LENGTH(name) for 'Rahim'

5

18

Find the Mistake (১০)

১০ মিনিট

Bug: LENGTH(employee_name name_length)

Missing AS / wrong syntax — use LENGTH(...) AS name_length

Bug: LENGTH('employee_name') FROM employees

Quotes → literal text length, not column

Bug: LENGTH employee_name)

Missing opening (

Bug: Thinking LENGTH always = visible chars

MySQL returns bytes

Bug: Ignoring space in length

Space adds bytes

Bug: Confusing '' and ' '

0 vs 1

Bug: Wrong column measured

Check which column you pass

Bug: Unclear alias

Use AS name_length

Bug: Misread Bangla result

Multi-byte UTF-8

Bug: Expect NUMBER magnitude from LENGTH(12345)

Measures string form of number

19

Common LENGTH Mistakes

৮ মিনিট

সবসময় visible characters ভাবা
Bytes ভুলে যাওয়া
Space ignore
Empty ও space এক ভাবা
Column quotes-এ
Parentheses ভুলে
Unicode result ভুল পড়া
Wrong column
খারাপ alias
Numeric = magnitude ভাবা

Pattern

WRONG → WHY? → CORRECT → EXPECTED

20

Case Study — Customer DQ

১২ মিনিট

customers: customer_id, customer_name, username, customer_code — ১৫+ rows (short/long, English/Bangla, empty, spaces)।

SELECT customer_name, LENGTH(customer_name) AS name_length, username, LENGTH(username) AS username_length, customer_code, LENGTH(customer_code) AS code_length FROM customers;

Largest name bytes?
Smallest username?
Longest code?
Unusual values?
Rule violations?
Space effect?
Bangla vs English?

21

Mini Project — Length Auditor

১৫ মিনিট

২০+ records: record_id, customer_name, username, product_code, employee_code।

SELECT record_id, customer_name, LENGTH(customer_name) AS name_length, username, LENGTH(username) AS username_length, product_code, LENGTH(product_code) AS code_length, employee_code, LENGTH(employee_code) AS emp_code_length FROM audit_data;

Name length
Username length
Product code
Employee code
Spot unusual
English vs Bangla
With/without spaces

22

Professional Use

৬ মিনিট

Analyst: DQ reports, text profiling
Engineer: validation, ETL checks
BI: quality dashboards
DB Dev: unexpected text sizes

23

Performance Notes

৫ মিনিট

LENGTH সহজ — কিন্তু millions row-এ এখনও cost আছে
অপ্রয়োজনীয় বারবার হিসাব এড়াও
ETL vs analysis — কখন calculate করবে ঠিক করো
Validation ≠ storage capacity lesson

24

Pattern Library

৬ মিনিট

SELECT LENGTH('Hello');
SELECT LENGTH(name) FROM employees;
SELECT LENGTH(name) AS name_length FROM employees;
SELECT LENGTH('');
SELECT LENGTH(' ');
SELECT LENGTH(product_code) FROM products;
SELECT LENGTH(username) FROM users;
SELECT LENGTH('বাংলা');  -- bytes, often > visible chars

25

Classroom Activities (১০)

১০ মিনিট

Act 1 — Manual: Hello?

5

Act 2 — HelloWorld vs Hello World?

10 vs 11

Act 3 — '' vs ' '?

0 vs 1

Act 4 — English ABC vs Bangla বাংলা?

ASCII 3 vs multi-byte higher

Act 5 — Long usernames to investigate?

Inspect largest LENGTH values

Act 6 — Product code expected 8 bytes?

Compare LENGTH(product_code)

Act 7 — EMP001 length?

6

Act 8 — Name with two spaces?

Extra bytes

Act 9 — Predict Data Science

12

Act 10 — Build length audit SELECT

SELECT col, LENGTH(col) AS …

26

Interview (২৫)

১২ মিনিট

IV 1. LENGTH কী?

String-এর byte length · MySQL · ইউজ: size check

IV 2. কী return করে?

Bytes · ভুল: always chars · ফলো: Unicode

IV 3. Characters নাকি bytes?

Bytes · ভুল: always visible · ফলো: Bangla

IV 4. Unicode কেন বড়?

Multi-byte UTF-8 · ভুল: bug · ফলো: demo

IV 5. Column length?

LENGTH(column) · ফলো: AS alias

IV 6. Alias?

AS name_length · ফলো: reports

IV 7. Empty string?

0 · ভুল: NULL · ফলো: space

IV 8. Space length?

1 · ভুল: 0 · ফলো: ''

IV 9. Space কেন গুরুত্বপূর্ণ?

Byte যোগ করে · ফলো: Rahim Khan

IV 10. Numbers?

String form-এ convert · সাবধানে · ফলো: '12345'

IV 11. Data quality?

অস্বাভাবিক long/short খুঁজে · ইউজ: profiling

IV 12. Validate codes?

Expected byte length · ইউজ: product_code

IV 13. Username profile?

LENGTH(username) · ইউজ: rules

IV 14. Bangla আলাদা কেন?

Multi-byte · ভুল: same as English · ফলো: LENGTH

IV 15. Byte vs char count?

LENGTH=bytes · char count অন্য module · ইউজ: clarity

IV 16. Unicode সতর্কতা?

Result > visible · ফলো: test

IV 17. Unusually long?

Compare LENGTH values · ইউজ: DQ

IV 18. ETL?

Source profiling / validation · ইউজ: pipelines

IV 19. Reporting?

Length columns in audit report · ইউজ: BI

IV 20. Common mistakes?

Quotes, spaces, bytes vs chars · ফলো: list

IV 21. Empty behavior?

LENGTH('')=0

IV 22. One space?

LENGTH(' ')=1

IV 23. Why not quote column?

Becomes literal · ফলো: LENGTH(col)

IV 24. LENGTH('employee_name')?

Measures 13-letter word

IV 25. Real use?

Username/code size audit

27

MCQ (২০)

১০ মিনিট

MCQ 1. LENGTH returns in MySQL? A) rows B) bytes C) tables D) indexes

B

MCQ 2. LENGTH('Hello')? A) 4 B) 5 C) 6 D) 0

B

MCQ 3. LENGTH('')? A) 1 B) 0 C) NULL D) -1

B

MCQ 4. LENGTH(' ')? A) 0 B) 1 C) 2 D) error

B

MCQ 5. Space counted? A) no B) yes C) only TRIM D) only CONCAT

B

MCQ 6. Always = visible chars? A) yes B) no C) only Bangla D) only English

B

MCQ 7. LENGTH('Rahim Khan')? A) 9 B) 10 C) 11 D) 8

B

MCQ 8. LENGTH('RahimKhan')? A) 9 B) 10 C) 11 D) 8

A

MCQ 9. LENGTH('employee_name') measures? A) column B) literal text C) table D) PK

B

MCQ 10. Alias purpose? A) rename result B) join C) delete D) sort

A

MCQ 11. Bangla may return? A) always 1 B) multi-byte higher C) always 0 D) error

B

MCQ 12. Data quality use? A) find odd sizes B) DROP table C) JOIN D) UNION

A

MCQ 13. LENGTH('Data Science')? A) 11 B) 12 C) 10 D) 13

B

MCQ 14. LENGTH('Hello World')? A) 10 B) 11 C) 12 D) 5

B

MCQ 15. Numeric LENGTH(12345)? A) magnitude B) string form length C) always 1 D) error always

B

MCQ 16. Focus of class? A) CONCAT B) LENGTH only C) LIKE D) JOIN

B

MCQ 17. Empty vs space? A) same B) 0 vs 1 C) both NULL D) both 2

B

MCQ 18. Code validation? A) LENGTH(code) B) DROP C) AVG D) UNION

A

MCQ 19. Missing (? A) syntax error B) OK C) auto D) 0

A

MCQ 20. Unicode tip? A) ignore B) bytes ≠ chars C) LENGTH is chars D) only for INT

B

28

Viva (২০)

৮ মিনিট

Viva 1. LENGTH কী?

Byte length function

Viva 2. Return কী?

Bytes

Viva 3. Hello?

5

Viva 4. Empty?

0

Viva 5. Space?

1

Viva 6. Space কেন গুরুত্বপূর্ণ?

Byte যোগ

Viva 7. Column কীভাবে?

LENGTH(col)

Viva 8. Alias?

AS name_length

Viva 9. Quotes ভুল কেন?

Literal

Viva 10. Bangla?

Multi-byte সম্ভব

Viva 11. Visible chars?

LENGTH সেটা নয়

Viva 12. DQ use?

Unusual lengths

Viva 13. Codes?

Expected size check

Viva 14. Username?

Profile sizes

Viva 15. Number input?

String form

Viva 16. Decimal?

String form careful

Viva 17. ETL?

Profiling

Viva 18. Mistake space?

Forget counting

Viva 19. Mistake bytes?

Think chars

Viva 20. Real use?

Length auditor report

29

Exercises (১৫)

১২ মিনিট

Ex 1 — LENGTH('SQL')

3

Ex 2 — LENGTH('Hello')

5

Ex 3 — LENGTH('Data Science')

12

Ex 4 — Empty string

LENGTH('') → 0

Ex 5 — Single space

LENGTH(' ') → 1

Ex 6 — Employee name length

SELECT name, LENGTH(name) AS name_length FROM employees

Ex 7 — Username length

LENGTH(username)

Ex 8 — Product code length

LENGTH(product_code)

Ex 9 — Employee code length

LENGTH(employee_code)

Ex 10 — Meaningful alias

AS username_length

Ex 11 — Unusually long usernames

Inspect LENGTH(username) values (filter if known)

Ex 12 — Compare code lengths

SELECT code, LENGTH(code)

Ex 13 — English vs Bangla

LENGTH('ABC') vs LENGTH('বাংলা')

Ex 14 — With/without space

RahimKhan vs Rahim Khan

Ex 15 — Length-audit report

Multiple LENGTH columns in one SELECT

30

Cheat Sheet + Memory Map

৫ মিনিট

LENGTH() → MySQL → returns BYTES
LENGTH('ABC') → 3
LENGTH('Hello') → 5
LENGTH('') → 0
LENGTH(' ') → 1

LENGTH = Bytes
≠ always Visible Characters (esp. Unicode)

LENGTH()
  → STRING → Count BYTES
  → English / Space / Unicode
  → Data Analysis → Quality → Validation

MySQL-এর LENGTH() function কোনো string কত bytes ব্যবহার করছে তা জানায়।

CONCAT / CHAR_LENGTH / TRIM / LIKE ইত্যাদি — অন্য module-এর বিষয়।

Text কত বড়? — SQL LENGTH()

বিষয়সূচী

কেন LENGTH()?

Real-life Use Cases

LENGTH() কী?

প্রথম Simple Examples

Manual Counting

Table Columns

Space Matters

Empty vs Space

সবচেয়ে গুরুত্বপূর্ণ: Bytes

LENGTH vs Visible Characters

Product Data

Employee Codes

Data Quality Checking

Field Rules (conceptual)

LENGTH with Numbers

LENGTH with Decimals

Predict the Output (১৫)

Find the Mistake (১০)

Common LENGTH Mistakes

Case Study — Customer DQ

Mini Project — Length Auditor

Professional Use

Performance Notes

Pattern Library

Classroom Activities (১০)

Interview (২৫)

MCQ (২০)

Viva (২০)

Exercises (১৫)

Cheat Sheet + Memory Map