SUBSTRING and LEFT/RIGHT in MySQL - Step-by-Step Execution

Concept Flow - SUBSTRING and LEFT/RIGHT

Start with input string

↓

Choose function: SUBSTRING / LEFT / RIGHT

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Determine start position and length

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Extract substring accordingly

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Return extracted substring

This flow shows how MySQL extracts parts of a string using SUBSTRING, LEFT, or RIGHT functions by specifying start positions and lengths.

Execution Sample

MySQL

SELECT SUBSTRING('Hello World', 7, 5) AS sub,
       LEFT('Hello World', 5) AS left_part,
       RIGHT('Hello World', 5) AS right_part;

Extracts 'World' using SUBSTRING, 'Hello' using LEFT, and 'World' using RIGHT from the string 'Hello World'.

Execution Table

Step	Function	Input String	Parameters	Extracted Substring	Explanation
1	SUBSTRING	'Hello World'	start=7, length=5	'World'	Start at 7th char, take 5 chars
2	LEFT	'Hello World'	length=5	'Hello'	Take first 5 chars from left
3	RIGHT	'Hello World'	length=5	'World'	Take last 5 chars from right
4	END				All functions executed, extraction complete

💡 All substring extractions done as per parameters, no more steps.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Input String	'Hello World'	'Hello World'	'Hello World'	'Hello World'	'Hello World'
SUBSTRING Result	NULL	'World'	'World'	'World'	'World'
LEFT Result	NULL	NULL	'Hello'	'Hello'	'Hello'
RIGHT Result	NULL	NULL	NULL	'World'	'World'

Key Moments - 3 Insights

Why does SUBSTRING('Hello World', 7, 5) return 'World'?

Because the 7th character in 'Hello World' is 'W', and taking 5 characters from there gives 'World' as shown in execution_table row 1.

What happens if LEFT('Hello World', 5) is used?

It extracts the first 5 characters from the left, which is 'Hello', as shown in execution_table row 2.

How does RIGHT('Hello World', 5) differ from LEFT?

RIGHT takes characters from the end (right side) of the string, so it returns 'World' as in execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution table, what substring does SUBSTRING('Hello World', 7, 5) extract?

A 'World'

B 'Hello'

C 'World!'

D 'Hello W'

💡 Hint

Check execution_table row 1 under Extracted Substring column.

At which step does LEFT('Hello World', 5) get executed and what is the result?

A Step 1, 'World'

B Step 2, 'Hello'

C Step 3, 'World'

D Step 2, 'World'

💡 Hint

Refer to execution_table row 2 for LEFT function details.

If we change RIGHT('Hello World', 3), what would be the extracted substring?

A 'orld'

B 'ld'

C 'rld'

D 'World'

💡 Hint

RIGHT extracts from the end; 3 characters means last 3 letters of 'Hello World'.

Concept Snapshot

SUBSTRING(str, start, length): Extracts substring from 'start' position for 'length' chars.
LEFT(str, length): Extracts first 'length' chars from left.
RIGHT(str, length): Extracts last 'length' chars from right.
Positions start at 1.
If length exceeds string, returns up to string end.
Useful for slicing strings in MySQL.

Full Transcript

This lesson shows how to extract parts of a string in MySQL using SUBSTRING, LEFT, and RIGHT functions. SUBSTRING takes a string, a start position, and a length to return a substring. LEFT returns the first N characters from the start of the string. RIGHT returns the last N characters from the end. Positions start counting at 1. For example, SUBSTRING('Hello World', 7, 5) returns 'World' because it starts at the 7th character and takes 5 characters. LEFT('Hello World', 5) returns 'Hello', the first 5 characters. RIGHT('Hello World', 5) returns 'World', the last 5 characters. These functions help you get parts of strings easily in SQL queries.

Practice

1. What does the MySQL function LEFT('Hello World', 5) return?

easy

A. 'World'

B. 'Hello'

C. 'Hello World'

D. 'WorldHello'

Solution

1. Step 1: Understand LEFT function

   The LEFT function extracts a specified number of characters from the start (left side) of a string.

2. Step 2: Apply LEFT to 'Hello World'

   Extracting 5 characters from the start of 'Hello World' gives 'Hello'.

3. Final Answer:

   'Hello' -> Option B

4. Quick Check:

   LEFT('Hello World', 5) = 'Hello' [OK]

Hint: LEFT gets characters from the start of the string [OK]

Common Mistakes:

• Confusing LEFT with RIGHT function
• Expecting the whole string instead of part
• Using wrong number of characters

2. Which of the following is the correct syntax to extract 3 characters starting from the 2nd character in MySQL?

easy

A. RIGHT('abcdef', 2, 3)

B. LEFT('abcdef', 2, 3)

C. SUBSTRING('abcdef', 2, 3)

D. SUBSTRING('abcdef', 3, 2)

Solution

1. Step 1: Recall SUBSTRING syntax

   SUBSTRING(string, start_position, length) extracts length characters starting at start_position.

2. Step 2: Check each option

   SUBSTRING('abcdef', 2, 3) uses SUBSTRING with start=2 and length=3, which is correct. LEFT and RIGHT do not accept three arguments.

3. Final Answer:

   SUBSTRING('abcdef', 2, 3) -> Option C

4. Quick Check:

   Correct SUBSTRING syntax = SUBSTRING('abcdef', 2, 3) [OK]

Hint: SUBSTRING needs start and length; LEFT/RIGHT only need length [OK]

Common Mistakes:

• Using LEFT or RIGHT with three arguments
• Swapping start and length values
• Starting count from zero instead of one

3. What is the result of this query?
SELECT SUBSTRING('Database', 5, 3) AS part1, LEFT('Database', 4) AS part2, RIGHT('Database', 2) AS part3;

medium

A. part1='bas', part2='Data', part3='se'

B. part1='base', part2='Data', part3='se'

C. part1='tab', part2='D', part3='se'

D. part1='bas', part2='Datab', part3='se'

Solution

1. Step 1: Calculate SUBSTRING('Database', 5, 3)

   Starting at position 5: 'D'(1), 'a'(2), 't'(3), 'a'(4), 'b'(5), 'a'(6), 's'(7), 'e'(8). Characters 5 to 7 are 'b', 'a', 's' -> 'bas'.

2. Step 2: Calculate LEFT('Database', 4) and RIGHT('Database', 2)

   LEFT takes first 4 chars: 'D', 'a', 't', 'a' -> 'Data'. RIGHT takes last 2 chars: 's', 'e' -> 'se'.

3. Final Answer:

   part1='bas', part2='Data', part3='se' -> Option A

4. Quick Check:

   SUBSTRING, LEFT, RIGHT results match part1='bas', part2='Data', part3='se' [OK]

Hint: Count characters carefully starting at 1 for SUBSTRING [OK]

Common Mistakes:

• Counting start position from zero
• Mixing up LEFT and RIGHT results
• Extracting wrong substring length

4. Identify the error in this query:
SELECT LEFT('Example', 3, 2) AS result;

medium

A. LEFT function does not accept three arguments

B. String 'Example' is too short

C. Missing FROM clause

D. RIGHT function should be used instead

Solution

1. Step 1: Check LEFT function syntax

   LEFT(string, length) accepts only two arguments: the string and number of characters from the left.

2. Step 2: Analyze the query

   The query uses three arguments in LEFT, which is invalid syntax and causes an error.

3. Final Answer:

   LEFT function does not accept three arguments -> Option A

4. Quick Check:

   LEFT syntax error = LEFT function does not accept three arguments [OK]

Hint: LEFT only needs string and length, no extra arguments [OK]

Common Mistakes:

• Adding extra arguments to LEFT or RIGHT
• Confusing LEFT with SUBSTRING syntax
• Ignoring syntax errors in function calls

5. You have a column full_name with values like 'John Smith'. Which query extracts the last 5 characters of full_name to get the last name part using MySQL functions?

hard

A. SELECT SUBSTRING(full_name, 6, 5) AS last_name FROM users;

B. SELECT LEFT(full_name, 5) AS last_name FROM users;

C. SELECT SUBSTRING(full_name, -5) AS last_name FROM users;

D. SELECT RIGHT(full_name, 5) AS last_name FROM users;

Solution

1. Step 1: Understand the requirement

   We want the last 5 characters of the full_name, which is the last name part 'Smith' in 'John Smith'.

2. Step 2: Choose the correct function

   RIGHT(string, length) extracts characters from the end. RIGHT(full_name, 5) returns last 5 characters.

3. Step 3: Check other options

   LEFT extracts from start, SUBSTRING with negative start is invalid in MySQL, and SUBSTRING with fixed start 6 may fail if names vary.

4. Final Answer:

   SELECT RIGHT(full_name, 5) AS last_name FROM users; -> Option D

5. Quick Check:

   RIGHT gets last characters = SELECT RIGHT(full_name, 5) AS last_name FROM users; [OK]

Hint: Use RIGHT to get characters from the end of a string [OK]

Common Mistakes:

• Using LEFT instead of RIGHT for last characters
• Using SUBSTRING with negative start (not supported)
• Assuming fixed position for last name