SUBSTRING and LEFT/RIGHT in MySQL - Time & Space Complexity

Time Complexity: SUBSTRING and LEFT/RIGHT

O(k * n)

Understanding Time Complexity

When working with text data in databases, we often extract parts of strings using functions like SUBSTRING, LEFT, and RIGHT.

We want to understand how the time it takes to run these functions changes as the size of the input text grows.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

SELECT 
  SUBSTRING(description, 1, 10) AS first_ten_chars,
  LEFT(description, 5) AS first_five_chars,
  RIGHT(description, 5) AS last_five_chars
FROM products;
    

This code extracts parts of the 'description' text from each product: the first 10 characters, the first 5 characters, and the last 5 characters.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Extracting a substring from each string in the 'description' column.
• How many times: Once per row in the 'products' table.

How Execution Grows With Input

Extracting a fixed-length substring takes time proportional to the length of the substring, not the full string.

Input Size (length of description)	Approx. Operations per row
10	About 10 operations (extracting 10 chars)
100	Still about 10 operations (only first 10 chars extracted)
1000	Still about 10 operations (fixed substring length)

Pattern observation: The time to extract a fixed substring stays about the same no matter how long the full string is.

Final Time Complexity

Time Complexity: O(k * n)

This means the time grows linearly with the number of rows (n), and with the fixed substring length (k), which is usually small and constant.

Common Mistake

[X] Wrong: "Extracting a substring takes time proportional to the full string length."

[OK] Correct: Since we only extract a fixed number of characters, the operation depends on that fixed length, not the entire string size.

Interview Connect

Understanding how string functions scale helps you write efficient queries and explain performance in interviews confidently.

Self-Check

"What if we extract a substring whose length depends on the full string length? How would the time complexity change?"

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