Substring and overlay functions in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the substring function do in PostgreSQL?

The substring function extracts a part of a string based on position and length. For example, substring('hello' from 2 for 3) returns 'ell'.

beginner

How do you use the overlay function in PostgreSQL?

The overlay function replaces a part of a string with another string. Syntax: overlay(original_string placing new_string from start_position for length). It returns the modified string.

beginner

Example: What is the result of substring('abcdef' from 3 for 2)?

It returns 'cd' because it starts at position 3 and takes 2 characters.

beginner

Example: What does overlay('abcdef' placing 'XY' from 2 for 3) return?

It returns 'aXYef'. It replaces 3 characters starting at position 2 with 'XY'.

intermediate

Can substring use regular expressions in PostgreSQL?

Yes, substring can extract parts of a string using a regular expression pattern. For example, substring('abc123def' from '\d+') returns '123'.

What does substring('hello world' from 7 for 5) return?

A 'world'

B 'hello'

C 'world!'

D 'hello world'

It extracts 5 characters starting at position 7, which is 'world'.

Which function replaces part of a string with another string in PostgreSQL?

A overlay

B substring

C replace

D concat

overlay replaces a substring with another string at a specified position.

What is the result of overlay('abcdef' placing 'XY' from 3 for 2)?

A 'XYcdef'

B 'aXYdef'

C 'abcXYf'

D 'abXYef'

It replaces 2 characters starting at position 3 with 'XY', resulting in 'abXYef'.

Can substring extract text using a regular expression?

A Only in MySQL

B No

C Yes

D Only with overlay

PostgreSQL's substring supports regex patterns to extract matching parts.

What happens if substring length is longer than the string?

A Returns an error

B Returns substring up to string end

C Returns empty string

D Returns full string twice

It returns the substring from start position to the end without error.

Explain how to extract a part of a string using the substring function in PostgreSQL.

Think about specifying where to start and how many characters to take.

Describe how the overlay function works and give an example.

It’s like cutting out a piece and putting a new piece in its place.

Practice

1. What does the PostgreSQL substring function do when given a string and a start position?

easy

A. Returns the length of the string

B. Extracts part of the string starting from the given position to the end

C. Replaces part of the string starting from the given position

D. Converts the string to uppercase

Solution

1. Step 1: Understand substring function purpose

   The substring function extracts a portion of a string starting at a specified position.

2. Step 2: Analyze given parameters

   When only a start position is given, substring returns from that position to the end of the string.

3. Final Answer:

   Extracts part of the string starting from the given position to the end -> Option B

4. Quick Check:

   substring extracts substring from position [OK]

Hint: Substring cuts string from start position to end if length missing [OK]

Common Mistakes:

• Confusing substring with overlay
• Thinking substring replaces text
• Assuming substring returns string length

2. Which of the following is the correct syntax to replace part of a string using the overlay function in PostgreSQL?

easy

A. overlay(string placing new_substring from start for length)

B. overlay(string from start for length placing new_substring)

C. overlay(string for length from start placing new_substring)

D. overlay(new_substring placing string from start for length)

Solution

1. Step 1: Recall overlay syntax

   The correct syntax is overlay(string placing new_substring from start for length).

2. Step 2: Match options to syntax

   overlay(string placing new_substring from start for length) matches the correct order: string placing new_substring from start for length.

3. Final Answer:

   overlay(string placing new_substring from start for length) -> Option A

4. Quick Check:

   Overlay syntax = string placing new_substring from start for length [OK]

Hint: Remember: overlay(string placing new from start for length) [OK]

Common Mistakes:

• Mixing order of 'from' and 'placing'
• Omitting 'for length' keyword
• Swapping string and new_substring positions

3. What is the result of this query?
SELECT overlay('abcdef' placing 'XYZ' from 2 for 3);

medium

A. 'abXYZf'

B. 'aXYZdef'

C. 'XYZdef'

D. 'aXYZef'

Solution

1. Step 1: Understand overlay parameters

   We replace 3 characters starting at position 2 in 'abcdef' with 'XYZ'. Positions start at 1.

2. Step 2: Apply replacement

   Characters at positions 2,3,4 ('b','c','d') are replaced by 'XYZ', resulting in 'aXYZef'.

3. Final Answer:

   'aXYZef' -> Option D

4. Quick Check:

   Overlay replaces substring from pos 2 length 3 with 'XYZ' [OK]

Hint: Overlay replaces exact length starting at position [OK]

Common Mistakes:

• Replacing wrong substring length
• Starting replacement at wrong position
• Appending instead of replacing

4. Identify the error in this query:
SELECT overlay('hello' placing 'y' for 2 from 3);

medium

A. String literals not quoted

B. Missing length parameter

C. Incorrect order of 'for' and 'from' keywords

D. Overlay function does not exist

Solution

1. Step 1: Check overlay syntax order

   The correct syntax is overlay(string placing new_substring from start for length). The query uses 'placing' then 'for' then 'from', which is wrong order.

2. Step 2: Identify error cause

   The keywords 'for' and 'from' are swapped, causing syntax error.

3. Final Answer:

   Incorrect order of 'for' and 'from' keywords -> Option C

4. Quick Check:

   Overlay syntax order matters: from then for then placing [OK]

Hint: Overlay syntax: from before for before placing [OK]

Common Mistakes:

• Swapping 'for' and 'from' keywords
• Omitting quotes around strings
• Using overlay without required keywords

5. Given the string 'PostgreSQL', which query correctly replaces 'gre' with 'XYZ' using overlay and then extracts the substring starting at position 5 with length 4?

hard

A. SELECT substring(overlay('PostgreSQL' placing 'XYZ' from 5 for 3) from 5 for 4);

B. SELECT substring(overlay('PostgreSQL' from 5 for 3 placing 'XYZ') from 5 for 4);

C. SELECT overlay(substring('PostgreSQL' from 5 for 3) placing 'XYZ' from 5 for 4);

D. SELECT overlay('PostgreSQL' from 5 for 4 placing 'XYZ');

Solution

1. Step 1: Replace 'gre' with 'XYZ' using overlay

   'gre' starts at position 5 and has length 3. Correct overlay syntax is overlay(string placing new_substring from start for length). SELECT substring(overlay('PostgreSQL' placing 'XYZ' from 5 for 3) from 5 for 4); matches this.

2. Step 2: Extract substring from position 5 length 4

   Apply substring on the result starting at position 5 for length 4. SELECT substring(overlay('PostgreSQL' placing 'XYZ' from 5 for 3) from 5 for 4); applies substring correctly after overlay.

3. Final Answer:

   SELECT substring(overlay('PostgreSQL' placing 'XYZ' from 5 for 3) from 5 for 4); -> Option A

4. Quick Check:

   Overlay then substring with correct syntax = SELECT substring(overlay('PostgreSQL' placing 'XYZ' from 5 for 3) from 5 for 4); [OK]

Hint: Overlay syntax: placing after string, then substring after overlay [OK]

Common Mistakes:

• Mixing overlay keyword order
• Applying substring before overlay
• Wrong positions or lengths in functions