Substring and overlay functions in PostgreSQL

Introduction

Substring and overlay functions help you extract or replace parts of text easily, like cutting or pasting pieces of a string.

You want to get a part of a word or sentence, like the first 3 letters of a name.

You need to replace a part of a text with something else, like fixing a typo inside a sentence.

You want to check or compare a small piece inside a bigger text.

You want to remove or change a section of a string without rewriting the whole thing.

Syntax

PostgreSQL

SUBSTRING(string FROM start FOR length)
OVERLAY(string PLACING new_substring FROM start FOR length)

Positions start at 1, not 0, so the first character is position 1.

If length is omitted in SUBSTRING, it takes all characters from start to the end.

Examples

Extracts the first 5 characters: 'Hello'.

PostgreSQL

SELECT SUBSTRING('Hello World' FROM 1 FOR 5);

Extracts from position 7 to the end: 'World'.

PostgreSQL

SELECT SUBSTRING('Hello World' FROM 7);

Replaces 'World' with 'Friend', resulting in 'Hello Friend'.

PostgreSQL

SELECT OVERLAY('Hello World' PLACING 'Friend' FROM 7 FOR 5);

Replaces 'bcd' with '123', resulting in 'a123ef'.

PostgreSQL

SELECT OVERLAY('abcdef' PLACING '123' FROM 2 FOR 3);

Sample Program

This program shows how to extract parts of a string and how to replace parts using substring and overlay functions.

PostgreSQL

SELECT 'Original text: Hello World' AS description;
SELECT SUBSTRING('Hello World' FROM 1 FOR 5) AS substring_example;
SELECT SUBSTRING('Hello World' FROM 7) AS substring_to_end;
SELECT OVERLAY('Hello World' PLACING 'Friend' FROM 7 FOR 5) AS overlay_example;
SELECT OVERLAY('abcdef' PLACING '123' FROM 2 FOR 3) AS overlay_partial;

Important Notes

Substring function runs in O(n) time where n is the length of the substring extracted.

Overlay also runs in O(n) time where n is the length of the string being modified.

Common mistake: forgetting that string positions start at 1, not 0.

Use substring when you want to get part of a string; use overlay when you want to replace part of a string.

Summary

Substring extracts a part of a string starting at a given position.

Overlay replaces part of a string with another string starting at a given position.

Positions start at 1, and length is optional in substring.

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