Substring and overlay functions in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to extract the first 5 characters from the string 'HelloWorld'.

PostgreSQL

SELECT SUBSTRING('HelloWorld' FROM 1 FOR [1]);

Drag options to blanks, or click blank then click option'

A1

B10

C0

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using 10 extracts the whole string, not just the first 5 characters.

Using 0 or 1 will not extract the correct substring length.

The SUBSTRING function extracts a part of the string starting at position 1 for 5 characters.

2fill in blank

medium

Complete the code to replace 3 characters starting at position 2 in 'abcdef' with 'XYZ'.

PostgreSQL

SELECT OVERLAY('abcdef' PLACING [1] FROM 2 FOR 3);

Drag options to blanks, or click blank then click option'

A'XYZ'

B'123'

C'abc'

D'def'

Attempts:

3 left

💡 Hint

Common Mistakes

Using '123' or other strings changes the output incorrectly.

Using 'abc' or 'def' does not replace the correct part.

The OVERLAY function replaces 3 characters starting at position 2 with 'XYZ'.

3fill in blank

hard

Fix the error in the code to extract substring starting at position 3 with length 4 from 'PostgreSQL'.

PostgreSQL

SELECT SUBSTRING('PostgreSQL' [1] 3 FOR 4);

Drag options to blanks, or click blank then click option'

AAT

BSTART

CFROM

DPOSITION

Attempts:

3 left

💡 Hint

Common Mistakes

Using AT or POSITION causes syntax errors.

Using START is not valid in PostgreSQL SUBSTRING.

The correct syntax uses FROM to specify the start position in SUBSTRING.

4fill in blank

hard

Fill both blanks to extract 4 characters starting at position 2 from 'Database' and replace them with '1234'.

PostgreSQL

SELECT OVERLAY('Database' PLACING [1] FROM [2] FOR 4);

Drag options to blanks, or click blank then click option'

A'1234'

B'Data'

C2

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong start position changes the replaced part.

Using incorrect PLACING string changes the output.

The OVERLAY replaces 4 characters starting at position 2 with '1234'.

5fill in blank

hard

Fill all three blanks to extract substring starting at position 4 for 3 characters from 'Programming' and replace it with 'XYZ'.

PostgreSQL

SELECT OVERLAY(SUBSTRING('Programming' FROM [1] FOR [2]) PLACING [3] FROM 1 FOR 3);

Drag options to blanks, or click blank then click option'

A4

B3

C'XYZ'

D'Pro'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong start position or length in SUBSTRING.

Using incorrect string in PLACING changes the result.

First, SUBSTRING extracts 3 characters starting at position 4, then OVERLAY replaces them with 'XYZ'.

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