REPLACE function in MySQL - Step-by-Step Execution

Concept Flow - REPLACE function

Input String

↓

Search for substring

Yes↓

Replace substring with new text

↓

Return new string

No↓

Return original string

The REPLACE function looks for a substring in a string and replaces all its occurrences with new text, returning the updated string.

Execution Sample

MySQL

SELECT REPLACE('hello world', 'world', 'MySQL');

This query replaces 'world' with 'MySQL' in the string 'hello world'.

Execution Table

Step	Input String	Search Substring	Replace With	Action	Result
1	'hello world'	'world'	'MySQL'	Search for 'world' in 'hello world'	Found at position 7
2	'hello world'	'world'	'MySQL'	Replace 'world' with 'MySQL'	'hello MySQL'
3	'hello MySQL'	'world'	'MySQL'	No more 'world' found	'hello MySQL'

💡 No more occurrences of 'world' found, replacement complete.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
Input String	'hello world'	'hello world'	'hello MySQL'	'hello MySQL'
Result String	''	''	'hello MySQL'	'hello MySQL'

Key Moments - 2 Insights

Why does REPLACE return the original string if the search substring is not found?

Because as shown in execution_table row 3, when the search substring is not found, REPLACE does no changes and returns the original string.

Does REPLACE change only the first occurrence or all occurrences?

REPLACE changes all occurrences of the search substring, but in this example only one occurrence exists, so only one replacement happens (see execution_table row 2).

Visual Quiz - 3 Questions

Look at the execution_table, what is the result after step 2?

A 'hello world'

B 'MySQL world'

C 'hello MySQL'

D 'hello'

💡 Hint

Check the 'Result' column in execution_table row 2.

At which step does REPLACE find no more occurrences of the search substring?

A Step 1

B Step 3

C Step 2

D No step

💡 Hint

Look at the 'Action' column in execution_table row 3.

If the input string was 'world world', how many replacements would REPLACE perform?

A Two

B One

C Zero

D Three

💡 Hint

REPLACE replaces all occurrences of the search substring.

Concept Snapshot

REPLACE(str, from_substr, to_substr)
- Searches str for all occurrences of from_substr
- Replaces each with to_substr
- Returns the new string
- If from_substr not found, returns original string
- Replaces all occurrences, not just first

Full Transcript

The REPLACE function in MySQL takes three inputs: the original string, the substring to find, and the substring to replace it with. It searches the original string for all occurrences of the search substring. If found, it replaces each occurrence with the new substring. If the search substring is not found, it returns the original string unchanged. For example, replacing 'world' with 'MySQL' in 'hello world' results in 'hello MySQL'. This process continues until no more occurrences are found. REPLACE always replaces all occurrences, not just the first one.

Practice

1. What does the REPLACE function do in MySQL?

easy

A. It deletes the entire string if a substring is found.

B. It splits the string into multiple parts.

C. It adds a substring at the end of the string.

D. It replaces all occurrences of a specified substring within a string.

Solution

1. Step 1: Understand the purpose of REPLACE

   The REPLACE function searches for all occurrences of a substring inside a string and replaces them with another substring.

2. Step 2: Compare options with the function behavior

   Only It replaces all occurrences of a specified substring within a string. correctly describes this behavior. Options A, C, and D describe different string operations.

3. Final Answer:

   It replaces all occurrences of a specified substring within a string. -> Option D

4. Quick Check:

   REPLACE changes parts of text inside strings = B [OK]

Hint: REPLACE swaps all found text parts inside a string [OK]

Common Mistakes:

• Thinking REPLACE deletes the whole string
• Confusing REPLACE with string splitting
• Assuming REPLACE adds text instead of replacing

2. Which of the following is the correct syntax to replace 'cat' with 'dog' in the string 'catapult' using MySQL?

easy

A. REPLACE('catapult', 'cat', 'dog')

B. REPLACE('catapult', 'dog', 'cat')

C. REPLACE('catapult', 'catapult', 'dog')

D. REPLACE('catapult', 'dog', 'dog')

Solution

1. Step 1: Recall REPLACE syntax

   The syntax is REPLACE(original_string, string_to_find, string_to_replace_with).

2. Step 2: Match parameters to the question

   We want to replace 'cat' with 'dog' in 'catapult', so the correct call is REPLACE('catapult', 'cat', 'dog').

3. Final Answer:

   REPLACE('catapult', 'cat', 'dog') -> Option A

4. Quick Check:

   REPLACE(original, find, replace) = A [OK]

Hint: Syntax is REPLACE(text, find, replace) always [OK]

Common Mistakes:

• Swapping find and replace arguments
• Replacing the whole string instead of substring
• Using wrong order of parameters

3. What is the result of this query?

SELECT REPLACE('hello world', 'l', 'x');

medium

A. 'hello world'

B. 'hexxo worlx'

C. 'hexxo worxd'

D. 'hexxo worll'

Solution

1. Step 1: Identify what is replaced

   All occurrences of 'l' in 'hello world' are replaced with 'x'.

2. Step 2: Perform the replacement

   'hello world' becomes 'hexxo worlx' because every 'l' changes to 'x'.

3. Final Answer:

   'hexxo worlx' -> Option B

4. Quick Check:

   Replace all 'l' with 'x' = 'hexxo worlx' [OK]

Hint: Replace every found letter, not just first [OK]

Common Mistakes:

• Replacing only first occurrence
• Not replacing at all
• Replacing wrong characters

4. Identify the error in this query:

SELECT REPLACE('database', 'a');

medium

A. Missing third argument for replacement string.

B. The first argument should be a column name, not a string.

C. REPLACE cannot be used in SELECT statements.

D. The function name should be REPLACESTR.

Solution

1. Step 1: Check REPLACE function arguments

   REPLACE requires three arguments: original string, string to find, and string to replace with.

2. Step 2: Identify missing argument

   The query only has two arguments, missing the replacement string.

3. Final Answer:

   Missing third argument for replacement string. -> Option A

4. Quick Check:

   REPLACE needs 3 arguments = A [OK]

Hint: REPLACE needs 3 arguments: original, find, replace [OK]

Common Mistakes:

• Forgetting the replacement string
• Using wrong function name
• Thinking REPLACE can't be in SELECT

5. Given a table products with a column code containing values like 'AB-123', 'AB-456', and 'CD-789', which query replaces the prefix 'AB-' with 'XY-' only in the code column?

hard

A. SELECT REPLACE(code, 'AB', 'XY') AS new_code FROM products;

B. SELECT REPLACE('code', 'AB-', 'XY-') AS new_code FROM products;

C. SELECT REPLACE(code, 'AB-', 'XY-') AS new_code FROM products;

D. SELECT REPLACE(code, 'XY-', 'AB-') AS new_code FROM products;

Solution

1. Step 1: Understand the goal

   We want to replace the prefix 'AB-' with 'XY-' in the column code values.

2. Step 2: Analyze each option

   SELECT REPLACE(code, 'AB-', 'XY-') AS new_code FROM products; correctly uses REPLACE on the column code replacing 'AB-' with 'XY-'. SELECT REPLACE('code', 'AB-', 'XY-') AS new_code FROM products; treats 'code' as a string literal, not a column. SELECT REPLACE(code, 'XY-', 'AB-') AS new_code FROM products; reverses replacement. SELECT REPLACE(code, 'AB', 'XY') AS new_code FROM products; replaces 'AB' without the dash, which may affect unintended parts.

3. Final Answer:

   SELECT REPLACE(code, 'AB-', 'XY-') AS new_code FROM products; -> Option C

4. Quick Check:

   Replace prefix 'AB-' with 'XY-' in column = D [OK]

Hint: Use column name as first argument, exact substring to replace [OK]

Common Mistakes:

• Using quotes around column name
• Replacing wrong substring
• Swapping find and replace strings