CONCAT and CONCAT_WS in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of CONCAT with NULL values

What is the output of this MySQL query?

SELECT CONCAT('Hello', NULL, 'World');

MySQL

SELECT CONCAT('Hello', NULL, 'World');

A NULL

B 'HelloWorld'

C 'HelloNULLWorld'

D 'Hello World'

💡 Hint

Remember how CONCAT treats NULL values in MySQL.

In MySQL, CONCAT returns NULL if any argument is NULL.

❓ query_result

intermediate

Using CONCAT_WS with NULL values

What is the output of this MySQL query?

SELECT CONCAT_WS('-', '2024', NULL, '06', '15');

MySQL

SELECT CONCAT_WS('-', '2024', NULL, '06', '15');

A NULL

B '2024-NULL-06-15'

C '2024--06-15'

D '2024-06-15'

💡 Hint

CONCAT_WS skips NULL values instead of treating them as strings.

CONCAT_WS skips NULL values and does not add separators for them.

📝 Syntax

advanced

Identify the syntax error in CONCAT usage

Which option contains a syntax error in using CONCAT in MySQL?

A SELECT CONCAT('A' 'B', 'C');

B SELECT CONCAT('A', , 'C');

C SELECT CONCAT('A', 'B' 'C');

D SELECT CONCAT('A', 'B', 'C');

💡 Hint

Look for missing arguments or commas.

Option B has two commas with no argument between them, causing a syntax error.

❓ optimization

advanced

Optimizing string concatenation with CONCAT_WS

You want to concatenate first name, middle name, and last name with spaces between them. Middle name can be NULL. Which query is the most efficient and correct to avoid extra spaces?

A SELECT CONCAT(first_name, ' ', middle_name, ' ', last_name) FROM users;

B SELECT CONCAT(first_name, middle_name, last_name) FROM users;

C SELECT CONCAT_WS(' ', first_name, middle_name, last_name) FROM users;

D SELECT CONCAT_WS('', first_name, middle_name, last_name) FROM users;

💡 Hint

Think about how NULL values and separators are handled.

CONCAT_WS skips NULL values and adds the separator only between non-NULL values, avoiding extra spaces.

🧠 Conceptual

expert

Behavior difference between CONCAT and CONCAT_WS

Which statement correctly describes the difference between CONCAT and CONCAT_WS in MySQL?

A CONCAT returns NULL if any argument is NULL; CONCAT_WS skips NULL arguments and does not return NULL.

B CONCAT_WS returns NULL if any argument is NULL; CONCAT skips NULL arguments.

C Both CONCAT and CONCAT_WS return NULL if any argument is NULL.

D Both CONCAT and CONCAT_WS skip NULL arguments and never return NULL.

💡 Hint

Think about how each function treats NULL values.

CONCAT returns NULL if any argument is NULL, but CONCAT_WS skips NULL arguments and concatenates the rest.

Practice

1. What is the main difference between CONCAT and CONCAT_WS in MySQL?

easy

A. CONCAT returns NULL if any argument is NULL; CONCAT_WS skips NULLs and uses a separator.

B. CONCAT adds a separator between strings; CONCAT_WS joins strings without separator.

C. CONCAT only works with numbers; CONCAT_WS only works with strings.

D. CONCAT converts all inputs to uppercase; CONCAT_WS converts to lowercase.

Solution

1. Step 1: Understand CONCAT behavior

   CONCAT joins strings directly but returns NULL if any input is NULL.

2. Step 2: Understand CONCAT_WS behavior

   CONCAT_WS joins strings with a separator and ignores NULL values instead of returning NULL.

3. Final Answer:

   CONCAT returns NULL if any argument is NULL; CONCAT_WS skips NULLs and uses a separator. -> Option A

4. Quick Check:

   Difference in NULL handling and separator use = CONCAT returns NULL if any argument is NULL; CONCAT_WS skips NULLs and uses a separator. [OK]

Hint: Remember: CONCAT returns NULL if any NULL; CONCAT_WS skips NULLs [OK]

Common Mistakes:

• Thinking CONCAT_WS adds no separator
• Assuming CONCAT_WS returns NULL if any argument is NULL
• Confusing which function skips NULLs

2. Which of the following is the correct syntax to join first_name and last_name with a space using CONCAT_WS?

easy

A. CONCAT_WS(' ', first_name + last_name)

B. CONCAT_WS(first_name, last_name, ' ')

C. CONCAT_WS(first_name + ' ' + last_name)

D. CONCAT_WS(' ', first_name, last_name)

Solution

1. Step 1: Identify CONCAT_WS syntax

   CONCAT_WS takes the separator as the first argument, then the strings to join.

2. Step 2: Match correct argument order

   CONCAT_WS(' ', first_name, last_name) correctly uses a space as separator, then first_name and last_name as strings.

3. Final Answer:

   CONCAT_WS(' ', first_name, last_name) -> Option D

4. Quick Check:

   Separator first, then strings = CONCAT_WS(' ', first_name, last_name) [OK]

Hint: Separator is always first argument in CONCAT_WS [OK]

Common Mistakes:

• Putting separator last instead of first
• Using + operator inside CONCAT_WS
• Passing concatenated string as single argument

3. Given a table users with columns first_name, middle_name, and last_name, what is the output of this query?

SELECT CONCAT_WS(' ', first_name, middle_name, last_name) AS full_name FROM users WHERE id = 1;

Assuming the row with id=1 has first_name='John', middle_name=NULL, and last_name='Doe'.

medium

A. 'John NULL Doe'

B. 'John Doe'

C. 'John Doe'

D. NULL

Solution

1. Step 1: Understand CONCAT_WS with NULL values

   CONCAT_WS skips NULL values and joins remaining strings with the separator.

2. Step 2: Apply to given data

   Since middle_name is NULL, it is skipped. The result joins 'John' and 'Doe' with a space.

3. Final Answer:

   'John Doe' -> Option B

4. Quick Check:

   NULL skipped, space separator used = 'John Doe' [OK]

Hint: CONCAT_WS skips NULLs, so no extra spaces for NULL columns [OK]

Common Mistakes:

• Expecting NULL to appear as text
• Assuming NULL causes entire result to be NULL
• Thinking NULL inserts extra spaces

4. Identify the error in this query:

SELECT CONCAT('Name: ', first_name, middle_name, last_name) AS full_name FROM users;

medium

A. The query will return NULL for full_name if any column is NULL.

B. The syntax is incorrect; CONCAT requires only two arguments.

C. NULL cannot be used inside CONCAT; it causes a syntax error.

D. The query will concatenate NULL as the string 'NULL'.

Solution

1. Step 1: Understand CONCAT behavior with NULL

   CONCAT returns NULL if any argument is NULL.

2. Step 2: Analyze the query

   The query uses CONCAT on columns that may contain NULL, so the entire result for full_name will be NULL if any column is NULL.

3. Final Answer:

   The query will return NULL for full_name if any column is NULL. -> Option A

4. Quick Check:

   CONCAT returns NULL if any argument is NULL = The query will return NULL for full_name if any column is NULL. [OK]

Hint: CONCAT returns NULL if any argument is NULL, no error [OK]

Common Mistakes:

• Thinking CONCAT throws syntax error with NULL
• Expecting NULL to appear as 'NULL' string
• Assuming CONCAT accepts unlimited arguments without NULL effect

5. You want to create a full address string from columns street, city, and zipcode in a table, separated by commas. Some rows have NULL in zipcode. Which query correctly creates the address without extra commas for NULL values?

hard

A. SELECT CONCAT_WS(', ', street, city) || ', ' || zipcode AS full_address FROM addresses;

B. SELECT CONCAT(street, ', ', city, ', ', zipcode) AS full_address FROM addresses;

C. SELECT CONCAT_WS(', ', street, city, zipcode) AS full_address FROM addresses;

D. SELECT CONCAT(street, city, zipcode) AS full_address FROM addresses;

Solution

1. Step 1: Understand handling of NULLs and separators

   CONCAT_WS joins strings with a separator and skips NULL values, avoiding extra separators.

2. Step 2: Analyze each option

   SELECT CONCAT_WS(', ', street, city, zipcode) AS full_address FROM addresses; uses CONCAT_WS with ', ' separator and all columns, so NULL zipcode is skipped without extra commas. SELECT CONCAT(street, ', ', city, ', ', zipcode) AS full_address FROM addresses; uses CONCAT which returns NULL if any argument is NULL, causing missing addresses. SELECT CONCAT_WS(', ', street, city) || ', ' || zipcode AS full_address FROM addresses; uses invalid syntax with || operator in MySQL. SELECT CONCAT(street, city, zipcode) AS full_address FROM addresses; concatenates without separators and returns NULL if any NULL present.

3. Final Answer:

   SELECT CONCAT_WS(', ', street, city, zipcode) AS full_address FROM addresses; -> Option C

4. Quick Check:

   Use CONCAT_WS to skip NULLs and add separators correctly = SELECT CONCAT_WS(', ', street, city, zipcode) AS full_address FROM addresses; [OK]

Hint: Use CONCAT_WS with separator to skip NULLs and avoid extra commas [OK]

Common Mistakes:

• Using CONCAT which returns NULL if any NULL argument
• Trying to concatenate with || which is not MySQL syntax
• Manually adding separators causing extra commas