NULLIF function in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return NULL if the two values are equal.

MySQL

SELECT NULLIF(10, [1]);

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

A5

B10

C15

D20

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a different number than the first argument will return the first argument instead of NULL.

The NULLIF function returns NULL if both arguments are equal. Here, 10 equals 10, so the result is NULL.

2fill in blank

medium

Complete the code to return the second value if the first is not equal to it.

MySQL

SELECT NULLIF(5, [1]);

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

ANULL

B5

C10

D5.0

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing the same value as the first argument will return NULL, not the first argument.

NULLIF returns the first argument if it is not equal to the second. Here, 5 is not equal to 10, so it returns 5.

3fill in blank

hard

Fix the error in the code to correctly use NULLIF with column names.

MySQL

SELECT NULLIF(price, [1]) FROM products;

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

Aprice

BNULL

C'price'

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Putting column names in single quotes treats them as strings, causing errors or wrong results.

Column names should not be in quotes. Using price without quotes refers to the column correctly.

4fill in blank

hard

Fill both blanks to return NULL if quantity equals 0, else return quantity.

MySQL

SELECT NULLIF([1], [2]) AS result FROM inventory;

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

Aquantity

B0

CNULL

D'0'

Attempts:

3 left

💡 Hint

Common Mistakes

Using '0' as a string instead of 0 as a number.

Using NULL as the second argument, which is invalid.

NULLIF(quantity, 0) returns NULL if quantity is 0, else returns quantity.

5fill in blank

hard

Fill all three blanks to return NULL if status equals 'inactive', else return status in uppercase.

MySQL

SELECT NULLIF(UPPER([1]), [2]) AS status_upper FROM users WHERE [3] = 'inactive';

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

Astatus

B'INACTIVE'

D'inactive'

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'inactive' in NULLIF comparison causes mismatch.

Using quotes incorrectly around column names.

We convert status to uppercase and compare it to 'INACTIVE' to return NULL if equal. The WHERE clause filters rows where status is 'inactive'.

Practice

1. What does the NULLIF function do in MySQL?

easy

A. Returns the sum of two expressions

B. Returns NULL if two expressions are equal, otherwise returns the first expression

C. Returns the first non-NULL expression

D. Returns the second expression if it is NULL

Solution

1. Step 1: Understand the purpose of NULLIF

   NULLIF compares two expressions and returns NULL if they are equal.

2. Step 2: Identify the return value when expressions differ

   If the expressions are not equal, NULLIF returns the first expression.

3. Final Answer:

   Returns NULL if two expressions are equal, otherwise returns the first expression -> Option B

4. Quick Check:

   NULLIF returns NULL on equality [OK]

Hint: NULLIF returns NULL only when values match exactly [OK]

Common Mistakes:

• Confusing NULLIF with COALESCE
• Thinking it sums values
• Assuming it returns second expression

2. Which of the following is the correct syntax for the NULLIF function in MySQL?

easy

A. NULLIF(expression1 OR expression2)

B. NULLIF(expression1 = expression2)

C. NULLIF(expression1, expression2, expression3)

D. NULLIF(expression1, expression2)

Solution

1. Step 1: Recall the syntax format of NULLIF

   NULLIF takes exactly two expressions separated by a comma inside parentheses.

2. Step 2: Check each option's syntax

   NULLIF(expression1, expression2) matches the correct syntax: NULLIF(expression1, expression2). Others have wrong number of arguments or invalid operators.

3. Final Answer:

   NULLIF(expression1, expression2) -> Option D

4. Quick Check:

   NULLIF syntax uses two expressions separated by comma [OK]

Hint: NULLIF always uses two expressions separated by a comma [OK]

Common Mistakes:

• Using comparison operator inside NULLIF
• Providing more than two arguments
• Using logical operators inside NULLIF

3. Given the table sales with columns id and amount, what will this query return?

SELECT id, NULLIF(amount, 0) AS adjusted_amount FROM sales;

If the table data is:
id | amount
1 | 100
2 | 0
3 | 50

medium

A. [{id:1, adjusted_amount:100}, {id:2, adjusted_amount:0}, {id:3, adjusted_amount:50}]

B. [{id:1, adjusted_amount:NULL}, {id:2, adjusted_amount:0}, {id:3, adjusted_amount:NULL}]

C. [{id:1, adjusted_amount:100}, {id:2, adjusted_amount:NULL}, {id:3, adjusted_amount:50}]

D. [{id:1, adjusted_amount:NULL}, {id:2, adjusted_amount:NULL}, {id:3, adjusted_amount:NULL}]

Solution

1. Step 1: Understand NULLIF(amount, 0)

   NULLIF returns NULL if amount equals 0, else returns amount.

2. Step 2: Apply to each row

   Row 1: amount=100, not 0, returns 100.
   Row 2: amount=0, equals 0, returns NULL.
   Row 3: amount=50, not 0, returns 50.

3. Final Answer:

   [{id:1, adjusted_amount:100}, {id:2, adjusted_amount:NULL}, {id:3, adjusted_amount:50}] -> Option C

4. Quick Check:

   NULLIF returns NULL only when amount is zero [OK]

Hint: NULLIF returns NULL when first value equals second [OK]

Common Mistakes:

• Assuming NULLIF returns 0 instead of NULL
• Confusing order of arguments
• Thinking NULLIF changes non-zero values

4. Identify the error in this query:

SELECT id, NULLIF(amount) FROM sales;

medium

A. NULLIF requires exactly two arguments

B. NULLIF cannot be used in SELECT clause

C. amount column does not exist

D. NULLIF should be written as NULLIF(amount = 0)

Solution

1. Step 1: Check NULLIF function usage

   NULLIF requires two arguments: expression1 and expression2.

2. Step 2: Analyze the query

   The query uses NULLIF with only one argument, which is invalid syntax.

3. Final Answer:

   NULLIF requires exactly two arguments -> Option A

4. Quick Check:

   NULLIF needs two expressions separated by comma [OK]

Hint: NULLIF always needs two arguments [OK]

Common Mistakes:

• Using NULLIF with one argument
• Writing NULLIF with comparison inside
• Assuming NULLIF works like ISNULL

5. You want to replace zero values in the score column with NULL in a query result. Which query correctly uses NULLIF to achieve this?

hard

A. SELECT id, NULLIF(score, 0) AS score_or_null FROM results;

B. SELECT id, NULLIF(0, score) AS score_or_null FROM results;

C. SELECT id, NULLIF(score) AS score_or_null FROM results;

D. SELECT id, NULLIF(score = 0) AS score_or_null FROM results;

Solution

1. Step 1: Understand the goal

   We want to convert zero scores to NULL, keep others unchanged.

2. Step 2: Apply NULLIF correctly

   NULLIF(score, 0) returns NULL if score is 0, else returns score.
   SELECT id, NULLIF(score, 0) AS score_or_null FROM results; matches this logic.

3. Step 3: Check other options

   SELECT id, NULLIF(0, score) AS score_or_null FROM results; reverses arguments, changing logic.
   SELECT id, NULLIF(score) AS score_or_null FROM results; has missing argument.
   SELECT id, NULLIF(score = 0) AS score_or_null FROM results; uses invalid syntax.

4. Final Answer:

   SELECT id, NULLIF(score, 0) AS score_or_null FROM results; -> Option A

5. Quick Check:

   NULLIF(score, 0) replaces zeros with NULL [OK]

Hint: Use NULLIF(column, 0) to convert zeros to NULL [OK]

Common Mistakes:

• Swapping argument order
• Using only one argument
• Using comparison inside NULLIF