NULLIF function behavior in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

PostgreSQL

SELECT NULLIF(5, [1]);

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

A3

B7

C5

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a value different from 5 will return the first value instead of NULL.

The NULLIF function returns NULL if both arguments are equal. Here, 5 equals 5, so the result is NULL.

2fill in blank

medium

Complete the code to return the first value when the two values are different.

PostgreSQL

SELECT NULLIF('apple', [1]);

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

A'orange'

B'apple'

C'apple '

D'APPLE'

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing the same string will return NULL instead of the first value.

NULLIF returns the first value if the two values are different. 'apple' and 'orange' are different, so it returns 'apple'.

3fill in blank

hard

Fix the error in the code to correctly use NULLIF with a column named score.

PostgreSQL

SELECT NULLIF(score, [1]) FROM results;

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

Ascore

B'score'

CNULL

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around column names causes the function to compare to a string, not the column value.

To compare the column score to itself, use the column name without quotes. Using quotes treats it as a string literal.

4fill in blank

hard

Fill both blanks to return NULL if age equals 18, otherwise return age.

PostgreSQL

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

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

Aage

B18

C'18'

Duser_age

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around 18 makes it a string, which may not match the numeric column.

NULLIF compares the first argument to the second. Using age and 18 (number) correctly returns NULL if age is 18.

5fill in blank

hard

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

PostgreSQL

SELECT COALESCE(NULLIF([1], [2]), [3]) AS status_check FROM accounts;

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

Astatus

B'active'

CUPPER(status)

DLOWER(status)

Attempts:

3 left

💡 Hint

Common Mistakes

Using LOWER instead of UPPER changes the output case incorrectly.

Not quoting 'active' treats it as a column, causing errors.

NULLIF returns NULL if status equals 'active'. COALESCE then returns the uppercase status if NULLIF returns NULL.

Practice

1. What does the NULLIF function return when both its arguments are equal?

easy

A. It returns an error.

B. It returns the second argument.

C. It returns the first argument.

D. It returns NULL.

Solution

1. Step 1: Understand the purpose of NULLIF

   The NULLIF function compares two values and returns NULL if they are equal.

2. Step 2: Apply the function behavior

   If both arguments are the same, NULLIF returns NULL instead of any value.

3. Final Answer:

   It returns NULL. -> Option D

4. Quick Check:

   Equal values = NULL [OK]

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

Common Mistakes:

• Thinking it returns the second argument when equal
• Assuming it always returns the first argument
• Believing it throws an error on equal values

2. Which of the following is the correct syntax to use NULLIF in PostgreSQL?

easy

A. NULLIF(value1; value2)

B. NULLIF value1, value2

C. NULLIF(value1, value2)

D. NULLIF[value1, value2]

Solution

1. Step 1: Recall the function syntax

   NULLIF is a standard SQL function that takes two arguments inside parentheses separated by a comma.

2. Step 2: Identify the correct syntax

   Only NULLIF(value1, value2) uses parentheses and comma correctly.

3. Final Answer:

   NULLIF(value1, value2) -> Option C

4. Quick Check:

   Function syntax = parentheses + comma [OK]

Hint: Use parentheses and comma for SQL functions [OK]

Common Mistakes:

• Omitting parentheses
• Using semicolon instead of comma
• Using square brackets instead of parentheses

3. What is the result of this query?

SELECT NULLIF(5, 5) AS result1, NULLIF(7, 5) AS result2;

medium

A. result1 = NULL, result2 = 7

B. result1 = 5, result2 = 7

C. result1 = NULL, result2 = NULL

D. result1 = 5, result2 = NULL

Solution

1. Step 1: Evaluate NULLIF(5, 5)

   Since both arguments are equal, NULLIF(5, 5) returns NULL.

2. Step 2: Evaluate NULLIF(7, 5)

   Arguments differ, so NULLIF(7, 5) returns the first argument, 7.

3. Final Answer:

   result1 = NULL, result2 = 7 -> Option A

4. Quick Check:

   Equal args = NULL, different args = first value [OK]

Hint: NULLIF returns NULL if equal, else first value [OK]

Common Mistakes:

• Assuming NULLIF always returns first argument
• Confusing which argument is returned when different
• Expecting both results to be NULL

4. Identify the error in this query:

SELECT NULLIF(10 10) AS result;

medium

A. Missing comma between arguments

B. NULLIF cannot take numeric arguments

C. NULLIF requires three arguments

D. Using wrong function name

Solution

1. Step 1: Check function argument syntax

   Arguments in NULLIF must be separated by a comma.

2. Step 2: Identify the missing comma

   The query has NULLIF(10 10) missing a comma between 10 and 10.

3. Final Answer:

   Missing comma between arguments -> Option A

4. Quick Check:

   Arguments need comma separator [OK]

Hint: Check commas between function arguments [OK]

Common Mistakes:

• Forgetting commas between arguments
• Thinking NULLIF needs more than two arguments
• Assuming numeric arguments are invalid

5. You want to avoid division by zero errors in this query:

SELECT value / divisor FROM numbers;

How can you use NULLIF to prevent errors when divisor is zero?

hard

A. SELECT NULLIF(value, 0) / divisor FROM numbers;

B. SELECT value / NULLIF(divisor, 0) FROM numbers;

C. SELECT value / divisor WHERE divisor != 0;

D. SELECT value / divisor NULLIF divisor, 0 FROM numbers;

Solution

1. Step 1: Understand division by zero problem

   Dividing by zero causes an error. We want to replace zero divisor with NULL to avoid error.

2. Step 2: Use NULLIF to replace zero divisor

   NULLIF(divisor, 0) returns NULL if divisor is zero, preventing division by zero error.

3. Step 3: Check options

   SELECT value / NULLIF(divisor, 0) FROM numbers; correctly uses value / NULLIF(divisor, 0). SELECT NULLIF(value, 0) / divisor FROM numbers; replaces value, not divisor. SELECT value / divisor WHERE divisor != 0; filters rows but does not use NULLIF. SELECT value / divisor NULLIF divisor, 0 FROM numbers; has syntax error.

4. Final Answer:

   SELECT value / NULLIF(divisor, 0) FROM numbers; -> Option B

5. Quick Check:

   Replace zero divisor with NULL using NULLIF [OK]

Hint: Use NULLIF(divisor, 0) to avoid division by zero [OK]

Common Mistakes:

• Replacing numerator instead of divisor
• Filtering rows instead of using NULLIF
• Writing invalid syntax with NULLIF