NULLIF function behavior in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of NULLIF when values are equal?

Consider the query:

SELECT NULLIF(5, 5) AS result;

What is the value of result?

PostgreSQL

SELECT NULLIF(5, 5) AS result;

A Error

B 5

C 0

D NULL

💡 Hint

NULLIF returns NULL if both arguments are equal.

The NULLIF function returns NULL when the two arguments are equal. Since both are 5, the result is NULL.

❓ query_result

intermediate

What does NULLIF return when values differ?

What is the output of this query?

SELECT NULLIF('apple', 'orange') AS result;

PostgreSQL

SELECT NULLIF('apple', 'orange') AS result;

A 'apple'

B NULL

C 'orange'

D Error

💡 Hint

NULLIF returns the first argument if they are not equal.

Since 'apple' and 'orange' are different, NULLIF returns the first argument 'apple'.

🧠 Conceptual

advanced

How does NULLIF behave with NULL arguments?

What is the result of this query?

SELECT NULLIF(NULL, NULL) AS result;

PostgreSQL

SELECT NULLIF(NULL, NULL) AS result;

A TRUE

B Error

C NULL

D FALSE

💡 Hint

NULL compared to anything is unknown, but NULLIF treats NULL arguments specially.

NULLIF returns NULL if the two arguments are equal. However, NULL compared to NULL is unknown, so they are not considered equal. Therefore, NULLIF returns the first argument, which is NULL.

❓ query_result

advanced

What is the output of NULLIF with expressions?

Evaluate the query:

SELECT NULLIF(10/2, 5) AS result;

PostgreSQL

SELECT NULLIF(10/2, 5) AS result;

A 5

B NULL

C 2

D Error

💡 Hint

Calculate the first expression and compare to the second argument.

10 divided by 2 is 5, which equals the second argument. NULLIF returns NULL when both arguments are equal.

📝 Syntax

expert

Which NULLIF usage causes a syntax error?

Identify the option that will cause a syntax error in PostgreSQL:

A SELECT NULLIF(1);

B SELECT NULLIF('a', 'b');

C SELECT NULLIF(1, 1) FROM dual;

D SELECT NULLIF(NULL, 0);

💡 Hint

Check the number of arguments NULLIF requires.

NULLIF requires exactly two arguments. Option A provides only one argument, causing a syntax error.

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