NULLIF function in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the NULLIF(expr1, expr2) function do in MySQL?

It compares expr1 and expr2. If they are equal, it returns NULL. Otherwise, it returns expr1.

beginner

What will NULLIF(5, 5) return?

It returns NULL because both values are equal.

beginner

What will NULLIF('apple', 'orange') return?

It returns 'apple' because the two values are not equal.

intermediate

Why is NULLIF useful in SQL queries?

It helps avoid division by zero or handle special cases by returning NULL when two values match, which can prevent errors or unwanted results.

intermediate

How does NULLIF differ from a simple CASE statement?

While CASE can do many checks, NULLIF is a shortcut for returning NULL if two expressions are equal, making code shorter and clearer.

What does NULLIF(10, 10) return?

A NULL

B 10

C 0

D Error

NULLIF returns NULL when both arguments are equal.

If NULLIF('cat', 'dog') is used, what is the result?

A 'dog'

B NULL

C 'cat'

D Error

Since the two values are different, NULLIF returns the first value, 'cat'.

Which of these is a common use of NULLIF?

A To replace NULL values with zero

B To return NULL when two values are equal

C To concatenate two strings

D To count rows in a table

NULLIF returns NULL if the two expressions are equal.

What will SELECT NULLIF(0, 0) + 1; return?

A Error

B 1

C 0

D NULL

Since both values are equal, NULLIF returns NULL. Adding 1 to NULL results in NULL.

Which SQL function is a shortcut for: CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END?

A NULLIF

B ISNULL

C IFNULL

D COALESCE

NULLIF(expr1, expr2) does exactly that: returns NULL if equal, else expr1.

Explain how the NULLIF function works and give a simple example.

Think about comparing two values and what happens if they match.

Describe a practical situation where using NULLIF can help avoid errors in SQL queries.

Consider what happens if you divide by zero or want to ignore certain values.

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