NULLIF function in MySQL - Step-by-Step Execution

Concept Flow - NULLIF function

Start

↓

Evaluate expr1

↓

Evaluate expr2

↓

Compare expr1 and expr2

↓

Return NULL

↓

End

NULLIF compares two expressions; if they are equal, it returns NULL, otherwise it returns the first expression.

Execution Sample

MySQL

SELECT NULLIF(5, 5) AS result1, NULLIF(5, 3) AS result2;

This query returns NULL for the first expression because 5 equals 5, and returns 5 for the second because 5 does not equal 3.

Execution Table

Step	Expression 1	Expression 2	Comparison Result	Return Value
1	5	5	Equal	NULL
2	5	3	Not Equal	5
3	-	-	-	Execution ends

💡 All expressions evaluated; NULLIF returns NULL if expressions are equal, else returns first expression.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
expr1	5	5	5	5
expr2	5 or 3	5	3	3
return_value	undefined	NULL	5	5

Key Moments - 2 Insights

Why does NULLIF(5, 5) return NULL instead of 5?

Because the two expressions are equal (see execution_table row 1), NULLIF returns NULL as per its rule.

What happens if the two expressions are not equal?

NULLIF returns the first expression unchanged (see execution_table row 2), so NULLIF(5, 3) returns 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the return value when expr1=5 and expr2=5?

A 5

B NULL

C 3

D Error

💡 Hint

Check execution_table row 1 under 'Return Value' column.

At which step does NULLIF return the first expression instead of NULL?

A Step 2

B Step 1

C Step 3

D Never

💡 Hint

See execution_table row 2 where expressions are not equal.

If expr1 is 10 and expr2 is 10, what would NULLIF return?

A 10

B 0

C NULL

D Error

💡 Hint

Recall NULLIF returns NULL when both expressions are equal (concept_flow).

Concept Snapshot

NULLIF(expr1, expr2)
- Compares expr1 and expr2
- Returns NULL if equal
- Returns expr1 if not equal
- Useful to avoid division by zero or replace values
- Simple conditional return function

Full Transcript

The NULLIF function in MySQL compares two expressions. If they are equal, it returns NULL. Otherwise, it returns the first expression. For example, NULLIF(5, 5) returns NULL because both values are equal. But NULLIF(5, 3) returns 5 because the values differ. This function helps replace specific values with NULL easily. The execution flow starts by evaluating both expressions, comparing them, and then returning NULL or the first expression based on equality.

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