IS DISTINCT FROM for NULL-safe comparison in PostgreSQL - Step-by-Step Execution

Concept Flow - IS DISTINCT FROM for NULL-safe comparison

Compare two values

↓

Are both NULL?

Yes→Return FALSE (not distinct)

No↓

Are values equal?

Yes→Return FALSE (not distinct)

No↓

Return TRUE (distinct)

This flow checks if two values are different, treating NULLs as equal only if both are NULL.

Execution Sample

PostgreSQL

SELECT 1 IS DISTINCT FROM 1 AS result1,
       1 IS DISTINCT FROM 2 AS result2,
       NULL IS DISTINCT FROM NULL AS result3,
       NULL IS DISTINCT FROM 1 AS result4;

This query compares pairs of values using IS DISTINCT FROM to safely handle NULLs.

Execution Table

Step	Comparison	Values Compared	Are Both NULL?	Are Values Equal?	Result
1	1 IS DISTINCT FROM 1	1 and 1	No	Yes	FALSE
2	1 IS DISTINCT FROM 2	1 and 2	No	No	TRUE
3	NULL IS DISTINCT FROM NULL	NULL and NULL	Yes	N/A	FALSE
4	NULL IS DISTINCT FROM 1	NULL and 1	No	No	TRUE

💡 All comparisons evaluated; NULL-safe logic applied to determine distinctness.

Variable Tracker

Variable	Step 1	Step 2	Step 3	Step 4
Values Compared	1 and 1	1 and 2	NULL and NULL	NULL and 1
Are Both NULL?	No	No	Yes	No
Are Values Equal?	Yes	No	N/A	No
Result	FALSE	TRUE	FALSE	TRUE

Key Moments - 2 Insights

Why does NULL IS DISTINCT FROM NULL return FALSE instead of TRUE?

Because IS DISTINCT FROM treats two NULLs as equal, so they are not distinct. See execution_table row 3.

How is IS DISTINCT FROM different from the usual <> operator when NULLs are involved?

The usual <> returns NULL (unknown) if either side is NULL, but IS DISTINCT FROM returns TRUE or FALSE safely. See execution_table rows 3 and 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of comparing 1 IS DISTINCT FROM 2?

A FALSE

B NULL

C TRUE

D Error

💡 Hint

Check execution_table row 2 under Result column.

At which step does the comparison find both values NULL?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Are Both NULL?' column in execution_table.

If we replaced IS DISTINCT FROM with <> in step 4 (NULL <> 1), what would be the result?

A TRUE

B NULL

C FALSE

D Error

💡 Hint

Recall that <> returns NULL if either side is NULL, unlike IS DISTINCT FROM.

Concept Snapshot

IS DISTINCT FROM compares two values treating NULLs safely.
Returns TRUE if values differ or one is NULL and the other is not.
Returns FALSE if values are equal or both NULL.
Unlike <> operator, it never returns NULL.
Useful for NULL-safe equality checks in PostgreSQL.

Full Transcript

The IS DISTINCT FROM operator in PostgreSQL compares two values and returns TRUE if they are different, including when one is NULL and the other is not. It returns FALSE if the values are equal or both are NULL. This operator is NULL-safe, meaning it treats two NULLs as equal, unlike the standard <> operator which returns NULL when comparing with NULLs. The execution flow first checks if both values are NULL, then if they are equal, and finally returns TRUE if neither condition is met. This makes IS DISTINCT FROM very useful for comparisons where NULLs might appear and you want a clear TRUE or FALSE result.

Practice

1. What does the IS DISTINCT FROM operator do in PostgreSQL when comparing two values?

easy

A. It returns true if the values are different or one is NULL and the other is not.

B. It returns true only if both values are NULL.

C. It returns false if the values are different.

D. It treats NULL as equal to any value.

Solution

1. Step 1: Understand the behavior of IS DISTINCT FROM

   This operator treats NULL as a real value and compares two values safely even if one or both are NULL.

2. Step 2: Analyze the comparison results

   It returns true if the values differ or if one is NULL and the other is not, otherwise false.

3. Final Answer:

   It returns true if the values are different or one is NULL and the other is not. -> Option A

4. Quick Check:

   IS DISTINCT FROM returns true for different or NULL vs non-NULL [OK]

Hint: Think: NULL counts as a real value in IS DISTINCT FROM [OK]

Common Mistakes:

• Assuming NULL equals NULL
• Confusing IS DISTINCT FROM with = operator
• Thinking it returns true only when both are NULL

2. Which of the following is the correct syntax to check if column a is distinct from column b in PostgreSQL?

easy

A. a <> b OR a IS NULL OR b IS NULL

B. a != b

C. a IS DISTINCT FROM b

D. a IS NOT DISTINCT FROM b

Solution

1. Step 1: Recall the correct syntax for NULL-safe comparison

   The operator IS DISTINCT FROM is used as a IS DISTINCT FROM b to compare two values safely including NULLs.

2. Step 2: Evaluate other options

   a != b uses standard inequality which fails with NULLs. a <> b OR a IS NULL OR b IS NULL is a complex expression but not the standard syntax. a IS NOT DISTINCT FROM b checks for NOT DISTINCT, which is the opposite.

3. Final Answer:

   a IS DISTINCT FROM b -> Option C

4. Quick Check:

   Use IS DISTINCT FROM for NULL-safe inequality [OK]

Hint: Use IS DISTINCT FROM for NULL-safe comparisons [OK]

Common Mistakes:

• Using != which fails with NULLs
• Writing complex OR conditions instead of operator
• Confusing IS DISTINCT FROM with IS NOT DISTINCT FROM

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

SELECT id FROM items WHERE value IS DISTINCT FROM NULL;

Assuming items has rows:
(1, NULL), (2, 10), (3, NULL), (4, 20)

medium

A. [1, 2, 3, 4]

B. [2, 4]

C. [1, 3]

D. [] (empty list)

Solution

1. Step 1: Understand the condition value IS DISTINCT FROM NULL

   This condition returns true if value is not NULL, because NULL is treated as a real value.

2. Step 2: Apply condition to each row

   Rows with value 10 and 20 are distinct from NULL, so ids 2 and 4 match.

3. Final Answer:

   [2, 4] -> Option B

4. Quick Check:

   IS DISTINCT FROM NULL filters non-NULL values [OK]

Hint: IS DISTINCT FROM NULL means NOT NULL [OK]

Common Mistakes:

• Thinking it returns rows with NULL values
• Confusing IS DISTINCT FROM with IS NULL
• Assuming it returns all rows

4. You wrote this query but it returns no rows:

SELECT * FROM users WHERE email IS DISTINCT FROM NULL;

What is the likely mistake?

medium

A. The email column has only NULL values

B. Using IS DISTINCT FROM NULL instead of IS NOT NULL

C. IS DISTINCT FROM cannot be used with NULL

D. The table users is empty

Solution

1. Step 1: Understand what email IS DISTINCT FROM NULL means

   This condition returns true for all rows where email is NOT NULL.

2. Step 2: Analyze why no rows are returned

   If no rows are returned, it means no rows satisfy the condition, so all email values are NULL or the table is empty.

3. Step 3: Eliminate other options

   Using IS DISTINCT FROM NULL instead of IS NOT NULL is incorrect because IS DISTINCT FROM NULL is valid. The table users is empty is possible but less likely. IS DISTINCT FROM cannot be used with NULL is false because the operator supports NULL.

4. Final Answer:

   The email column has only NULL values -> Option A

5. Quick Check:

   No rows means no non-NULL emails [OK]

Hint: No rows means condition never true; check NULL presence [OK]

Common Mistakes:

• Thinking IS DISTINCT FROM NULL is invalid syntax
• Assuming table is empty without checking
• Confusing IS DISTINCT FROM NULL with IS NULL

5. You want to find all pairs of rows in table records where col1 and col2 differ, treating NULLs as real values. Which query correctly finds these pairs?

hard

A. SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 IS NOT DISTINCT FROM r2.col1 AND r1.col2 IS NOT DISTINCT FROM r2.col2;

B. SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 <> r2.col1 OR r1.col2 <> r2.col2;

C. SELECT * FROM records r1 JOIN records r2 ON r1.id = r2.id WHERE r1.col1 IS DISTINCT FROM r2.col1 AND r1.col2 IS DISTINCT FROM r2.col2;

D. SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 IS DISTINCT FROM r2.col1 OR r1.col2 IS DISTINCT FROM r2.col2;

Solution

1. Step 1: Understand the goal

   We want pairs where col1 or col2 differ, treating NULLs as real values.

2. Step 2: Analyze each option

   SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 IS DISTINCT FROM r2.col1 OR r1.col2 IS DISTINCT FROM r2.col2; uses IS DISTINCT FROM with OR, correctly finding pairs differing in either column. SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 <> r2.col1 OR r1.col2 <> r2.col2; uses <> which fails with NULLs. SELECT * FROM records r1 JOIN records r2 ON r1.id = r2.id WHERE r1.col1 IS DISTINCT FROM r2.col1 AND r1.col2 IS DISTINCT FROM r2.col2; joins on equal ids, so no pairs. SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 IS NOT DISTINCT FROM r2.col1 AND r1.col2 IS NOT DISTINCT FROM r2.col2; finds pairs where columns are NOT distinct, opposite of goal.

3. Final Answer:

   SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 IS DISTINCT FROM r2.col1 OR r1.col2 IS DISTINCT FROM r2.col2 -> Option D

4. Quick Check:

   Use IS DISTINCT FROM with OR for NULL-safe difference [OK]

Hint: Use IS DISTINCT FROM with OR to find any column differences [OK]

Common Mistakes:

• Using <> which fails with NULLs
• Joining on equal ids instead of different ids
• Using IS NOT DISTINCT FROM which finds equal rows