IS DISTINCT FROM for NULL-safe comparison in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select rows where column_a is distinct from column_b, including NULL-safe comparison.

PostgreSQL

SELECT * FROM table_name WHERE column_a [1] column_b;

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

AIS NOT NULL

B=

C!=

DIS DISTINCT FROM

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' operator which returns NULL when comparing with NULL values.

Using '!=' which does not handle NULLs properly.

The IS DISTINCT FROM operator compares two values and treats NULLs as distinct values, unlike the standard = operator.

2fill in blank

medium

Complete the code to select rows where column_a is NOT distinct from column_b, including NULL-safe comparison.

PostgreSQL

SELECT * FROM table_name WHERE column_a [1] column_b;

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

AIS NOT DISTINCT FROM

BIS DISTINCT FROM

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' which returns NULL when comparing NULLs.

Using '!=' which does not handle NULLs properly.

The IS NOT DISTINCT FROM operator returns true when values are equal or both NULL, providing NULL-safe equality comparison.

3fill in blank

hard

Fix the error in the WHERE clause to correctly compare column_a and column_b with NULL-safe comparison.

PostgreSQL

SELECT * FROM table_name WHERE column_a [1] column_b;

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

A=

B!=

CIS DISTINCT FROM

DIS NOT NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving '=' which causes NULL comparison issues.

Using '!=' which does not handle NULLs correctly.

Using IS DISTINCT FROM ensures NULL-safe comparison, unlike '=' which returns NULL if either side is NULL.

4fill in blank

hard

Fill both blanks to select rows where column_x is distinct from column_y and column_z is NOT distinct from column_w.

PostgreSQL

SELECT * FROM table_name WHERE column_x [1] column_y AND column_z [2] column_w;

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

AIS DISTINCT FROM

B=

CIS NOT DISTINCT FROM

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' or '!=' which do not handle NULLs properly.

Mixing up IS DISTINCT FROM and IS NOT DISTINCT FROM.

The first blank uses IS DISTINCT FROM to find rows where column_x and column_y differ including NULLs. The second blank uses IS NOT DISTINCT FROM to find rows where column_z and column_w are equal or both NULL.

5fill in blank

hard

Fill all three blanks to select rows where column_a is distinct from column_b, column_c is NOT distinct from column_d, and column_e is distinct from NULL.

PostgreSQL

SELECT * FROM table_name WHERE column_a [1] column_b AND column_c [2] column_d AND column_e [3] NULL;

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

AIS DISTINCT FROM

BIS NOT DISTINCT FROM

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' to compare with NULL which returns NULL instead of true/false.

Confusing IS DISTINCT FROM and IS NOT DISTINCT FROM.

The first blank uses IS DISTINCT FROM to compare column_a and column_b safely. The second blank uses IS NOT DISTINCT FROM for column_c and column_d. The third blank uses IS DISTINCT FROM to compare column_e with NULL safely.

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