Boolean type behavior in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Boolean comparison in WHERE clause

Given the table users with a boolean column is_active, what rows will be returned by this query?

SELECT * FROM users WHERE is_active = TRUE;

PostgreSQL

CREATE TABLE users (id SERIAL PRIMARY KEY, name TEXT, is_active BOOLEAN);
INSERT INTO users (name, is_active) VALUES ('Alice', TRUE), ('Bob', FALSE), ('Carol', NULL);

A Only rows where is_active is TRUE (e.g., Alice)

B Rows where is_active is TRUE or NULL (Alice and Carol)

C All rows regardless of is_active value

D Only rows where is_active is FALSE (Bob)

💡 Hint

Remember that in SQL, NULL means unknown and does not equal TRUE or FALSE.

The condition 'is_active = TRUE' filters only rows where is_active is TRUE. Rows with FALSE or NULL do not satisfy this condition.

❓ query_result

intermediate

Boolean logic with NULL values

What is the result of this query?

SELECT TRUE AND NULL AS result;

A FALSE

B NULL

C TRUE

D Syntax error

💡 Hint

Think about how SQL treats unknown values in boolean logic.

In SQL, TRUE AND NULL results in NULL because NULL represents unknown, so the result is unknown.

📝 Syntax

advanced

Correct boolean literal usage in PostgreSQL

Which option uses the correct boolean literal in PostgreSQL to insert a TRUE value?

A INSERT INTO flags (flag) VALUES ('true');

B INSERT INTO flags (flag) VALUES ('yes');

C INSERT INTO flags (flag) VALUES (1);

D INSERT INTO flags (flag) VALUES (TRUE);

💡 Hint

PostgreSQL accepts boolean literals TRUE and FALSE without quotes.

PostgreSQL boolean literals are TRUE and FALSE without quotes. Using strings or numbers may cause errors or implicit casts.

❓ query_result

advanced

Boolean expression evaluation in SELECT

What is the output of this query?

SELECT
  CASE WHEN FALSE OR NULL THEN 'Yes' ELSE 'No' END AS answer;

A 'Yes'

B NULL

C 'No'

D Syntax error

💡 Hint

Remember how OR works with NULL in SQL's three-valued logic.

FALSE OR NULL evaluates to NULL (unknown), so the CASE condition is false, returning 'No'.

🧠 Conceptual

expert

Behavior of boolean aggregates with NULLs

Consider a table checks with a boolean column passed containing TRUE, FALSE, and NULL values. What does the aggregate function bool_and(passed) return if any row has FALSE, but some rows have NULL?

A FALSE

B TRUE

C NULL

D Error

💡 Hint

bool_and returns TRUE only if all non-null inputs are TRUE.

bool_and returns FALSE if any input is FALSE, ignoring NULLs. So presence of FALSE causes FALSE result.

Practice

1. In PostgreSQL, which of the following is a valid boolean value stored in a boolean column?

easy

A. TRUE

B. 2

C. 'foo'

D. 'NULL'

Solution

1. Step 1: Understand boolean data type values

   PostgreSQL boolean columns accept TRUE, FALSE, or NULL (unknown).

2. Step 2: Identify valid boolean literals

   TRUE and FALSE are valid boolean literals; 2, 'foo', and 'NULL' are not valid boolean literals in PostgreSQL.

3. Final Answer:

   TRUE -> Option A

4. Quick Check:

   Boolean values = TRUE or FALSE [OK]

Hint: Boolean columns accept TRUE, FALSE, or NULL only [OK]

Common Mistakes:

• Using 1 or 0 as boolean values directly
• Using strings like 'yes' or 'no' as boolean
• Confusing NULL with a boolean TRUE or FALSE

2. Which of the following is the correct way to declare a boolean column named is_active in a PostgreSQL table?

easy

A. CREATE TABLE users (is_active BOOLEAN(1));

B. CREATE TABLE users (is_active BOOLEAN);

C. CREATE TABLE users (is_active BOOL(1));

D. CREATE TABLE users (is_active BOOLIAN);

Solution

1. Step 1: Check correct boolean type syntax

   PostgreSQL uses BOOLEAN or BOOL as valid types; BOOLEAN(1) is invalid.

2. Step 2: Identify syntax errors

   A uses invalid BOOLEAN(1) syntax; B has a typo 'BOOLIAN'; C uses invalid BOOL(1) syntax.

3. Final Answer:

   CREATE TABLE users (is_active BOOLEAN); -> Option B

4. Quick Check:

   BOOLEAN keyword is correct type [OK]

Hint: Use BOOLEAN or BOOL, no size or typo allowed [OK]

Common Mistakes:

• Typo in BOOLEAN keyword
• Adding size like BOOLEAN(1)
• Using unsupported types like BOOLIAN

3. Given the table settings with a boolean column enabled, what will this query return?

SELECT enabled FROM settings WHERE enabled = FALSE;

medium

A. All rows where enabled is TRUE

B. All rows where enabled is NULL

C. All rows where enabled is FALSE

D. Syntax error

Solution

1. Step 1: Understand the WHERE clause condition

   The query filters rows where the boolean column enabled equals FALSE.

2. Step 2: Determine which rows match

   Only rows with enabled = FALSE will be returned; TRUE or NULL rows are excluded.

3. Final Answer:

   All rows where enabled is FALSE -> Option C

4. Quick Check:

   WHERE enabled = FALSE returns FALSE rows [OK]

Hint: WHERE boolean_column = FALSE returns only FALSE rows [OK]

Common Mistakes:

• Assuming NULL rows are included
• Confusing TRUE and FALSE values
• Thinking it returns all rows

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

SELECT * FROM users WHERE is_admin = TRUE;

The is_admin column is boolean but some rows have NULL. What is the likely issue?

medium

A. TRUE is not a valid boolean value in PostgreSQL

B. The query syntax is incorrect

C. The column should be text, not boolean

D. NULL values cause the condition to fail, so no rows with NULL appear

Solution

1. Step 1: Understand boolean NULL behavior in WHERE clause

   Comparing boolean column to TRUE excludes NULL rows because NULL means unknown, not TRUE.

2. Step 2: Explain why no rows returned

   Rows with NULL in is_admin do not satisfy is_admin = TRUE, so they are excluded.

3. Final Answer:

   NULL values cause the condition to fail, so no rows with NULL appear -> Option D

4. Quick Check:

   NULL in boolean = TRUE returns no rows [OK]

Hint: NULL in boolean columns excludes rows in equality checks [OK]

Common Mistakes:

• Thinking NULL equals TRUE
• Assuming TRUE is invalid boolean
• Believing syntax error caused no rows

5. You want to select all rows from tasks where the boolean column completed is either FALSE or NULL. Which query correctly does this?

hard

A. SELECT * FROM tasks WHERE completed = FALSE OR completed IS NULL;

B. SELECT * FROM tasks WHERE completed = FALSE AND completed IS NULL;

C. SELECT * FROM tasks WHERE NOT completed;

D. SELECT * FROM tasks WHERE completed != TRUE;

Solution

1. Step 1: Understand boolean NULL and FALSE filtering

   To get rows where completed is FALSE or NULL, use OR with completed = FALSE and completed IS NULL.

2. Step 2: Analyze each option

   SELECT * FROM tasks WHERE completed = FALSE AND completed IS NULL; uses AND, which cannot be both FALSE and NULL simultaneously. SELECT * FROM tasks WHERE NOT completed; excludes NULL rows. SELECT * FROM tasks WHERE completed != TRUE; excludes TRUE and NULL (since NULL != TRUE is unknown), includes only FALSE.

3. Final Answer:

   SELECT * FROM tasks WHERE completed = FALSE OR completed IS NULL; -> Option A

4. Quick Check:

   Use OR to include FALSE or NULL rows [OK]

Hint: Use OR with IS NULL to include unknown boolean values [OK]

Common Mistakes:

• Using AND instead of OR
• Ignoring NULL rows in boolean filters
• Using NOT completed excludes NULL rows