Boolean type behavior in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What values can a PostgreSQL Boolean type store?

A PostgreSQL Boolean type can store three values: true, false, and NULL (unknown).

beginner

How does PostgreSQL interpret the string 't' when cast to Boolean?

PostgreSQL interprets the string 't' as true when cast to Boolean.

intermediate

What happens when you use a Boolean column in a WHERE clause in PostgreSQL?

The WHERE clause treats true as passing the condition, false as failing, and NULL as unknown, so rows with NULL in that column are excluded unless explicitly handled.

intermediate

Can you use Boolean values in arithmetic operations in PostgreSQL?

No, Boolean values cannot be directly used in arithmetic operations. You must cast them to integers (true = 1, false = 0) first.

beginner

How do you explicitly cast a Boolean to text in PostgreSQL?

Use the CAST operator or :: syntax, for example: SELECT true::text; returns 't'.

Which of these is NOT a valid Boolean literal in PostgreSQL?

A 'xyz'

B 'true'

C 'f'

D 'false'

PostgreSQL accepts 'true', 'false', 't', 'f' as Boolean literals, but 'xyz' is not recognized as a Boolean.

What does the expression SELECT NULL::boolean; return?

A Error

B false

C true

D NULL

Casting NULL to boolean results in NULL, representing an unknown Boolean value.

In a WHERE clause, what happens to rows where the Boolean column is NULL?

A They are excluded

B They are included

C They cause an error

D They are converted to false

Rows with NULL in a Boolean column are excluded because NULL means unknown, which does not satisfy the WHERE condition.

How can you convert a Boolean true to integer 1 in PostgreSQL?

A true * 1

B true + 1

C CAST(true AS integer)

D true::text

You must explicitly cast Boolean to integer using CAST or :: syntax; arithmetic operations on Boolean are not allowed.

Which of these is the correct way to cast Boolean false to text?

A CAST(false AS integer)

B false::text

C CAST(false AS boolean)

D false + ''

Casting Boolean false to text uses false::text or CAST(false AS text).

Explain how PostgreSQL handles Boolean values and their behavior in queries.

Think about how true, false, and NULL affect filtering and casting.

Describe how to convert Boolean values to other types in PostgreSQL and why it might be necessary.

Consider when you want to display or calculate with Boolean values.

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