COALESCE for NULL handling in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the COALESCE function do in SQL?

COALESCE returns the first non-NULL value from a list of expressions. It helps handle NULL values by providing a fallback value.

beginner

How does COALESCE handle multiple NULL values?

COALESCE checks each value in order and returns the first one that is not NULL. If all values are NULL, it returns NULL.

beginner

Write a COALESCE example to return 'Unknown' if a column 'name' is NULL.

SELECT COALESCE(name, 'Unknown') FROM table_name;

intermediate

Why is COALESCE preferred over CASE WHEN for NULL handling?

COALESCE is simpler and more readable for checking multiple NULL values compared to writing multiple CASE WHEN conditions.

intermediate

Can COALESCE handle different data types in its arguments?

All arguments in COALESCE must be of compatible data types or implicitly convertible to a common type; otherwise, it causes an error.

What will COALESCE(NULL, NULL, 'Hello', 'World') return?

A 'Hello'

B NULL

C 'World'

D Error

COALESCE returns the first non-NULL value, which is 'Hello' here.

If all arguments to COALESCE are NULL, what is the result?

A Empty string

B Error

C 0

D NULL

COALESCE returns NULL if all arguments are NULL.

Which SQL function is best for replacing NULL values with a default?

A COUNT()

B SUM()

C COALESCE()

D AVG()

COALESCE is designed to replace NULL values with the first non-NULL value.

What happens if COALESCE arguments have incompatible data types?

A Returns first argument

B Raises an error

C Returns NULL

D Converts all to string

COALESCE requires compatible data types; otherwise, it raises an error.

Which of these is a valid COALESCE usage?

A COALESCE(name, 'No Name')

B COALESCE()

C COALESCE(5, 'text')

D COALESCE(name, 0)

COALESCE(name, 'No Name') is valid if 'name' is text or nullable text. Mixing incompatible types like number and text causes errors.

Explain how COALESCE helps handle NULL values in SQL queries.

Think about what happens when a column has missing data.

Describe a scenario where using COALESCE improves query results.

Imagine showing user names but some are missing.

Practice

1. What does the COALESCE function do in PostgreSQL?

easy

A. Returns the first non-NULL value from the list of arguments

B. Returns the last value in the list regardless of NULL

C. Counts the number of NULL values in a column

D. Deletes rows with NULL values

Solution

1. Step 1: Understand COALESCE purpose

   COALESCE checks each argument in order and returns the first one that is not NULL.

2. Step 2: Compare options with definition

   Only Returns the first non-NULL value from the list of arguments correctly describes this behavior; others describe unrelated functions.

3. Final Answer:

   Returns the first non-NULL value from the list of arguments -> Option A

4. Quick Check:

   COALESCE = first non-NULL value [OK]

Hint: COALESCE picks first non-NULL value in order [OK]

Common Mistakes:

• Thinking COALESCE counts NULLs
• Assuming it returns last value
• Confusing with DELETE or COUNT functions

2. Which of the following is the correct syntax to use COALESCE in PostgreSQL?

easy

A. COALESCE(value1; value2; ...; valueN)

B. COALESCE[value1, value2, ..., valueN]

C. COALESCE(value1, value2, ..., valueN)

D. COALESCE{value1, value2, ..., valueN}

Solution

1. Step 1: Recall function call syntax in SQL

   Functions use parentheses with comma-separated arguments.

2. Step 2: Match options to syntax rules

   Only COALESCE(value1, value2, ..., valueN) uses parentheses and commas correctly; others use invalid separators or brackets.

3. Final Answer:

   COALESCE(value1, value2, ..., valueN) -> Option C

4. Quick Check:

   Function syntax = parentheses + commas [OK]

Hint: Use parentheses and commas for SQL functions [OK]

Common Mistakes:

• Using semicolons instead of commas
• Using square or curly brackets
• Omitting parentheses

3. Given the table users with columns id, nickname, and fullname, what will this query return?

SELECT id, COALESCE(nickname, fullname, 'Anonymous') AS display_name FROM users;

Assuming a row has nickname = NULL and fullname = 'John Doe'.

medium

A. NULL

B. 'John Doe'

C. 'Anonymous'

D. nickname value

Solution

1. Step 1: Evaluate COALESCE arguments in order

   First argument is nickname which is NULL, so move to next.

2. Step 2: Check fullname value

   fullname is 'John Doe', which is not NULL, so COALESCE returns 'John Doe'.

3. Final Answer:

   'John Doe' -> Option B

4. Quick Check:

   First non-NULL = 'John Doe' [OK]

Hint: COALESCE returns first non-NULL value in order [OK]

Common Mistakes:

• Assuming NULL is returned if first value is NULL
• Ignoring order of arguments
• Choosing default 'Anonymous' incorrectly

4. Identify the error in this query:

SELECT id, COALESCE nickname, fullname, 'Guest' AS display_name FROM users;

medium

A. AS keyword misplaced

B. Wrong keyword instead of COALESCE

C. No error, query is correct

D. Missing parentheses around COALESCE arguments

Solution

1. Step 1: Check COALESCE function syntax

   COALESCE requires parentheses around its arguments.

2. Step 2: Analyze given query

   Query uses COALESCE without parentheses, causing syntax error.

3. Final Answer:

   Missing parentheses around COALESCE arguments -> Option D

4. Quick Check:

   Functions need parentheses [OK]

Hint: Always use parentheses with COALESCE arguments [OK]

Common Mistakes:

• Omitting parentheses for functions
• Misplacing AS keyword
• Confusing COALESCE with other functions

5. You want to display a user's preferred contact method from columns email, phone, and address. If all are NULL, show 'No contact info'. Which query correctly uses COALESCE to achieve this?

hard

A. SELECT id, COALESCE(email, phone, address, 'No contact info') AS contact FROM users;

B. SELECT id, COALESCE(email, phone, address) OR 'No contact info' AS contact FROM users;

C. SELECT id, COALESCE(email, phone, address) ?? 'No contact info' AS contact FROM users;

D. SELECT id, COALESCE(email, phone, address, NULL) AS contact FROM users;

Solution

1. Step 1: Understand COALESCE usage for defaults

   COALESCE returns first non-NULL; adding 'No contact info' as last argument provides default.

2. Step 2: Evaluate each option's correctness

   SELECT id, COALESCE(email, phone, address, 'No contact info') AS contact FROM users; correctly lists all contact columns plus default string. Options B and C use invalid operators, D ends with NULL, no default.

3. Final Answer:

   SELECT id, COALESCE(email, phone, address, 'No contact info') AS contact FROM users; -> Option A

4. Quick Check:

   Default last in COALESCE list [OK]

Hint: Put default last inside COALESCE parentheses [OK]

Common Mistakes:

• Using OR or ?? instead of COALESCE
• Not providing a default value
• Ending COALESCE with NULL instead of default