COALESCE and NULLIF as CASE shortcuts in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of COALESCE with NULL values

Given the table Employees with columns id, name, and nickname, what is the output of this query?

SELECT id, COALESCE(nickname, name) AS display_name FROM Employees ORDER BY id;

Assume the data:

id | name    | nickname
1  | Alice   | NULL
2  | Bob     | Bobby
3  | Charlie | NULL

SQL

SELECT id, COALESCE(nickname, name) AS display_name FROM Employees ORDER BY id;

A [{"id":1,"display_name":null},{"id":2,"display_name":"Bobby"},{"id":3,"display_name":null}]

B [{"id":1,"display_name":"Alice"},{"id":2,"display_name":"Bobby"},{"id":3,"display_name":"Charlie"}]

C [{"id":1,"display_name":"nickname"},{"id":2,"display_name":"Bobby"},{"id":3,"display_name":"nickname"}]

D [{"id":1,"display_name":"Alice"},{"id":2,"display_name":"Bob"},{"id":3,"display_name":"Charlie"}]

💡 Hint

COALESCE returns the first non-NULL value from its arguments.

COALESCE(nickname, name) returns nickname if it is not NULL; otherwise, it returns name. For id=1 and 3, nickname is NULL, so name is used. For id=2, nickname is 'Bobby', so it is used.

❓ query_result

intermediate

Using NULLIF to avoid division by zero

Consider a table Sales with columns region, total_sales, and total_customers. What is the output of this query?

SELECT region, total_sales / NULLIF(total_customers, 0) AS avg_sale_per_customer FROM Sales ORDER BY region;

Given data:

region | total_sales | total_customers
East   | 1000        | 10
West   | 500         | 0
North  | 750         | 15

SQL

SELECT region, total_sales / NULLIF(total_customers, 0) AS avg_sale_per_customer FROM Sales ORDER BY region;

A [{"region":"East","avg_sale_per_customer":100},{"region":"North","avg_sale_per_customer":50},{"region":"West","avg_sale_per_customer":null}]

B [{"region":"East","avg_sale_per_customer":100},{"region":"North","avg_sale_per_customer":50},{"region":"West","avg_sale_per_customer":0}]

C [{"region":"East","avg_sale_per_customer":100},{"region":"North","avg_sale_per_customer":50},{"region":"West","avg_sale_per_customer":"Infinity"}]

D [{"region":"East","avg_sale_per_customer":100},{"region":"North","avg_sale_per_customer":50},{"region":"West","avg_sale_per_customer":500}]

💡 Hint

NULLIF returns NULL if the second argument matches the first, preventing division by zero.

NULLIF(total_customers, 0) returns NULL when total_customers is 0, so division by zero is avoided and result is NULL for West region.

📝 Syntax

advanced

Identify the syntax error in COALESCE usage

Which of the following SQL statements will cause a syntax error?

A SELECT COALESCE(NULL, NULL, 'fallback') AS result;

B SELECT COALESCE('value1', 'value2', NULL) AS result;

C SELECT COALESCE(NULL, 'default') AS result;

D SELECT COALESCE(NULL 'default') AS result;

💡 Hint

Check the commas separating arguments in COALESCE.

Option D misses a comma between NULL and 'default', causing a syntax error.

🧠 Conceptual

advanced

Understanding NULLIF behavior

What is the result of the expression NULLIF('abc', 'abc') in SQL?

A 'abc'

B Error

C NULL

D '' (empty string)

💡 Hint

NULLIF returns NULL if both arguments are equal.

NULLIF returns NULL when both arguments are equal, otherwise returns the first argument.

❓ optimization

expert

Optimizing CASE with COALESCE

You want to replace this CASE statement with a simpler expression:

SELECT CASE WHEN col1 IS NOT NULL THEN col1 ELSE col2 END AS result FROM table1;

Which of the following is the best equivalent SQL expression?

A SELECT COALESCE(col1, col2) AS result FROM table1;

B SELECT NULLIF(col1, col2) AS result FROM table1;

C SELECT CASE WHEN col1 = col2 THEN col1 ELSE col2 END AS result FROM table1;

D SELECT IFNULL(col2, col1) AS result FROM table1;

💡 Hint

COALESCE returns the first non-NULL value.

COALESCE(col1, col2) returns col1 if it is not NULL, else col2, matching the CASE logic.

Practice

1. What does the SQL function COALESCE do?

easy

A. Returns the first non-NULL value from its arguments

B. Returns NULL if two values are equal

C. Always returns the last value in the list

D. Converts NULL values to zero

Solution

1. Step 1: Understand COALESCE behavior

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

2. Step 2: Compare with other options

   NULLIF returns NULL if two values are equal, which is different. COALESCE does not convert NULL to zero automatically.

3. Final Answer:

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

4. Quick Check:

   COALESCE = first non-NULL [OK]

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

Common Mistakes:

• Confusing COALESCE with NULLIF
• Thinking COALESCE converts NULL to zero
• Assuming COALESCE returns last value

2. Which of the following is the correct syntax for using NULLIF in SQL?

easy

A. NULLIF(value1, value2)

B. NULLIF(value1 = value2)

C. NULLIF(value1; value2)

D. NULLIF(value1, value2, value3)

Solution

1. Step 1: Recall NULLIF syntax

   NULLIF takes exactly two arguments separated by a comma: NULLIF(value1, value2).

2. Step 2: Eliminate incorrect options

   NULLIF(value1 = value2) uses an equality inside parentheses, which is invalid syntax. NULLIF(value1; value2) uses a semicolon instead of a comma. NULLIF(value1, value2, value3) has three arguments, which is not allowed.

3. Final Answer:

   NULLIF(value1, value2) -> Option A

4. Quick Check:

   NULLIF syntax = NULLIF(val1, val2) [OK]

Hint: NULLIF takes exactly two comma-separated values [OK]

Common Mistakes:

• Using semicolon instead of comma
• Passing more than two arguments
• Using an expression inside NULLIF parentheses

3. Given the table Employees with columns id, bonus (nullable), what is the result of this query?

SELECT id, COALESCE(bonus, 100) AS bonus_amount FROM Employees WHERE id = 1;

Assuming the row with id=1 has bonus = NULL.

medium

A. No rows returned

B. [{"id": 1, "bonus_amount": 100}]

C. [{ "id": 1, "bonus_amount": 0 }]

D. [{ "id": 1, "bonus_amount": NULL }]

Solution

1. Step 1: Understand COALESCE with NULL bonus

   Since bonus is NULL for id=1, COALESCE returns the second argument, 100.

2. Step 2: Check query output

   The query selects id=1 and bonus_amount as 100, so the result is [{"id": 1, "bonus_amount": 100}].

3. Final Answer:

   [{"id": 1, "bonus_amount": 100}] -> Option B

4. Quick Check:

   COALESCE(NULL, 100) = 100 [OK]

Hint: COALESCE returns first non-NULL, here 100 [OK]

Common Mistakes:

• Assuming NULL is returned instead of 100
• Confusing COALESCE with NULLIF
• Expecting zero instead of 100

4. Identify the error in this SQL snippet:

SELECT id, NULLIF(bonus) AS result FROM Employees;

medium

A. Missing WHERE clause

B. NULLIF cannot be used in SELECT clause

C. NULLIF requires two arguments, only one given

D. NULLIF returns wrong data type

Solution

1. Step 1: Check NULLIF function usage

   NULLIF requires exactly two arguments, but only one (bonus) is provided here.

2. Step 2: Validate other options

   NULLIF can be used in SELECT. WHERE clause is optional. NULLIF does not cause data type errors by itself.

3. Final Answer:

   NULLIF requires two arguments, only one given -> Option C

4. Quick Check:

   NULLIF needs two values [OK]

Hint: NULLIF always needs two arguments [OK]

Common Mistakes:

• Using NULLIF with one argument
• Thinking WHERE clause is mandatory
• Assuming NULLIF changes data type

5. You want to replace a value with NULL if it equals zero, otherwise keep it as is. Which SQL expression using NULLIF achieves this?

hard

A. CASE WHEN value = 0 THEN value ELSE NULL END

B. COALESCE(value, 0)

C. NULLIF(0, value)

D. NULLIF(value, 0)

Solution

1. Step 1: Understand NULLIF behavior

   NULLIF(value, 0) returns NULL if value equals 0, else returns value unchanged.

2. Step 2: Compare other options

   COALESCE returns first non-NULL, not conditional replacement. CASE in CASE WHEN value = 0 THEN value ELSE NULL END returns value when zero, opposite of requirement. NULLIF(0, value) returns NULL if 0 equals value, which is reversed logic.

3. Final Answer:

   NULLIF(value, 0) -> Option D

4. Quick Check:

   NULLIF(value, 0) replaces zero with NULL [OK]

Hint: NULLIF(value, 0) returns NULL if value is zero [OK]

Common Mistakes:

• Using COALESCE instead of NULLIF
• Reversing argument order in NULLIF
• Writing CASE with wrong THEN/ELSE logic