COALESCE and NULLIF as CASE shortcuts in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the first non-null value between column1 and column2.

SQL

SELECT COALESCE(column1, [1]) AS result FROM table_name;

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

A'default'

Bcolumn3

CNULL

Dcolumn2

Attempts:

3 left

💡 Hint

Common Mistakes

Using a literal string instead of a column name.

Using NULL as the second argument which always returns NULL if column1 is NULL.

The COALESCE function returns the first non-null value from the list. Here, it checks column1 first, then column2.

2fill in blank

medium

Complete the code to return NULL if column1 equals column2, otherwise return column1.

SQL

SELECT NULLIF(column1, [1]) AS result FROM table_name;

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

Acolumn2

Bcolumn3

C'value'

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing column1 with a literal instead of column2.

Using COALESCE instead of NULLIF.

NULLIF returns NULL if the two arguments are equal. Here, it compares column1 and column2.

3fill in blank

hard

Fix the error in the code to return the first non-null value among column1, column2, and 'N/A'.

SQL

SELECT COALESCE([1], column2, 'N/A') AS result FROM table_name;

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

Acolumn3

B'N/A'

Ccolumn1

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using a literal or NULL as the first argument instead of column1.

Putting 'N/A' before columns which prevents checking columns.

The first argument to COALESCE should be column1 to check it first for a non-null value.

4fill in blank

hard

Fill both blanks to return 'Unknown' if column1 equals column2, else return column1 or 'Unknown' if column1 is NULL.

SQL

SELECT COALESCE(NULLIF(column1, [1]), [2]) AS result FROM table_name;

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

Acolumn2

B'Unknown'

Ccolumn3

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using NULL instead of 'Unknown' as fallback.

Comparing column1 with a wrong column.

NULLIF(column1, column2) returns NULL if equal, then COALESCE returns 'Unknown' if NULL, else column1.

5fill in blank

hard

Fill all three blanks to return the uppercase of column1 if UPPER(column1) is not equal to column2 and column1 is not NULL, else return 'DEFAULT'.

SQL

SELECT COALESCE(NULLIF([1], [2]), [3]) AS result FROM table_name;

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

AUPPER(column1)

Bcolumn2

C'DEFAULT'

Dcolumn1

Attempts:

3 left

💡 Hint

Common Mistakes

Using column1 instead of UPPER(column1).

Using NULL instead of 'DEFAULT' as fallback.

NULLIF compares UPPER(column1) and column2; if equal returns NULL, then COALESCE returns 'DEFAULT' if NULL.

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