GREATEST and LEAST functions in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of GREATEST with NULL values

What is the output of this PostgreSQL query?

SELECT GREATEST(5, NULL, 3);

PostgreSQL

SELECT GREATEST(5, NULL, 3);

A Error

B 5

C 3

D NULL

💡 Hint

Remember how GREATEST handles NULL values in PostgreSQL.

In PostgreSQL, GREATEST returns NULL if any argument is NULL unless you explicitly handle NULLs.

❓ query_result

intermediate

LEAST function with mixed data types

What is the result of this query?

SELECT LEAST('10', '2', '30');

Note: The values are strings.

PostgreSQL

SELECT LEAST('10', '2', '30');

A '10'

B Error

C '30'

D '2'

💡 Hint

Strings are compared lexicographically in PostgreSQL.

LEAST compares strings lexicographically. '10' < '2' < '30' because the first characters '1' < '2' < '3', so '10' is the smallest.

📝 Syntax

advanced

Identify the syntax error in GREATEST usage

Which option contains a syntax error when using the GREATEST function?

A SELECT GREATEST(1, 2, 3);

B SELECT GREATEST(5, 10);

C SELECT GREATEST(1, 2,);

D SELECT GREATEST(7);

💡 Hint

Check for trailing commas in function arguments.

Option C has a trailing comma after 2, which is a syntax error in PostgreSQL function calls.

❓ optimization

advanced

Optimizing LEAST function with many columns

You want to find the smallest value among 10 numeric columns in a large table. Which approach is most efficient?

A Use LEAST(col1, col2, ..., col10) in SELECT clause directly.

B Use a CASE expression comparing each column pairwise.

C Unpivot columns into rows and use MIN() aggregation.

D Create a user-defined function to loop through columns.

💡 Hint

Consider built-in function efficiency versus complex expressions.

LEAST is optimized internally and faster than manual CASE or unpivoting for this use case.

🧠 Conceptual

expert

Behavior of GREATEST with mixed NULL and NOT NULL columns

Given a table with columns a, b, c where b can be NULL, what does this query return?

SELECT GREATEST(a, b, c) FROM table WHERE id = 1;

Assuming a=5, b=NULL, c=3 for id=1.

PostgreSQL

SELECT GREATEST(a, b, c) FROM table WHERE id = 1;

A 3

B NULL

C 5

D Error

💡 Hint

How does GREATEST handle NULL values in columns?

GREATEST returns NULL if any argument is NULL, so the presence of b=NULL causes the result to be NULL.

Practice

1. What does the GREATEST function do in PostgreSQL?

easy

A. Counts the number of values in a list

B. Returns the smallest value from a list of values

C. Returns the largest value from a list of values

D. Calculates the average of values in a list

Solution

1. Step 1: Understand the function purpose

   The GREATEST function compares all given values and finds the largest one.

2. Step 2: Compare with other options

   LEAST returns the smallest, counting and averaging are different functions.

3. Final Answer:

   Returns the largest value from a list of values -> Option C

4. Quick Check:

   GREATEST = largest value [OK]

Hint: GREATEST picks the biggest value from all inputs [OK]

Common Mistakes:

• Confusing GREATEST with LEAST
• Thinking it counts or averages values
• Assuming it returns a boolean

2. Which of the following is the correct syntax to find the smallest value among 5, 10, and 3 using PostgreSQL?

easy

A. SELECT LEAST[5, 10, 3];

B. SELECT LEAST(5, 10, 3);

C. SELECT LEAST{5, 10, 3};

D. SELECT LEAST 5, 10, 3;

Solution

1. Step 1: Recall correct function call syntax

   PostgreSQL functions use parentheses with comma-separated values.

2. Step 2: Check each option

   Only SELECT LEAST(5, 10, 3); uses parentheses correctly; others use brackets, braces, or no parentheses.

3. Final Answer:

   SELECT LEAST(5, 10, 3); -> Option B

4. Quick Check:

   Function calls need parentheses [OK]

Hint: Use parentheses () for function arguments in SQL [OK]

Common Mistakes:

• Using square brackets or braces instead of parentheses
• Omitting parentheses around arguments
• Separating arguments incorrectly

3. What will be the result of this query?

SELECT GREATEST(7, 2, 9, 4) AS max_val, LEAST(7, 2, 9, 4) AS min_val;

medium

A. max_val = 9, min_val = 2

B. max_val = 7, min_val = 4

C. max_val = 4, min_val = 2

D. max_val = 9, min_val = 4

Solution

1. Step 1: Evaluate GREATEST function

   Among 7, 2, 9, 4, the largest value is 9.

2. Step 2: Evaluate LEAST function

   Among the same values, the smallest is 2.

3. Final Answer:

   max_val = 9, min_val = 2 -> Option A

4. Quick Check:

   GREATEST=9, LEAST=2 [OK]

Hint: GREATEST picks max, LEAST picks min from values [OK]

Common Mistakes:

• Mixing up max and min values
• Choosing middle values instead of extremes
• Ignoring all values except first or last

4. Identify the error in this query:

SELECT GREATEST(4, 8, 'ten') AS max_value;

medium

A. Missing closing parenthesis

B. No error, query runs fine

C. GREATEST function requires at least four arguments

D. Mixing integer and string types causes an error

Solution

1. Step 1: Check argument types

   GREATEST compares values of the same type; mixing integers and strings causes a type error.

2. Step 2: Verify syntax and argument count

   Parentheses are correct and function accepts any number of arguments greater than zero.

3. Final Answer:

   Mixing integer and string types causes an error -> Option D

4. Quick Check:

   Type mismatch causes error [OK]

Hint: All arguments must be same type for GREATEST/LEAST [OK]

Common Mistakes:

• Assuming mixed types work without error
• Thinking function needs fixed number of arguments
• Ignoring type errors in SQL

5. You have a table scores with columns math, science, and english. Which query correctly finds the highest and lowest score for each student?

hard

A. SELECT student_id, GREATEST(math, science, english) AS highest, LEAST(math, science, english) AS lowest FROM scores;

B. SELECT student_id, MAX(math, science, english) AS highest, MIN(math, science, english) AS lowest FROM scores;

C. SELECT student_id, GREATEST(math, science, english) AS lowest, LEAST(math, science, english) AS highest FROM scores;

D. SELECT student_id, MAX(math) AS highest, MIN(english) AS lowest FROM scores;

Solution

1. Step 1: Understand GREATEST and LEAST usage

   They compare multiple columns per row to find max and min values.

2. Step 2: Analyze each option

   SELECT student_id, GREATEST(math, science, english) AS highest, LEAST(math, science, english) AS lowest FROM scores; correctly uses GREATEST and LEAST on columns per student. SELECT student_id, MAX(math, science, english) AS highest, MIN(math, science, english) AS lowest FROM scores; uses MAX/MIN incorrectly with multiple columns. SELECT student_id, GREATEST(math, science, english) AS lowest, LEAST(math, science, english) AS highest FROM scores; swaps highest and lowest. SELECT student_id, MAX(math) AS highest, MIN(english) AS lowest FROM scores; only compares single columns, not all three.

3. Final Answer:

   SELECT student_id, GREATEST(math, science, english) AS highest, LEAST(math, science, english) AS lowest FROM scores; -> Option A

4. Quick Check:

   GREATEST/LEAST compare columns per row [OK]

Hint: Use GREATEST/LEAST to compare multiple columns in one row [OK]

Common Mistakes:

• Using MAX/MIN with multiple columns incorrectly
• Swapping highest and lowest functions
• Comparing only one column instead of all