GREATEST and LEAST in MySQL - Step-by-Step Execution

Concept Flow - GREATEST and LEAST

Start with values

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Compare values pairwise

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GREATEST: Select largest value

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LEAST: Select smallest value

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Return result

GREATEST compares values and returns the largest; LEAST returns the smallest among given values.

Execution Sample

MySQL

SELECT GREATEST(3, 7, 5) AS max_val, LEAST(3, 7, 5) AS min_val;

This query finds the largest and smallest values among 3, 7, and 5.

Execution Table

Step	Values Compared	GREATEST Result	LEAST Result	Explanation
1	3 and 7	7	3	Compare first two values: 7 is greater, 3 is smaller
2	7 and 5	7	3	Compare previous greatest 7 with 5: 7 remains greatest; least stays 3
3	All values compared	7	3	Final greatest is 7, least is 3

💡 All values compared, final greatest and least values returned

Variable Tracker

Variable	Start	After 1	After 2	Final
GREATEST	N/A	7	7	7
LEAST	N/A	3	3	3

Key Moments - 2 Insights

Why does GREATEST return 7 and not 5 or 3?

Because in the execution_table rows 1 and 2, 7 is compared with other values and remains the largest.

Why does LEAST return 3 and not 5 or 7?

Because 3 is the smallest value found after comparing all values as shown in execution_table rows 1 and 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the GREATEST result after step 1?

A 5

B 3

C 7

D None

💡 Hint

Check the 'GREATEST Result' column in execution_table row 1

At which step does the LEAST value become final?

A Step 1

B Step 3

C Step 2

D Never

💡 Hint

Look at the 'LEAST Result' column and the exit_note in execution_table

If we change the values to (10, 2, 8), what would GREATEST return?

A 10

B 8

C 2

D None

💡 Hint

GREATEST returns the largest value among the inputs, similar to the tracked variable in variable_tracker

Concept Snapshot

GREATEST(value1, value2, ...) returns the largest value.
LEAST(value1, value2, ...) returns the smallest value.
They compare all given values pairwise.
Useful for quick max/min among multiple values.
NULL input returns NULL if any value is NULL.

Full Transcript

This visual execution trace shows how MySQL functions GREATEST and LEAST work by comparing multiple values step-by-step. Starting with the values 3, 7, and 5, the functions compare pairs to find the largest and smallest values. After comparing 3 and 7, 7 is the greatest and 3 the least. Then 7 is compared with 5, and 7 remains greatest while 3 remains least. Finally, the functions return 7 as the greatest and 3 as the least. This helps beginners see how these functions pick the max and min values from a list.

Practice

1. What does the MySQL function GREATEST(3, 7, 5) return?

easy

A. NULL

B. 3

C. 5

D. 7

Solution

1. Step 1: Understand the GREATEST function

   The GREATEST function returns the largest value among the given arguments.

2. Step 2: Compare the values 3, 7, and 5

   Among 3, 7, and 5, the largest number is 7.

3. Final Answer:

   7 -> Option D

4. Quick Check:

   GREATEST(3, 7, 5) = 7 [OK]

Hint: GREATEST picks the highest number from the list [OK]

Common Mistakes:

• Confusing GREATEST with LEAST
• Thinking it returns the first value
• Assuming it returns a sum

2. Which of the following is the correct syntax to find the smallest value among 10, 20, and 5 using MySQL?

easy

A. MIN(10, 20, 5)

B. LEAST(10, 20, 5)

C. SMALLEST(10, 20, 5)

D. LOWEST(10, 20, 5)

Solution

1. Step 1: Recall the function to find smallest value

   In MySQL, the LEAST function returns the smallest value from a list of arguments.

2. Step 2: Check the syntax options

   LEAST(10, 20, 5) is the correct syntax. MIN is an aggregate function and requires a column, not a list of values. SMALLEST and LOWEST are not valid MySQL functions.

3. Final Answer:

   LEAST(10, 20, 5) -> Option B

4. Quick Check:

   LEAST(10, 20, 5) is correct syntax [OK]

Hint: Use LEAST() for smallest, GREATEST() for largest values [OK]

Common Mistakes:

• Using MIN() with values instead of columns
• Using non-existent functions like SMALLEST()
• Confusing function names

3. Given the table Scores with columns math, science, and english, what will the query SELECT GREATEST(math, science, english) AS top_score FROM Scores WHERE id = 1; return if the row with id = 1 has values (85, 90, 88)?

medium

A. 90

B. 85

C. 88

D. NULL

Solution

1. Step 1: Identify the values for id = 1

   The row has math=85, science=90, english=88.

2. Step 2: Apply GREATEST to these values

   GREATEST(85, 90, 88) returns the largest value, which is 90.

3. Final Answer:

   90 -> Option A

4. Quick Check:

   GREATEST(85, 90, 88) = 90 [OK]

Hint: GREATEST returns the highest among given column values [OK]

Common Mistakes:

• Selecting the first column value instead of greatest
• Confusing GREATEST with LEAST
• Assuming NULL if any value is less

4. Consider this query: SELECT LEAST(score1, score2, score3) FROM Results; It returns NULL for some rows even though all scores are numbers. What is the likely cause?

medium

A. Syntax error in the query

B. LEAST function only works with positive numbers

C. One or more scores are NULL in those rows

D. LEAST returns the largest value, so NULL is unexpected

Solution

1. Step 1: Understand LEAST behavior with NULLs

   LEAST returns NULL if any argument is NULL because NULL means unknown value.

2. Step 2: Check the cause of NULL results

   If some rows have NULL in score1, score2, or score3, LEAST will return NULL for those rows.

3. Final Answer:

   One or more scores are NULL in those rows -> Option C

4. Quick Check:

   LEAST returns NULL if any input is NULL [OK]

Hint: Check for NULLs in inputs; LEAST returns NULL if any argument is NULL [OK]

Common Mistakes:

• Assuming LEAST ignores NULLs
• Thinking LEAST only works with positive numbers
• Misreading LEAST as GREATEST

5. You want to find the highest and lowest scores among three exams for each student in a table Students with columns exam1, exam2, and exam3. Which query correctly returns the student ID, highest score as max_score, and lowest score as min_score?

hard

A. SELECT student_id, GREATEST(exam1, exam2, exam3) AS max_score, LEAST(exam1, exam2, exam3) AS min_score FROM Students;

B. SELECT student_id, MAX(exam1, exam2, exam3) AS max_score, MIN(exam1, exam2, exam3) AS min_score FROM Students;

C. SELECT student_id, GREATEST(exam1, exam2, exam3) AS min_score, LEAST(exam1, exam2, exam3) AS max_score FROM Students;

D. SELECT student_id, MAX(exam1, exam2, exam3) AS min_score, MIN(exam1, exam2, exam3) AS max_score FROM Students;

Solution

1. Step 1: Understand GREATEST and LEAST usage

   GREATEST returns the highest value among columns, LEAST returns the lowest.

2. Step 2: Check the query syntax

   SELECT student_id, GREATEST(exam1, exam2, exam3) AS max_score, LEAST(exam1, exam2, exam3) AS min_score FROM Students; correctly uses GREATEST and LEAST with column names and aliases them properly. MAX and MIN are aggregate functions and cannot take multiple columns as arguments directly.

3. Final Answer:

   Option A query -> Option A

4. Quick Check:

   Use GREATEST and LEAST for column-wise max/min [OK]

Hint: Use GREATEST/LEAST for row-wise max/min, not MAX/MIN [OK]

Common Mistakes:

• Using MAX/MIN with multiple columns incorrectly
• Swapping GREATEST and LEAST aliases
• Syntax errors with aggregate functions