FIRST_VALUE and LAST_VALUE in SQL - Step-by-Step Execution

Concept Flow - FIRST_VALUE and LAST_VALUE

Start Query

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Partition Data (optional)

↓

Order Rows within Partition

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Apply FIRST_VALUE/LAST_VALUE Window Function

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Return Result Set with New Columns

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End Query

The query starts by optionally partitioning data, then ordering rows, applying FIRST_VALUE or LAST_VALUE to get first or last values in the window, and returns the result.

Execution Sample

SQL

SELECT
  department,
  employee,
  salary,
  FIRST_VALUE(salary) OVER (PARTITION BY department ORDER BY salary DESC) AS highest_salary,
  LAST_VALUE(salary) OVER (PARTITION BY department ORDER BY salary DESC ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS lowest_salary
FROM employees;

This query shows the highest and lowest salary per department using FIRST_VALUE and LAST_VALUE window functions.

Execution Table

Step	Row (department, employee, salary)	Window Order	FIRST_VALUE(salary)	LAST_VALUE(salary)	Notes
1	(Sales, Alice, 9000)	9000, 7000, 6000	9000	6000	First row in Sales partition, highest salary is 9000, last salary is 6000
2	(Sales, Bob, 7000)	9000, 7000, 6000	9000	6000	Second row, FIRST_VALUE stays 9000, LAST_VALUE includes full window with frame clause
3	(Sales, Carol, 6000)	9000, 7000, 6000	9000	6000	Third row, same FIRST_VALUE and LAST_VALUE
4	(HR, Dave, 8000)	8000, 7500	8000	7500	First row in HR, highest salary 8000, lowest 7500
5	(HR, Eve, 7500)	8000, 7500	8000	7500	Second row, values remain same
6	End of rows				Query completes after all rows processed

💡 All rows processed, window functions applied per partition and order.

Variable Tracker

Variable	Start	After Row 1	After Row 2	After Row 3	After Row 4	After Row 5	Final
department	N/A	Sales	Sales	Sales	HR	HR	N/A
employee	N/A	Alice	Bob	Carol	Dave	Eve	N/A
salary	N/A	9000	7000	6000	8000	7500	N/A
FIRST_VALUE(salary)	N/A	9000	9000	9000	8000	8000	N/A
LAST_VALUE(salary)	N/A	6000	6000	6000	7500	7500	N/A

Key Moments - 2 Insights

Why does LAST_VALUE need the frame clause 'ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING' to return the expected lowest salary?

Without the frame clause, LAST_VALUE defaults to the frame from the start of the partition to the current row, which can cause it to return the current row's value instead of the last in the full partition. The frame clause ensures the window covers all rows.

Why does FIRST_VALUE always return the same highest salary for all rows in the partition?

Because FIRST_VALUE returns the first value in the ordered window, and since the order is descending by salary, the first value is the highest salary, which stays the same for all rows in that partition.

Visual Quiz - 3 Questions

Look at the execution table, what is the FIRST_VALUE(salary) for the second row in the Sales department?

A 6000

B 7000

C 9000

D NULL

💡 Hint

Check the row 2 in the execution_table under FIRST_VALUE(salary)

At which step does the LAST_VALUE(salary) for HR department first appear as 7500?

A Step 4

B Step 5

C Step 3

D Step 2

💡 Hint

Look at the LAST_VALUE(salary) column for HR rows in execution_table

If we remove the frame clause from LAST_VALUE, what would likely happen to the LAST_VALUE(salary) output?

A It would always show the lowest salary

B It would show the salary of the current row

C It would cause a syntax error

D It would show NULL

💡 Hint

Refer to key_moments about the importance of the frame clause for LAST_VALUE

Concept Snapshot

FIRST_VALUE and LAST_VALUE are window functions.
They return the first or last value in an ordered window.
Use PARTITION BY to group rows.
Use ORDER BY to define order.
LAST_VALUE often needs a frame clause to cover full partition.
They help find first/last values without aggregation.

Full Transcript

This visual execution trace shows how FIRST_VALUE and LAST_VALUE window functions work in SQL. The query partitions employees by department and orders them by salary descending. FIRST_VALUE returns the highest salary in each department for every row. LAST_VALUE returns the lowest salary, but requires a frame clause to cover the entire partition; otherwise, it may return the current row's salary. The execution table walks through each row, showing how these values are computed and remain consistent across rows in the same partition. Key moments clarify why the frame clause is needed for LAST_VALUE and why FIRST_VALUE stays constant. The quiz tests understanding of these behaviors by referencing specific steps and values in the execution table. The snapshot summarizes the syntax and key rules for these window functions.

Practice

1. What does the SQL function FIRST_VALUE(column_name) OVER (ORDER BY column_name) return?

easy

A. The average value of the column

B. The first value in the ordered set of rows for that column

C. The last value in the ordered set of rows for that column

D. The total count of rows in the table

Solution

1. Step 1: Understand FIRST_VALUE function

   It returns the first value in the ordered set of rows defined by the OVER clause.

2. Step 2: Analyze the ORDER BY clause

   The ORDER BY inside OVER() defines the order to pick the first value from.

3. Final Answer:

   The first value in the ordered set of rows for that column -> Option B

4. Quick Check:

   FIRST_VALUE returns first row value [OK]

Hint: FIRST_VALUE picks the first row after ordering [OK]

Common Mistakes:

• Confusing FIRST_VALUE with LAST_VALUE
• Thinking it returns an aggregate like average
• Ignoring the ORDER BY inside OVER()

2. Which of the following is the correct syntax to get the last value of salary ordered by hire_date in SQL?

easy

A. SELECT LAST_VALUE(salary) OVER (ORDER BY hire_date) FROM employees;

B. SELECT LAST_VALUE(salary) FROM employees ORDER BY hire_date;

C. SELECT LAST_VALUE(salary) OVER (PARTITION BY hire_date) FROM employees;

D. SELECT LAST_VALUE(salary) OVER (ORDER BY hire_date ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) FROM employees;

Solution

1. Step 1: Understand LAST_VALUE default frame

   LAST_VALUE by default looks only at rows up to current row, so it may not return the true last value.

2. Step 2: Use frame clause to include all rows

   Adding ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING ensures LAST_VALUE looks at the entire partition.

3. Final Answer:

   SELECT LAST_VALUE(salary) OVER (ORDER BY hire_date ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) FROM employees; -> Option D

4. Quick Check:

   LAST_VALUE needs full frame to get last row [OK]

Hint: Add full frame clause to LAST_VALUE for correct last row [OK]

Common Mistakes:

• Omitting frame clause with LAST_VALUE
• Using PARTITION BY incorrectly
• Trying to use LAST_VALUE without OVER()

3. Given the table sales with columns region and amount, what will this query return?

SELECT region, amount, FIRST_VALUE(amount) OVER (PARTITION BY region ORDER BY amount DESC) AS max_amount FROM sales;

medium

A. The first amount in the whole table regardless of region

B. The lowest amount per region repeated on each row of that region

C. The highest amount per region repeated on each row of that region

D. Syntax error due to missing frame clause

Solution

1. Step 1: Analyze PARTITION BY and ORDER BY

   The window partitions rows by region and orders amounts descending within each region.

2. Step 2: Understand FIRST_VALUE with ordering

   FIRST_VALUE picks the first row's amount in each partition ordered by amount DESC, which is the max amount.

3. Final Answer:

   The highest amount per region repeated on each row of that region -> Option C

4. Quick Check:

   FIRST_VALUE with DESC order gives max per partition [OK]

Hint: Use ORDER BY DESC with FIRST_VALUE for max per group [OK]

Common Mistakes:

• Assuming FIRST_VALUE always returns lowest value
• Confusing PARTITION BY with filtering
• Expecting syntax error without frame clause here

4. Identify the error in this query:

SELECT employee_id, LAST_VALUE(salary) OVER (ORDER BY employee_id) AS last_sal FROM employees;

medium

A. LAST_VALUE needs a frame clause to get correct last value

B. Missing PARTITION BY clause causes error

C. ORDER BY cannot be used inside OVER()

D. salary column must be aggregated with GROUP BY

Solution

1. Step 1: Check LAST_VALUE default behavior

   By default, LAST_VALUE looks only up to current row, so it may not return the true last value.

2. Step 2: Add frame clause to fix

   Adding ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING makes LAST_VALUE consider all rows.

3. Final Answer:

   LAST_VALUE needs a frame clause to get correct last value -> Option A

4. Quick Check:

   LAST_VALUE without frame returns wrong last row [OK]

Hint: Add full frame to LAST_VALUE for correct last row [OK]

Common Mistakes:

• Thinking PARTITION BY is mandatory
• Believing ORDER BY is invalid inside OVER()
• Confusing LAST_VALUE with aggregate functions

5. You want to find the first and last sale amounts per customer ordered by sale date from the sales table. Which query correctly returns both values on each row?

hard

A. SELECT customer_id, sale_date, FIRST_VALUE(amount) OVER (PARTITION BY customer_id ORDER BY sale_date) AS first_sale, LAST_VALUE(amount) OVER (PARTITION BY customer_id ORDER BY sale_date ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS last_sale FROM sales;

B. SELECT customer_id, sale_date, FIRST_VALUE(amount) OVER (ORDER BY sale_date) AS first_sale, LAST_VALUE(amount) OVER (ORDER BY sale_date) AS last_sale FROM sales;

C. SELECT customer_id, sale_date, FIRST_VALUE(amount) AS first_sale, LAST_VALUE(amount) AS last_sale FROM sales GROUP BY customer_id;

D. SELECT customer_id, sale_date, MIN(amount) OVER (PARTITION BY customer_id) AS first_sale, MAX(amount) OVER (PARTITION BY customer_id) AS last_sale FROM sales;

Solution

1. Step 1: Use PARTITION BY customer_id

   This groups rows by customer to get first and last sales per customer.

2. Step 2: Use ORDER BY sale_date for chronological order

   Ordering by sale_date ensures first and last are by date.

3. Step 3: Add frame clause for LAST_VALUE

   LAST_VALUE needs ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING to get true last value.

4. Final Answer:

   Query in option A correctly returns first and last sale amounts per customer -> Option A

5. Quick Check:

   FIRST_VALUE and LAST_VALUE with proper frame and partition [OK]

Hint: Use full frame for LAST_VALUE with PARTITION and ORDER BY [OK]

Common Mistakes:

• Omitting frame clause for LAST_VALUE
• Using aggregate functions instead of window functions
• Not partitioning by customer_id