FIRST_VALUE and LAST_VALUE in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the FIRST_VALUE function do in SQL?

FIRST_VALUE returns the first value in an ordered set of rows within a window.

It helps you get the earliest or smallest value based on the order you define.

beginner

What is the purpose of the LAST_VALUE function in SQL?

LAST_VALUE returns the last value in an ordered set of rows within a window.

It helps you get the latest or largest value based on the order you define.

intermediate

How do FIRST_VALUE and LAST_VALUE differ from aggregate functions like MIN and MAX?

FIRST_VALUE and LAST_VALUE return values based on row order within a window, not just the minimum or maximum value.

They consider the order of rows, while MIN and MAX find the smallest or largest value regardless of order.

advanced

What happens if you use LAST_VALUE without adjusting the window frame in PostgreSQL?

By default, LAST_VALUE looks at the current row and all previous rows, so it may return the current row's value instead of the last in the partition.

You often need to set the window frame to ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING to get the true last value.

beginner

Write a simple SQL query using FIRST_VALUE to get the first sale date per customer from a sales table.

SELECT customer_id,
       sale_date,
       FIRST_VALUE(sale_date) OVER (PARTITION BY customer_id ORDER BY sale_date ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS first_sale_date
FROM sales;

This query shows each sale and the first sale date for that customer.

What does FIRST_VALUE return in a window function?

A The first value in the ordered window

B The smallest value in the entire table

C The last value in the ordered window

D The average value in the window

FIRST_VALUE returns the first value in the ordered set of rows defined by the window.

Which clause is essential to define the order for FIRST_VALUE and LAST_VALUE?

A GROUP BY

B ORDER BY

C WHERE

D HAVING

The ORDER BY clause inside the window function defines the order of rows.

If you want LAST_VALUE to return the last value in the entire partition, what should you do?

A Use <code>ORDER BY DESC</code> only

B Use <code>GROUP BY</code> clause

C Nothing, it works by default

D Use <code>ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING</code> frame

Adjusting the window frame ensures LAST_VALUE looks at all rows in the partition.

Which function would you use to get the earliest date in a group ignoring row order?

A MIN()

B FIRST_VALUE()

C LAST_VALUE()

D AVG()

MIN() returns the smallest value regardless of row order.

Can FIRST_VALUE and LAST_VALUE be used without a window function?

A Only <code>FIRST_VALUE</code> can be used without OVER()

B Yes, like aggregate functions

C No, they require an OVER() clause

D Only <code>LAST_VALUE</code> can be used without OVER()

Both require the OVER() clause to define the window.

Explain how FIRST_VALUE and LAST_VALUE work in SQL window functions.

Think about how they pick values from ordered rows in a group.

Describe a situation where you would need to adjust the window frame when using LAST_VALUE.

Consider how window frames affect which rows are visible to the function.

Practice

1. What does the FIRST_VALUE function do in PostgreSQL?

easy

A. Returns the first value in a sorted group of rows.

B. Returns the last value in a sorted group of rows.

C. Returns the average value of a column.

D. Returns the total count of rows.

Solution

1. Step 1: Understand the purpose of FIRST_VALUE

   FIRST_VALUE returns the first value in a sorted window or group of rows.

2. Step 2: Compare with other options

   Options B, C, and D describe different functions or aggregates, not FIRST_VALUE.

3. Final Answer:

   Returns the first value in a sorted group of rows. -> Option A

4. Quick Check:

   FIRST_VALUE = first value in sorted rows [OK]

Hint: FIRST_VALUE always picks the first row's value in order [OK]

Common Mistakes:

• Confusing FIRST_VALUE with LAST_VALUE
• Thinking it returns an aggregate like sum or count
• Ignoring the sorting order in the window

2. Which of the following is the correct syntax to get the last value of a column score ordered by date using LAST_VALUE in PostgreSQL?

easy

A. SELECT LAST_VALUE(score) FROM results ORDER BY date;

B. SELECT LAST_VALUE(score) OVER (ORDER BY date ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) FROM results;

C. SELECT LAST_VALUE(score) OVER (PARTITION BY date) FROM results;

D. SELECT LAST_VALUE(score) OVER (ORDER BY date) FROM results;

Solution

1. Step 1: Understand LAST_VALUE default frame

   LAST_VALUE by default looks from current row to end, so it may not return the last value overall without framing.

2. Step 2: Use explicit window frame to get true last value

   Adding ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING ensures the entire partition is considered.

3. Final Answer:

   SELECT LAST_VALUE(score) OVER (ORDER BY date ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) FROM results; -> Option B

4. Quick Check:

   LAST_VALUE needs full frame to get last value [OK]

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

Common Mistakes:

• Omitting window frame causing wrong last value
• Using PARTITION BY incorrectly
• Using LAST_VALUE without OVER() clause

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

SELECT region, month, revenue, FIRST_VALUE(revenue) OVER (PARTITION BY region ORDER BY month) AS first_rev FROM sales ORDER BY region, month;

medium

A. The total revenue summed for each region.

B. The revenue of the last month for each region repeated on every row of that region.

C. The revenue of the first month for each region repeated on every row of that region.

D. The revenue of the current month only.

Solution

1. Step 1: Analyze FIRST_VALUE with PARTITION and ORDER BY

   FIRST_VALUE(revenue) OVER (PARTITION BY region ORDER BY month) returns the first revenue value per region ordered by month.

2. Step 2: Understand output per row

   For each row in a region, the first month's revenue is repeated as first_rev.

3. Final Answer:

   The revenue of the first month for each region repeated on every row of that region. -> Option C

4. Quick Check:

   FIRST_VALUE repeats first row's value per partition [OK]

Hint: FIRST_VALUE repeats first sorted value per group [OK]

Common Mistakes:

• Thinking it returns last value instead
• Assuming it sums or aggregates
• Ignoring PARTITION BY effect

4. You wrote this query but it returns unexpected results:

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

What is the likely cause?

medium

A. LAST_VALUE cannot be used without PARTITION BY clause.

B. ORDER BY employee_id is invalid in window functions.

C. salary column must be aggregated with SUM before LAST_VALUE.

D. LAST_VALUE needs a window frame to include all rows; default frame causes wrong output.

Solution

1. Step 1: Recall LAST_VALUE default frame behavior

   By default, LAST_VALUE looks from current row to end, so it may not return the last row's value overall.

2. Step 2: Identify missing window frame

   Without specifying ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING, the last value is not the last row's value.

3. Final Answer:

   LAST_VALUE needs a window frame to include all rows; default frame causes wrong output. -> Option D

4. Quick Check:

   LAST_VALUE requires full frame for correct last row [OK]

Hint: Add full frame to LAST_VALUE to fix wrong results [OK]

Common Mistakes:

• Assuming ORDER BY alone fixes LAST_VALUE
• Thinking PARTITION BY is mandatory
• Trying to aggregate salary before LAST_VALUE

5. You want to find the first and last sale amount for each customer ordered by sale date. Which query correctly returns both values per customer in PostgreSQL?

hard

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

B. SELECT customer_id, sale_date, sale_amount, FIRST_VALUE(sale_amount), LAST_VALUE(sale_amount) FROM sales GROUP BY customer_id;

C. SELECT customer_id, sale_date, sale_amount, FIRST_VALUE(sale_amount) OVER (ORDER BY sale_date), LAST_VALUE(sale_amount) OVER (ORDER BY sale_date) FROM sales;

D. SELECT customer_id, sale_date, sale_amount, MIN(sale_amount) OVER (PARTITION BY customer_id), MAX(sale_amount) OVER (PARTITION BY customer_id) FROM sales;

Solution

1. Step 1: Use FIRST_VALUE with PARTITION and ORDER BY

   FIRST_VALUE(sale_amount) OVER (PARTITION BY customer_id ORDER BY sale_date) gets first sale per customer.

2. Step 2: Use LAST_VALUE with full window frame

   LAST_VALUE needs ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING to get last sale per customer.

3. Step 3: Check other options

   SELECT customer_id, sale_date, sale_amount, FIRST_VALUE(sale_amount), LAST_VALUE(sale_amount) FROM sales GROUP BY customer_id; lacks OVER() and ORDER BY, C lacks PARTITION BY, D uses MIN/MAX which is different.

4. Final Answer:

   SELECT customer_id, sale_date, sale_amount, FIRST_VALUE(sale_amount) OVER (PARTITION BY customer_id ORDER BY sale_date), LAST_VALUE(sale_amount) OVER (PARTITION BY customer_id ORDER BY sale_date ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) FROM sales; -> Option A

5. Quick Check:

   Combine FIRST_VALUE and LAST_VALUE with proper framing [OK]

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

Common Mistakes:

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