ORDER BY with NULLS FIRST and NULLS LAST in PostgreSQL

Introduction

We use ORDER BY to sort data. Sometimes, data has empty values called NULLs. NULLS FIRST and NULLS LAST help decide if these empty values come at the start or end of the list.

When you want to list customers by their last purchase date but show those who never purchased (NULL) at the top.

When sorting product prices but want products without a price (NULL) to appear last.

When showing employee records sorted by their manager ID, placing those without a manager (NULL) first.

When displaying event dates and you want events without a date (NULL) to appear at the end.

Syntax

PostgreSQL

SELECT column1, column2
FROM table_name
ORDER BY column_name [ASC|DESC] NULLS {FIRST|LAST};

ASC means ascending order (smallest to largest), DESC means descending (largest to smallest).

NULLS FIRST puts NULL values at the beginning, NULLS LAST puts them at the end.

Examples

Sort players by score from highest to lowest. Players without a score (NULL) appear at the end.

PostgreSQL

SELECT name, score
FROM players
ORDER BY score DESC NULLS LAST;

Sort products by price from lowest to highest. Products without a price (NULL) appear at the start.

PostgreSQL

SELECT product, price
FROM products
ORDER BY price ASC NULLS FIRST;

Sort employees by their manager ID, showing those without a manager (NULL) first.

PostgreSQL

SELECT employee, manager_id
FROM employees
ORDER BY manager_id ASC NULLS FIRST;

Sample Program

This creates a temporary table with some items and their ratings. Some ratings are NULL (unknown). The query sorts items by rating from highest to lowest, placing items with no rating at the end.

PostgreSQL

CREATE TEMP TABLE items (id SERIAL, name TEXT, rating INT);

INSERT INTO items (name, rating) VALUES
('Item A', 5),
('Item B', NULL),
('Item C', 3),
('Item D', NULL),
('Item E', 4);

SELECT name, rating
FROM items
ORDER BY rating DESC NULLS LAST;

Important Notes

By default, in PostgreSQL, ORDER BY ASC puts NULLs first, and ORDER BY DESC puts NULLs last.

Using NULLS FIRST or NULLS LAST lets you control this behavior explicitly.

This helps make your sorted lists clearer and easier to understand.

Summary

ORDER BY sorts data by columns.

NULLS FIRST and NULLS LAST decide where empty (NULL) values appear in the sorted list.

Use them to make your data lists more meaningful and easier to read.

Practice

1. What does ORDER BY column_name NULLS FIRST do in a PostgreSQL query?

easy

A. It ignores NULL values while sorting by column_name.

B. It sorts the rows by column_name and places NULL values at the end.

C. It causes a syntax error because NULLS FIRST is not valid.

D. It sorts the rows by column_name and places NULL values at the beginning.

Solution

1. Step 1: Understand ORDER BY behavior with NULLS FIRST

   The ORDER BY clause sorts rows by the specified column. Adding NULLS FIRST means any NULL values in that column appear before non-NULL values.

2. Step 2: Confirm the effect on sorting

   So, the rows with NULL in column_name come first, then the rest sorted normally.

3. Final Answer:

   It sorts the rows by column_name and places NULL values at the beginning. -> Option D

4. Quick Check:

   NULLS FIRST means NULLs appear first [OK]

Hint: NULLS FIRST puts empty values at the top of the list [OK]

Common Mistakes:

• Confusing NULLS FIRST with NULLS LAST
• Thinking NULLs are ignored in sorting
• Assuming NULLS FIRST causes errors

2. Which of the following is the correct syntax to sort a table by column score in ascending order with NULL values last?

easy

A. SELECT * FROM table ORDER BY score ASC NULLS LAST;

B. SELECT * FROM table ORDER BY score NULLS LAST ASC;

C. SELECT * FROM table ORDER BY NULLS LAST score ASC;

D. SELECT * FROM table ORDER BY score ASC LAST NULLS;

Solution

1. Step 1: Recall correct ORDER BY syntax with NULLS LAST

   The correct syntax places the column name first, then the sort order (ASC or DESC), followed by NULLS FIRST or NULLS LAST.

2. Step 2: Check each option

   SELECT * FROM table ORDER BY score ASC NULLS LAST; follows this order: score ASC NULLS LAST. Others mix the order or use invalid syntax.

3. Final Answer:

   SELECT * FROM table ORDER BY score ASC NULLS LAST; -> Option A

4. Quick Check:

   Column, order, then NULLS position [OK]

Hint: Order: column, ASC/DESC, then NULLS FIRST/LAST [OK]

Common Mistakes:

• Placing NULLS FIRST/LAST before column name
• Swapping ASC/DESC and NULLS keywords
• Using invalid keyword order causing syntax errors

3. Given the table students with column grade containing values (NULL, 85, 90, NULL, 75), what is the order of grade values after running:
SELECT grade FROM students ORDER BY grade DESC NULLS FIRST;?

medium

A. [NULL, NULL, 90, 85, 75]

B. [90, 85, 75, NULL, NULL]

C. [75, 85, 90, NULL, NULL]

D. [NULL, 90, 85, 75, NULL]

Solution

1. Step 1: Understand DESC with NULLS FIRST

   Sorting by grade DESC means highest grades first. Adding NULLS FIRST places NULL values before all grades.

2. Step 2: Arrange values accordingly

   NULLs come first: [NULL, NULL], then grades in descending order: 90, 85, 75.

3. Final Answer:

   [NULL, NULL, 90, 85, 75] -> Option A

4. Quick Check:

   DESC sorts high to low, NULLS FIRST puts NULLs upfront [OK]

Hint: NULLS FIRST always puts NULLs before sorted values [OK]

Common Mistakes:

• Placing NULLs at the end despite NULLS FIRST
• Sorting grades ascending instead of descending
• Mixing order of NULLs and values

4. You wrote this query but it gives a syntax error:
SELECT * FROM employees ORDER BY salary NULLS FIRST DESC;
What is the error and how to fix it?

medium

A. The column name salary is missing ASC or DESC.

B. NULLS FIRST cannot be used with DESC; remove NULLS FIRST.

C. The order of NULLS FIRST and DESC is wrong; use ORDER BY salary DESC NULLS FIRST.

D. You must use parentheses around NULLS FIRST and DESC.

Solution

1. Step 1: Identify correct keyword order in ORDER BY

   The correct syntax is: column name, then ASC or DESC, then NULLS FIRST or NULLS LAST.

2. Step 2: Fix the query

   Change ORDER BY salary NULLS FIRST DESC to ORDER BY salary DESC NULLS FIRST to fix the syntax error.

3. Final Answer:

   The order of NULLS FIRST and DESC is wrong; use ORDER BY salary DESC NULLS FIRST. -> Option C

4. Quick Check:

   Column, order, then NULLS position [OK]

Hint: Put ASC/DESC before NULLS FIRST/LAST [OK]

Common Mistakes:

• Placing NULLS FIRST before ASC/DESC
• Assuming NULLS FIRST is invalid with DESC
• Missing ASC/DESC keyword

5. You have a table products with a price column containing some NULLs. You want to list products sorted by price ascending, but show NULL prices at the end. Then, for products with the same price, sort by name ascending. Which query achieves this?

hard

A. SELECT * FROM products ORDER BY price NULLS LAST ASC, name ASC;

B. SELECT * FROM products ORDER BY price ASC NULLS LAST, name ASC;

C. SELECT * FROM products ORDER BY price ASC, name ASC NULLS LAST;

D. SELECT * FROM products ORDER BY price ASC NULLS FIRST, name ASC;

Solution

1. Step 1: Sort by price ascending with NULLs last

   Use ORDER BY price ASC NULLS LAST to sort prices low to high and put NULLs at the end.

2. Step 2: Add secondary sort by name ascending

   After sorting by price, add name ASC to sort products with the same price alphabetically.

3. Final Answer:

   SELECT * FROM products ORDER BY price ASC NULLS LAST, name ASC; -> Option B

4. Quick Check:

   Price ASC NULLS LAST, then name ASC [OK]

Hint: Chain ORDER BY: main column with NULLS, then tie-breaker column [OK]

Common Mistakes:

• Placing NULLS LAST before ASC
• Applying NULLS LAST to wrong column
• Using NULLS FIRST instead of NULLS LAST