ORDER BY with NULLS FIRST and NULLS LAST in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does ORDER BY column NULLS FIRST do in PostgreSQL?

It sorts the results by the specified column, placing all NULL values at the beginning of the sorted list before non-NULL values.

beginner

How does ORDER BY column NULLS LAST affect the sorting order?

It sorts the results by the specified column, placing all NULL values at the end of the sorted list after non-NULL values.

beginner

By default, where do NULL values appear when sorting in ascending order without specifying NULLS FIRST or NULLS LAST?

In ascending order, NULL values appear last by default.

beginner

By default, where do NULL values appear when sorting in descending order without specifying NULLS FIRST or NULLS LAST?

In descending order, NULL values appear first by default.

beginner

Write a PostgreSQL query to select all rows from a table employees ordered by salary ascending, but with NULL salaries shown first.

SELECT * FROM employees ORDER BY salary ASC NULLS FIRST;

In PostgreSQL, what does ORDER BY age DESC NULLS LAST do?

A Sorts by age descending, placing NULLs at the end

B Sorts by age ascending, placing NULLs at the beginning

C Sorts by age descending, placing NULLs at the beginning

D Sorts by age ascending, placing NULLs at the end

The query sorts ages from highest to lowest and puts NULL values after all non-NULL ages.

What is the default position of NULLs when sorting ascending without specifying NULLS FIRST or LAST?

A At the beginning

B At the end

C Randomly placed

D Depends on the data type

By default, NULLs appear last in ascending order.

Which clause explicitly places NULL values at the start when sorting?

A NULLS LAST

B NULLS TOP

C NULLS FIRST

D NULLS BEGIN

NULLS FIRST places NULL values before non-NULL values in the sort order.

If you want NULLs to appear last in descending order, which clause should you use?

A NULLS LAST

B No clause needed

C NULLS FIRST

D ORDER BY NULLS LAST DESC

NULLS LAST explicitly places NULLs after non-NULL values even in descending order.

What happens if you omit NULLS FIRST or NULLS LAST in an ORDER BY clause?

A NULLs are ignored

B NULLs are always first

C Query fails with error

D Default NULL placement depends on ASC or DESC

PostgreSQL uses default NULL placement: last for ASC, first for DESC.

Explain how PostgreSQL handles NULL values in ORDER BY clauses and how NULLS FIRST and NULLS LAST change this behavior.

Think about where NULLs appear by default and how the clauses move them.

Write a query to order a table by a column with NULL values shown first, and explain why you might want to do this.

Use ORDER BY column ASC NULLS FIRST.

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