ORDER BY with NULLS FIRST and NULLS LAST in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to order the results by the column score in ascending order with NULL values appearing first.

PostgreSQL

SELECT * FROM players ORDER BY score [1];

Drag options to blanks, or click blank then click option'

ANULLS FIRST

BASC

CNULLS LAST

DDESC

Attempts:

3 left

💡 Hint

Common Mistakes

Using only ASC without NULLS FIRST will put NULLs last by default.

Using DESC instead of NULLS FIRST changes the order direction.

Using ORDER BY score NULLS FIRST sorts the scores in ascending order by default and places NULL values at the beginning.

2fill in blank

medium

Complete the code to order the results by the column date_joined in descending order with NULL values appearing last.

PostgreSQL

SELECT * FROM members ORDER BY date_joined [1];

Drag options to blanks, or click blank then click option'

ANULLS FIRST

BNULLS LAST

CDESC NULLS LAST

DDESC

Attempts:

3 left

💡 Hint

Common Mistakes

Using only DESC puts NULLs first by default.

Using NULLS FIRST will put NULLs at the beginning, which is not desired here.

Using ORDER BY date_joined DESC NULLS LAST sorts dates from newest to oldest and places NULL values at the end.

3fill in blank

hard

Fix the error in the code to order by price ascending with NULL values last.

PostgreSQL

SELECT * FROM products ORDER BY price [1];

Drag options to blanks, or click blank then click option'

AASC NULLS FIRST

BNULLS LAST

CDESC NULLS LAST

DASC NULLS LAST

Attempts:

3 left

💡 Hint

Common Mistakes

Using only NULLS LAST without ASC or DESC causes syntax error.

Using DESC NULLS LAST changes the order direction.

The correct syntax to order ascending with NULLs last is ASC NULLS LAST.

4fill in blank

hard

Fill both blanks to order by rating descending with NULL values first.

PostgreSQL

SELECT * FROM reviews ORDER BY rating [1] [2];

Drag options to blanks, or click blank then click option'

ADESC

BNULLS FIRST

CASC

DNULLS LAST

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order direction and NULLS clause.

Using NULLS LAST when NULLS FIRST is required.

Ordering by rating DESC NULLS FIRST sorts ratings from highest to lowest and puts NULLs at the beginning.

5fill in blank

hard

Fill all three blanks to order by score ascending with NULL values last, then by name descending with NULL values first.

PostgreSQL

SELECT * FROM leaderboard ORDER BY score [1] [2], name [3];

Drag options to blanks, or click blank then click option'

AASC

BNULLS LAST

CDESC NULLS FIRST

DDESC

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to specify NULLS position for one or both columns.

Mixing up ASC and DESC directions.

The query orders first by score ASC NULLS LAST (lowest scores first, NULLs last), then by name DESC NULLS FIRST (names descending, 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