ORDER BY with NULL values behavior in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to order the results by the column 'age' in ascending order.

SQL

SELECT * FROM users ORDER BY [1];

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

Asalary

Bname

Cid

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a column unrelated to the sorting requirement.

Forgetting to specify the column name.

The ORDER BY clause sorts the results by the specified column. Here, we want to sort by age.

2fill in blank

medium

Complete the code to order the results by 'score' in descending order.

SQL

SELECT * FROM results ORDER BY [1] DESC;

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

Ascore

Bdate

Cplayer

Dlevel

Attempts:

3 left

💡 Hint

Common Mistakes

Using ascending order instead of descending.

Choosing the wrong column for sorting.

To sort from highest to lowest, use ORDER BY score DESC.

3fill in blank

hard

Fix the error in the code to order by 'salary' in ascending order with NULL values appearing last.

SQL

SELECT * FROM employees ORDER BY salary [1] NULLS LAST;

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

ADESC

BASC

CSORT

DORDER

Attempts:

3 left

💡 Hint

Common Mistakes

Using DESC instead of ASC.

Using invalid keywords like SORT or ORDER in this position.

To order salaries ascending and place NULLs last, use ORDER BY salary ASC NULLS LAST.

4fill in blank

hard

Fill both blanks to order by 'date' descending and place NULL values first.

SQL

SELECT * FROM events ORDER BY date [1] NULLS [2];

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

ADESC

BASC

CFIRST

DLAST

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing ASC with NULLS FIRST incorrectly.

Using NULLS LAST when NULLS FIRST is needed.

Ordering by date descending with NULLs first uses ORDER BY date DESC NULLS FIRST.

5fill in blank

hard

Fill all three blanks to order by 'rating' ascending, place NULLs last, and then by 'name' ascending.

SQL

SELECT * FROM products ORDER BY rating [1] NULLS [2], name [3];

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

ADESC

BASC

CLAST

DFIRST

Attempts:

3 left

💡 Hint

Common Mistakes

Using DESC instead of ASC for ascending order.

Using NULLS FIRST instead of NULLS LAST.

Forgetting to specify order for the second column.

This orders products by rating ascending with NULLs last, then by name ascending: ORDER BY rating ASC NULLS LAST, name ASC.

Practice

1. What does the ORDER BY clause do when sorting a column that contains NULL values by default in SQL?

easy

A. It treats NULL as zero and sorts accordingly.

B. It places NULL values at the beginning when sorting in ascending order.

C. It places NULL values at the end when sorting in ascending order.

D. It causes a syntax error if NULL values exist.

Solution

1. Step 1: Understand default ORDER BY behavior

   By default, when sorting ascending, SQL places NULL values last because NULL means unknown or missing data.

2. Step 2: Confirm NULL placement in ascending order

   Since NULL is considered neither high nor low, it appears after all actual values in ascending order.

3. Final Answer:

   It places NULL values at the end when sorting in ascending order. -> Option C

4. Quick Check:

   Default ascending sort puts NULL last [OK]

Hint: Remember: NULLs come last by default in ascending order [OK]

Common Mistakes:

• Thinking NULLs are treated as zero
• Assuming NULLs appear first by default
• Believing NULLs cause errors in ORDER BY

2. Which of the following is the correct syntax to sort a column named score in ascending order but place NULL values first?

easy

A. ORDER BY score ASC NULLS FIRST

B. ORDER BY score ASC NULLS LAST

C. ORDER BY score NULLS FIRST ASC

D. ORDER BY NULLS FIRST score ASC

Solution

1. Step 1: Recall correct syntax for NULL placement

   The correct syntax places NULLS FIRST or NULLS LAST after the sort direction (ASC or DESC).

2. Step 2: Match syntax with options

   ORDER BY score ASC NULLS FIRST uses ORDER BY score ASC NULLS FIRST, which is the correct order of keywords.

3. Final Answer:

   ORDER BY score ASC NULLS FIRST -> Option A

4. Quick Check:

   NULLS FIRST comes after ASC/DESC [OK]

Hint: Put NULLS FIRST or LAST after ASC/DESC in ORDER BY [OK]

Common Mistakes:

• Placing NULLS FIRST before column name
• Mixing order of ASC/DESC and NULLS FIRST
• Using NULLS FIRST without ASC or DESC

3. Given the table students with column grade containing values (85, NULL, 90, 70), what is the result of this query?

SELECT grade FROM students ORDER BY grade DESC NULLS LAST;

medium

A. [NULL, 90, 85, 70]

B. [90, 85, 70, NULL]

C. [90, 85, 70]

D. [70, 85, 90, NULL]

Solution

1. Step 1: Sort grades descending with NULLs last

   Descending order sorts from highest to lowest: 90, 85, 70. NULLs go last as specified.

2. Step 2: Arrange final output

   So the order is 90, 85, 70, then NULL at the end.

3. Final Answer:

   [90, 85, 70, NULL] -> Option B

4. Quick Check:

   DESC with NULLS LAST puts NULL at end [OK]

Hint: DESC sorts high to low; NULLS LAST puts NULL at end [OK]

Common Mistakes:

• Placing NULL at start despite NULLS LAST
• Ignoring NULLS LAST and using default NULL position
• Sorting ascending instead of descending

4. Identify the error in this SQL query:

SELECT name FROM employees ORDER BY salary NULLS FIRST ASC;

medium

A. The order of NULLS FIRST and ASC is incorrect.

B. NULLS FIRST cannot be used with ASC.

C. Missing comma between salary and NULLS FIRST.

D. The query is correct and will run without errors.

Solution

1. Step 1: Check correct syntax order for ORDER BY

   The correct syntax is ORDER BY salary ASC NULLS FIRST. NULLS FIRST must come after ASC or DESC.

2. Step 2: Identify error in given query

   The query places NULLS FIRST before ASC, which is invalid syntax.

3. Final Answer:

   The order of NULLS FIRST and ASC is incorrect. -> Option A

4. Quick Check:

   NULLS FIRST must follow ASC/DESC [OK]

Hint: NULLS FIRST/LAST always after ASC or DESC [OK]

Common Mistakes:

• Placing NULLS FIRST before ASC/DESC
• Adding commas in ORDER BY clause
• Assuming NULLS FIRST is invalid with ASC

5. You want to list products by price ascending but ensure that products with unknown price (NULL) appear at the top. Which query achieves this correctly?

hard

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

B. SELECT * FROM products ORDER BY price NULLS FIRST ASC;

C. SELECT * FROM products ORDER BY NULLS FIRST price ASC;

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

Solution

1. Step 1: Understand requirement for NULL placement

   We want ascending order but NULL values first, so use NULLS FIRST after ASC.

2. Step 2: Check syntax correctness

   SELECT * FROM products ORDER BY price ASC NULLS FIRST; uses ORDER BY price ASC NULLS FIRST, which is valid and meets the requirement.

3. Final Answer:

   SELECT * FROM products ORDER BY price ASC NULLS FIRST; -> Option D

4. Quick Check:

   ASC with NULLS FIRST puts NULLs on top [OK]

Hint: Use ASC NULLS FIRST to put NULLs at top in ascending sort [OK]

Common Mistakes:

• Placing NULLS FIRST before ASC
• Using NULLS LAST instead of NULLS FIRST
• Misordering keywords causing syntax errors