ORDER BY with NULL values behavior in SQL - Step-by-Step Execution

Concept Flow - ORDER BY with NULL values behavior

Start with unsorted data

↓

Apply ORDER BY clause

↓

Check for NULL values

↓

If NULLs, decide position

↓

NULLS FIRST or NULLS LAST

↓

Sort NULLs accordingly

↓

Sort non-NULL values

↓

Return sorted result

The ORDER BY clause sorts data, handling NULL values by placing them either first or last depending on the NULLS FIRST or NULLS LAST option.

Execution Sample

SQL

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

This query sorts players by score, placing NULL scores at the end.

Execution Table

Step	Row	score value	NULL check	Action	Position in sorted list
1	Alice	85	Not NULL	Consider for sorting	To be placed among non-NULLs
2	Bob	NULL	NULL	Place according to NULLS LAST	At the end
3	Charlie	92	Not NULL	Consider for sorting	To be placed among non-NULLs
4	Diana	70	Not NULL	Consider for sorting	To be placed among non-NULLs
5	Eve	NULL	NULL	Place according to NULLS LAST	At the end
6	Sorting non-NULL scores	-	-	Sort 70, 85, 92 ascending	Diana, Alice, Charlie
7	Final sorted list	-	-	Combine sorted non-NULLs + NULLs last	Diana, Alice, Charlie, Bob, Eve

💡 All rows processed; NULLs placed last as specified; sorting complete.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
score	unsorted	85 (Alice)	NULL (Bob)	92 (Charlie)	70 (Diana)	sorted: 70, 85, 92, NULL, NULL
NULL position	unknown	checked	checked	checked	checked	NULLS LAST

Key Moments - 2 Insights

Why do NULL values appear at the end of the sorted list?

Because the query uses NULLS LAST, the execution_table rows 2, 5, and 7 show NULL values are placed after all non-NULL values.

Are NULL values considered when sorting non-NULL values?

No, as shown in execution_table row 6, only non-NULL scores are sorted; NULLs are handled separately.

Visual Quiz - 3 Questions

Look at the execution_table, at which step are NULL values assigned their position?

A Step 2 and Step 5

B Step 1

C Step 6

D Step 7

💡 Hint

Check the 'NULL check' and 'Action' columns in rows 2 and 5.

According to variable_tracker, what is the final order of scores?

A NULL, 70, 85, 92, NULL

B 85, 70, 92, NULL, NULL

C 70, 85, 92, NULL, NULL

D 92, 85, 70, NULL, NULL

💡 Hint

Look at the 'score' row in variable_tracker under 'Final'.

If we change NULLS LAST to NULLS FIRST, where would NULL values appear?

A At the end of the list

B At the beginning of the list

C In the middle of the list

D NULL values would be ignored

💡 Hint

Refer to concept_flow where NULLS FIRST places NULLs before non-NULLs.

Concept Snapshot

ORDER BY sorts rows by column values.
NULL values can be placed first or last using NULLS FIRST or NULLS LAST.
Without specifying, NULL placement depends on the database.
Use ORDER BY column NULLS FIRST to put NULLs at the start.
Use ORDER BY column NULLS LAST to put NULLs at the end.

Full Transcript

This visual execution shows how the ORDER BY clause sorts data with NULL values. The query orders players by their score, placing NULL scores last as specified by NULLS LAST. Each row is checked: non-NULL scores are sorted ascending, while NULL scores are assigned positions at the end. The variable tracker shows the score values before and after sorting. Key moments clarify why NULLs appear last and how they are excluded from the numeric sort. The quiz tests understanding of NULL placement steps and final order. Remember, NULLS FIRST would place NULLs at the beginning instead.

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