NULLs in JOIN conditions in SQL - Step-by-Step Execution

Concept Flow - NULLs in JOIN conditions

Start with two tables

↓

Perform JOIN

↓

Compare join keys

↓

Check if keys are NULL

Yes No↓

No match

↓

Return joined rows or NULLs

↓

End

This flow shows how SQL JOIN compares keys, treats NULLs as unknown, and only matches rows when keys are equal and not NULL.

Execution Sample

SQL

SELECT A.id, B.id
FROM A
LEFT JOIN B ON A.id = B.id;

This query joins tables A and B on the id column, showing how NULLs affect matching.

Execution Table

Step	A.id	B.id	Join Condition (A.id = B.id)	Match Result	Output Row
1	1	1	1 = 1 (True)	Match	(1, 1)
2	2	NULL	2 = NULL (Unknown)	No Match	(2, NULL) from A with NULL in B columns
3	NULL	2	NULL = 2 (Unknown)	No Match	(NULL, NULL) from A with NULL in B columns
4	NULL	NULL	NULL = NULL (Unknown)	No Match	(NULL, NULL) from A with NULL in B columns
5	3	3	3 = 3 (True)	Match	(3, 3)

💡 All rows processed; NULL comparisons never match, so LEFT JOIN returns NULLs for B columns when no match.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
A.id	1	1	2	NULL	NULL	3	3
B.id	1	1	NULL	2	NULL	3	3
Join Condition Result	N/A	True	Unknown	Unknown	Unknown	True	True
Match Result	N/A	Match	No Match	No Match	No Match	Match	Match
Output Row	N/A	(1,1)	(2,NULL)	(NULL,NULL)	(NULL,NULL)	(3,3)	(3,3)

Key Moments - 2 Insights

Why does a join condition comparing NULL to any value not result in a match?

Because in SQL, NULL means unknown, so any comparison with NULL returns unknown (not true). The join only matches when the condition is true, so rows with NULL keys do not match (see execution_table rows 2-4).

Why does a LEFT JOIN still return rows from the left table even when there is no match?

LEFT JOIN returns all rows from the left table. When no match is found on the right, it fills right table columns with NULLs. This is shown in execution_table rows 2-4 where B columns are NULL.

Visual Quiz - 3 Questions

Look at the execution table, what is the match result when A.id is 2 and B.id is NULL?

A No Match

B Match

C Error

D Unknown but treated as Match

💡 Hint

Check row 2 in the execution_table under 'Match Result' column.

At which step does the join condition evaluate to true?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Join Condition (A.id = B.id)' column in the execution_table.

If the join was INNER JOIN instead of LEFT JOIN, what would happen to rows where A.id is 2 and B.id is NULL?

A They would appear with NULLs for B columns

B They would be excluded from the output

C They would cause an error

D They would match anyway

💡 Hint

INNER JOIN only returns rows where join condition is true; see execution_table rows 2-4 for no matches.

Concept Snapshot

SQL JOIN compares keys to match rows.
NULL means unknown, so NULL = value is never true.
JOIN matches only when condition is true.
LEFT JOIN returns all left rows, filling NULLs for no matches.
NULLs in join keys cause no matches, not errors.

Full Transcript

This visual execution shows how SQL JOIN handles NULLs in join conditions. We start with two tables A and B, each having some NULL values in the join key column 'id'. The JOIN compares A.id and B.id for equality. When either side is NULL, the comparison returns unknown, so no match occurs. For example, when A.id=2 and B.id=NULL, the join condition is unknown, so no match. LEFT JOIN returns all rows from A, and for no matches, fills B columns with NULLs. Only rows where both keys are equal and not NULL match. This explains why NULLs in join keys prevent matching rows in SQL JOINs.

Practice

1. What happens when you use an INNER JOIN on columns that contain NULL values in SQL?

easy

A. Rows with NULL in join columns are always included.

B. Rows with NULL in join columns are excluded from the result.

C. The query returns an error due to NULL values.

D. Rows with NULL are treated as equal and joined together.

Solution

1. Step 1: Understand how INNER JOIN works with = condition

   When joining tables, SQL compares join columns using =. If a column has NULL, it means unknown, so comparison fails.

2. Step 2: Effect of NULL in join columns

   Since NULL = NULL is not true, rows with NULL in join columns do not match and are excluded.

3. Final Answer:

   Rows with NULL in join columns are excluded from the result. -> Option B

4. Quick Check:

   NULL means unknown, so no match [OK]

Hint: Remember: NULL never equals NULL in joins [OK]

Common Mistakes:

• Assuming NULL equals NULL in join conditions
• Expecting NULL rows to appear in INNER JOIN results
• Thinking NULL causes query errors in JOIN

2. Which of the following SQL JOIN syntaxes correctly includes rows with NULL values in the join column from the left table?

easy

A. SELECT * FROM A INNER JOIN B ON A.id = B.id;

B. SELECT * FROM A RIGHT JOIN B ON A.id = B.id;

C. SELECT * FROM A LEFT JOIN B ON A.id = B.id;

D. SELECT * FROM A FULL JOIN B ON A.id = B.id WHERE A.id IS NOT NULL;

Solution

1. Step 1: Identify JOIN types that keep left table rows

   LEFT JOIN keeps all rows from the left table, including those with NULL in join columns.

2. Step 2: Check other options

   INNER JOIN excludes unmatched rows; RIGHT JOIN keeps right table rows; FULL JOIN keeps all but filtering with WHERE A.id IS NOT NULL removes left table NULLs.

3. Final Answer:

   SELECT * FROM A LEFT JOIN B ON A.id = B.id; -> Option C

4. Quick Check:

   LEFT JOIN keeps left rows even if join column is NULL [OK]

Hint: Use LEFT JOIN to keep left table NULL rows [OK]

Common Mistakes:

• Using INNER JOIN expecting NULL rows to appear
• Filtering NULLs after FULL JOIN with WHERE clause
• Confusing RIGHT JOIN with LEFT JOIN behavior

3. Given tables:
Table A: id
1
2
NULL

Table B: id
2
NULL

What is the result of this query?

SELECT A.id, B.id FROM A INNER JOIN B ON A.id = B.id;

medium

A. (1, 2), (2, 2), (NULL, NULL)

B. (NULL, NULL)

C. (1, 2), (2, 2)

D. (2, 2)

Solution

1. Step 1: Understand INNER JOIN with NULLs

   INNER JOIN matches rows where A.id = B.id. NULLs do not match anything, even NULL.

2. Step 2: Check matching rows

   Only A.id = 2 matches B.id = 2. Rows with NULL do not match.

3. Final Answer:

   (2, 2) -> Option D

4. Quick Check:

   NULLs do not match, only 2 joins [OK]

Hint: NULLs never match in INNER JOIN conditions [OK]

Common Mistakes:

• Including NULL = NULL as a match
• Assuming all rows join regardless of NULL
• Confusing INNER JOIN with OUTER JOIN behavior

4. You wrote this query:

SELECT * FROM A LEFT JOIN B ON A.id = B.id WHERE B.id IS NOT NULL;

But it returns no rows, even though you expect rows from A with NULL id. What is the likely problem?

medium

A. The WHERE B.id IS NOT NULL filters out all rows including those with NULL in A.id.

B. LEFT JOIN does not include rows with NULL in join columns.

C. You should use INNER JOIN instead of LEFT JOIN.

D. The join condition A.id = B.id causes a syntax error.

Solution

1. Step 1: Understand LEFT JOIN with WHERE filter

   LEFT JOIN keeps all A rows, but WHERE B.id IS NOT NULL filters only rows where B.id is NOT NULL after join.

2. Step 2: Effect on rows with NULL in A.id

   Rows with NULL in A.id do not match B.id, so B.id is NULL after the join. The WHERE B.id IS NOT NULL condition excludes these rows.

3. Final Answer:

   The WHERE B.id IS NOT NULL filters out all rows including those with NULL in A.id. -> Option A

4. Quick Check:

   WHERE filters after join, can exclude expected rows [OK]

Hint: Remember WHERE filters after JOIN, can exclude NULL rows [OK]

Common Mistakes:

• Thinking LEFT JOIN excludes NULL join columns
• Using INNER JOIN expecting NULL rows
• Assuming join condition causes syntax error

5. You want to join tables A and B on id columns, but include rows where id is NULL in either table. Which query correctly returns all rows from both tables, matching on id including NULLs treated as equal?

hard

A. SELECT * FROM A FULL JOIN B ON A.id = B.id OR (A.id IS NULL AND B.id IS NULL);

B. SELECT * FROM A FULL JOIN B ON A.id = B.id;

C. SELECT * FROM A INNER JOIN B ON A.id = B.id OR (A.id IS NULL AND B.id IS NULL);

D. SELECT * FROM A LEFT JOIN B ON A.id = B.id WHERE A.id IS NOT NULL;

Solution

1. Step 1: Understand FULL JOIN and NULL behavior

   FULL JOIN returns all rows from both tables, but A.id = B.id excludes NULL matches because NULL = NULL is false.

2. Step 2: Add explicit NULL check in join condition

   Using OR (A.id IS NULL AND B.id IS NULL) treats NULLs as equal, including those rows in join results.

3. Step 3: Check other options

   SELECT * FROM A FULL JOIN B ON A.id = B.id;

   excludes NULL matches;

   SELECT * FROM A INNER JOIN B ON A.id = B.id OR (A.id IS NULL AND B.id IS NULL);

   uses INNER JOIN, excluding unmatched rows;

   SELECT * FROM A LEFT JOIN B ON A.id = B.id WHERE A.id IS NOT NULL;

   filters out NULLs.

4. Final Answer:

   SELECT * FROM A FULL JOIN B ON A.id = B.id OR (A.id IS NULL AND B.id IS NULL); -> Option A

5. Quick Check:

   Explicit NULL checks in JOIN condition include NULL matches [OK]

Hint: Add IS NULL checks in JOIN to match NULLs explicitly [OK]

Common Mistakes:

• Assuming NULL = NULL matches automatically
• Using INNER JOIN instead of FULL JOIN for all rows
• Filtering NULLs unintentionally with WHERE clause