NULLs in JOIN conditions in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to join two tables on the column 'id'.

SQL

SELECT * FROM table1 JOIN table2 ON table1.id [1] table2.id;

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

A=

B!=

C<>

DLIKE

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' or '<>' which check for inequality.

Using 'LIKE' which is for pattern matching, not equality.

The correct operator to join tables on matching 'id' values is '='.

2fill in blank

medium

Complete the code to perform a LEFT JOIN that includes all rows from table1.

SQL

SELECT * FROM table1 [1] JOIN table2 ON table1.id = table2.id;

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

AINNER

BRIGHT

CLEFT

DFULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN which excludes unmatched rows.

Using RIGHT JOIN which keeps all rows from the right table.

A LEFT JOIN returns all rows from the left table (table1) and matched rows from the right table.

3fill in blank

hard

Fix the error in the JOIN condition to correctly handle NULL values in 'id'.

SQL

SELECT * FROM table1 JOIN table2 ON table1.id = table2.id OR (table1.id IS NULL [1] table2.id IS NULL);

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

AOR

BAND

CNOT

DXOR

Attempts:

3 left

💡 Hint

Common Mistakes

Using OR which matches rows incorrectly when only one side is NULL.

Using NOT or XOR which are not logical for this condition.

To treat NULLs as equal in the join, use AND to check both are NULL.

4fill in blank

hard

Fill both blanks to join tables treating NULLs as equal in 'key' columns.

SQL

SELECT * FROM t1 JOIN t2 ON (t1.key [1] t2.key) OR (t1.key IS NULL [2] t2.key IS NULL);

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

A=

B!=

CAND

DOR

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '=' for key comparison.

Using 'OR' instead of 'AND' for NULL checks.

Use '=' to compare keys and 'AND' to check both keys are NULL together.

5fill in blank

hard

Fill all three blanks to select rows where 'col1' matches or both are NULL, and 'col2' matches.

SQL

SELECT * FROM A JOIN B ON (A.col1 [1] B.col1 OR (A.col1 IS NULL [2] B.col1 IS NULL)) AND A.col2 [3] B.col2;

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

A=

B!=

CAND

DOR

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '=' for comparisons.

Using 'OR' instead of 'AND' for NULL checks.

Use '=' to compare columns, 'AND' to ensure both col1 are NULL together, and '=' for col2 match.

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