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.