Cartesian product and joins in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the SQL query to select all columns from two tables using a Cartesian product.

DBMS Theory

SELECT * FROM employees [1] departments;

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AJOIN

BLEFT JOIN

CINNER JOIN

DCROSS JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN without an ON condition causes an error.

Using LEFT JOIN or JOIN without specifying join conditions.

The CROSS JOIN keyword produces the Cartesian product of the two tables, combining every row of the first table with every row of the second table.

2fill in blank

medium

Complete the SQL query to join two tables on a matching column.

DBMS Theory

SELECT * FROM orders [1] customers ON orders.customer_id = customers.id;

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AINNER JOIN

BCROSS JOIN

CFULL JOIN

DRIGHT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using CROSS JOIN when a matching condition is needed.

Using FULL JOIN without understanding it returns all rows from both tables.

The INNER JOIN keyword returns rows when there is a match in both tables based on the join condition.

3fill in blank

hard

Fix the error in the SQL join query by completing the missing keyword.

DBMS Theory

SELECT * FROM products [1] ON products.category_id = categories.id;

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ACROSS JOIN

BINNER JOIN

CJOIN

DLEFT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using CROSS JOIN with an ON condition causes syntax errors.

Using JOIN without specifying INNER or LEFT can cause ambiguity.

The INNER JOIN keyword is required here to join the tables based on the condition after ON. Using just JOIN is ambiguous in some SQL dialects.

4fill in blank

hard

Fill both blanks to create a query that selects employee names and department names using an INNER JOIN.

DBMS Theory

SELECT employees.name, departments.name FROM employees [1] departments [2] employees.department_id = departments.id;

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AINNER JOIN

BON

CWHERE

DCROSS JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using WHERE instead of ON for join conditions.

Using CROSS JOIN when a condition is specified.

The INNER JOIN keyword joins the tables, and the ON keyword specifies the join condition.

5fill in blank

hard

Fill all three blanks to write a query that selects all orders and their customers, including orders without customers, using a LEFT JOIN.

DBMS Theory

SELECT orders.id, customers.name FROM orders [1] customers [2] orders.customer_id = customers.id [3];

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ALEFT JOIN

BON

CORDER BY orders.id

DWHERE customers.id IS NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN excludes orders without customers.

Using WHERE to filter join results instead of ON.

The LEFT JOIN returns all orders even if there is no matching customer. The ON specifies the join condition. ORDER BY sorts the results by order ID.