Free Practice Quiz Question List

4 SQL Joins Online Quiz Questions

Use this free practice quiz with 30 questions to review 4 SQL Joins, test your knowledge, and prepare for your next test or exam.

30 questions

Choose one

1 point

Given employees(employee_id, name, department_id) and departments(department_id, department_name), which ON condition matches employees to their departments using the stated key relationship?

A
ON employees.employee_id = departments.department_id
B
ON employees.department_id = departments.department_id
C
ON employees.name = departments.department_name
D
ON employees.department_id = departments.department_name

Choose one

1 point

A query uses FROM employees AS e JOIN departments AS d ON e.department_id = d.department_id. An employee has no matching department. What happens to that employee's row?

A
It keeps every employee and fills department columns with NULL when unmatched.
B
It keeps every department and fills employee columns with NULL when unmatched.
C
It returns only employees and departments whose department IDs match.
D
It returns every possible employee-department pair.

Choose one

1 point

You need a result that includes every employee, even employees without a matching department. Which join setup meets that requirement?

A
Use employees AS e LEFT JOIN departments AS d with the department-ID match condition.
B
Use employees AS e INNER JOIN departments AS d with the department-ID match condition.
C
Use employees AS e RIGHT JOIN departments AS d with the department-ID match condition.
D
Use employees AS e CROSS JOIN departments AS d.

Choose one

1 point

Using employees AS e and departments AS d, which approach returns only employees who have no matching department?

A
Use INNER JOIN and WHERE d.department_id IS NOT NULL.
B
Use LEFT JOIN and WHERE e.department_id IS NULL.
C
Use FULL OUTER JOIN and WHERE d.department_id IS NOT NULL.
D
Use LEFT JOIN and WHERE d.department_id IS NULL.

Choose one

1 point

For A RIGHT JOIN B ON A.key = B.key, what happens to a row in B that has no matching row in A?

A
Every row from the left table remains, with right-side columns set to NULL for unmatched rows.
B
Every row from the right table remains, with left-side columns set to NULL for unmatched rows.
C
Only matching row pairs remain.
D
Every possible pair of rows is returned.

Choose one

1 point

You need matched records plus unmatched records from both input tables. Which join type should you use?

A
It returns only matching pairs.
B
It preserves all rows from the left table but discards unmatched right rows.
C
It preserves all rows from both tables and uses NULL for columns on an unmatched side.
D
It returns every possible pair, whether or not the keys match.

Choose one

1 point

A products table has 8 rows and a sizes table has 5 rows. How many rows does products CROSS JOIN sizes return?

A
40 rows
B
13 rows
C
8 rows
D
5 rows

Choose one

1 point

Given orders(order_id, customer_id), customers(customer_id, name), order_lines(order_id, product_id, quantity), and products(product_id, product_name), which set of join conditions connects each order to its customer and its line-item products?

A
Join orders to products on order_id, then customers to order_lines on customer_id.
B
Join orders to customers on order_id, then order_lines to products on order_id.
C
Join orders to order_lines on product_id, then order_lines to products on order_id.
D
Join orders to customers on customer_id, orders to order_lines on order_id, and order_lines to products on product_id.

Choose one

1 point

An order has three matching rows in order_lines. A query joins orders to order_lines on order_id. How can that order appear in the result?

A
The order must appear only once because order_id is unique in orders.
B
The order can appear on multiple result rows, one per matching order line.
C
The join discards all but the first matching order line.
D
The join produces every product for that order, even products not in its order lines.

Choose one

1 point

A query uses employees AS e LEFT JOIN departments AS d ON e.department_id = d.department_id, followed by WHERE d.department_name = 'Sales'. Why might this fail to retain employees with no matching department, and how can the query be adjusted if those employees must remain?

A
The WHERE condition makes the join preserve more unmatched employees.
B
The query becomes a cross join because the filter refers to the right table.
C
The WHERE condition removes NULL-extended rows; moving the restriction into ON can preserve unmatched employees.
D
The filter has no effect on rows where d.department_name is NULL.

Choose one

1 point

You want every employee in the result, but you want only Sales departments to match. Where should you put the department-name restriction?

A
Put d.department_name = 'Sales' in the ON condition and keep the LEFT JOIN.
B
Keep the LEFT JOIN, but put d.department_name = 'Sales' in a WHERE condition.
C
Change the join to INNER JOIN and put d.department_name = 'Sales' in the ON condition.
D
Change the join to CROSS JOIN and put d.department_name = 'Sales' in the ON condition.

Choose one

1 point

Why might a query that joins two tables use SELECT e.name, d.department_name instead of SELECT *?

A
It forces unmatched rows to be excluded.
B
It automatically removes duplicate result rows.
C
It changes the join into an inner join.
D
It makes the result easier to interpret and avoids unnecessary or duplicate-named columns.

Choose one

1 point

Given employees(employee_id, name, department_id) and departments(department_id, department_name), which condition matches an employee to the referenced department?

A
Match employees.employee_id to departments.department_id.
B
Match employees.department_id to departments.department_id.
C
Match employees.name to departments.department_name.
D
Match employees.department_id to departments.department_name.

Choose one

1 point

A query uses INNER JOIN to connect employees with departments. What happens to an employee whose department_id has no matching row in departments?

A
Every employee, with NULL department fields for employees without a match.
B
Every department, with NULL employee fields for departments without a match.
C
Only employees and departments whose rows satisfy the join condition.
D
Every possible employee-department pair.

Choose one

1 point

The query starts with employees LEFT JOIN departments. If one employee has no matching department, how is that employee represented in the result?

A
Every employee remains; right-table columns are NULL when no department matches.
B
Every department remains; left-table columns are NULL when no employee matches.
C
Only employees with a matching department remain.
D
Every possible employee-department pair is returned.

Choose one

1 point

In employees RIGHT JOIN departments, which unmatched rows are guaranteed to remain?

A
Only matching rows remain, with no NULL values.
B
Every employees row remains, whether or not it matches.
C
Every row from both tables remains, whether or not it matches.
D
Every departments row remains; left-table columns are NULL when no employee matches.

Choose one

1 point

You need a result that retains every row from both employees and departments, including rows with no match. Which join type meets this requirement?

A
Only matched rows from the two tables.
B
Matched rows and unmatched rows from both tables, with NULLs on the missing side.
C
All rows from the left table, but no unmatched rows from the right table.
D
Every possible pair of rows, whether or not the join condition matches.

Choose one

1 point

A products table has 4 rows and a sizes table has 3 rows. How many rows does products CROSS JOIN sizes return?

A
7 rows
B
1 row
C
12 rows
D
4 rows

Choose one

1 point

An order query uses orders, customers, order_lines, and products. Which set of join relationships correctly follows the schema described for these tables?

A
Join orders to customers on customer_id, orders to order_lines on order_id, and order_lines to products on product_id.
B
Join orders to customers on order_id, orders to order_lines on product_id, and order_lines to products on customer_id.
C
Join orders to customers on product_id, orders to order_lines on customer_id, and order_lines to products on order_id.
D
Join every table to customers using customer_id, without conditions for the other relationships.

Choose one

1 point

An order has three matching rows in order_lines. When the query joins orders to order_lines, how many result rows represent that order?

A
One row, because each order can appear only once in a join result.
B
Two rows, because the order is represented once for the order and once for its lines.
C
Four rows, because the order row is repeated in addition to its three lines.
D
Three rows, one for each matching order line.

Choose one

1 point

A query uses employees LEFT JOIN departments, then filters with WHERE departments.department_name = 'Sales'. What happens to employees with no matching department?

A
It retains all employees because the query uses LEFT JOIN.
B
It removes employees without a matching department, so the unmatched left rows do not survive.
C
It turns all unmatched department values into the text 'Sales'.
D
It adds all departments with no employees to the result.

Choose one

1 point

You want every employee to remain in the result, but you want only matching departments named 'Sales' to be included. Where should the department-name restriction go?

A
Put the restriction in WHERE and add DISTINCT so unmatched employees remain.
B
Replace LEFT JOIN with INNER JOIN and filter in ON.
C
Put the department-name restriction in ON so employees without a qualifying match remain with NULL department fields.
D
Use CROSS JOIN and filter out employees after the join.

Choose one

1 point

Both joined tables have a column named department_id. What is a characteristic of writing the join condition as USING (department_id)?

A
It matches on the shared column and returns one copy of that matching column.
B
It returns two copies of the shared column, one from each table.
C
It creates every possible pair of rows and ignores the shared column.
D
It keeps only rows that have no match.

Choose one

1 point

Why is selecting named columns often preferable to SELECT * in a query that joins tables?

A
SELECT * guarantees that matching rows are not duplicated.
B
SELECT * automatically changes an outer join into an inner join.
C
Explicit columns prevent one-to-many relationships from producing repeated rows.
D
Naming the needed columns makes the result easier to interpret and avoids unnecessary or duplicate-named columns.

Choose one

1 point

Given employees and departments joined on employees.department_id = departments.department_id, what does an INNER JOIN return?

A
It returns every employee and fills department columns with NULL when there is no match.
B
It returns only employee-department row pairs that satisfy the join condition.
C
It returns every possible employee-department pair.
D
It returns every department, including those without employees.

Choose one

1 point

A products table has 3 rows and a sizes table has 4 rows. How many rows does products CROSS JOIN sizes return?

A
3
B
7
C
12
D
24

Choose one

1 point

A report must include every employee and every department, including employees without a department match and departments without employees. Which join behavior meets this requirement?

A
Keep all rows from both tables; use NULLs for columns from the side without a match.
B
Keep only rows with matches in both tables.
C
Keep all rows from the left table but discard unmatched rows from the right table.
D
Return every possible pair of rows from the two tables.

Choose one

1 point

Using employees AS e and departments AS d, which approach returns employees who have no matching department?

A
Use an inner join and filter with d.department_id IS NULL.
B
Use a right join and filter with e.department_id IS NULL.
C
Use a cross join and filter with d.department_id IS NULL.
D
Use a left join and filter with d.department_id IS NULL.

Choose one

1 point

An order has three matching rows in order_lines. In a query joining orders to order_lines, why might the order's details appear on three result rows?

A
The join removes the order whenever it has more than one line.
B
The query creates one row for every customer, regardless of orders.
C
Each matching order line contributes a result row, so the order details repeat.
D
The join combines all order lines into one result row automatically.

Choose one

1 point

You need every employee to remain in the result, but only want a right-table row to match when it meets an additional condition. Where should you put that right-table restriction?

A
Put the restriction in WHERE; it preserves all unmatched left rows.
B
Put the restriction in ON; this restricts right-side matches while preserving unmatched left rows.
C
Replace the left join with an inner join; it retains unmatched employees.
D
Put the restriction in SELECT; this changes which right-side rows match.