Subquery with EXISTS operator in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the EXISTS operator do in SQL?

EXISTS checks if a subquery returns any rows. It returns TRUE if the subquery has at least one row, otherwise FALSE.

beginner

Write a simple example of a query using EXISTS.

SELECT * FROM Customers WHERE EXISTS (SELECT 1 FROM Orders WHERE Orders.CustomerID = Customers.ID);
This returns customers who have at least one order.

intermediate

How is EXISTS different from IN in SQL?

EXISTS checks for the presence of rows and stops at the first match, making it efficient for some cases. IN compares values and may scan more rows.

intermediate

Can EXISTS subquery refer to the outer query tables?

Yes, EXISTS subqueries often use columns from the outer query to filter results, making them correlated subqueries.

intermediate

Why might you use EXISTS instead of JOIN?

EXISTS is useful when you only want to check if related rows exist, not to retrieve them. It can be faster and clearer for existence checks.

What does the EXISTS operator return if the subquery finds no rows?

A FALSE

B An error

C NULL

D TRUE

EXISTS returns FALSE if the subquery returns zero rows.

Which of the following is a valid use of EXISTS?

A SELECT * FROM Employees WHERE EXISTS Employees.DeptID = Departments.ID;

B SELECT * FROM Employees WHERE EXISTS (Departments.ID);

C SELECT * FROM Employees WHERE EXISTS Departments;

D SELECT * FROM Employees WHERE EXISTS (SELECT * FROM Departments WHERE Departments.ID = Employees.DeptID);

Option D correctly uses EXISTS with a subquery that relates to the outer query.

EXISTS subqueries are usually:

A Uncorrelated

B Always return multiple rows

C Correlated

D Used only with aggregate functions

EXISTS subqueries often refer to outer query columns, making them correlated.

Which is true about EXISTS performance?

A EXISTS always scans the entire subquery result

B EXISTS stops checking after finding the first matching row

C EXISTS is slower than IN in all cases

D EXISTS returns all rows from the subquery

EXISTS stops as soon as it finds one matching row, which can improve performance.

What does this query do? SELECT * FROM Products WHERE EXISTS (SELECT 1 FROM Orders WHERE Orders.ProductID = Products.ID);

A Returns products that have at least one order

B Returns all products

C Returns products with no orders

D Returns orders with matching products

It returns products that have at least one matching order.

Explain how the EXISTS operator works in SQL and give a simple example.

Think about checking if something exists before selecting.

Describe the difference between EXISTS and IN operators and when you might prefer EXISTS.

Consider performance and purpose of each operator.

Practice

1. What does the EXISTS operator do in an SQL query?

easy

A. Checks if a subquery returns any rows and returns TRUE or FALSE.

B. Counts the number of rows in a table.

C. Joins two tables based on a condition.

D. Deletes rows from a table.

Solution

1. Step 1: Understand the purpose of EXISTS

   The EXISTS operator checks if the subquery returns at least one row.

2. Step 2: Compare with other options

   Counting rows, joining tables, or deleting rows are different SQL operations unrelated to EXISTS.

3. Final Answer:

   Checks if a subquery returns any rows and returns TRUE or FALSE. -> Option A

4. Quick Check:

   EXISTS = TRUE if rows found [OK]

Hint: EXISTS means "is there at least one row?" [OK]

Common Mistakes:

• Thinking EXISTS counts rows instead of checking existence
• Confusing EXISTS with JOIN operations
• Assuming EXISTS deletes or modifies data

2. Which of the following is the correct syntax to use EXISTS in a WHERE clause?

easy

A. SELECT * FROM table WHERE EXISTS (SELECT column FROM table2);

B. SELECT * FROM table WHERE EXISTS = (SELECT column FROM table2);

C. SELECT * FROM table WHERE EXISTS IN (SELECT column FROM table2);

D. SELECT * FROM table WHERE EXISTS LIKE (SELECT column FROM table2);

Solution

1. Step 1: Review correct EXISTS syntax

   EXISTS is used as EXISTS (subquery) without any operator like =, IN, or LIKE.

2. Step 2: Identify incorrect options

   Options B, C, and D misuse operators (=, IN, LIKE) with EXISTS, causing syntax errors.

3. Final Answer:

   SELECT * FROM table WHERE EXISTS (SELECT column FROM table2); -> Option A

4. Quick Check:

   EXISTS uses only parentheses for subquery [OK]

Hint: EXISTS always followed by (subquery) without operators [OK]

Common Mistakes:

• Adding = or IN after EXISTS
• Using LIKE with EXISTS
• Forgetting parentheses around subquery

3. Given tables Customers(id, name) and Orders(id, customer_id), what does this query return?

SELECT name FROM Customers c WHERE EXISTS (SELECT 1 FROM Orders o WHERE o.customer_id = c.id);

medium

A. All customers including those without orders.

B. All customers who have placed at least one order.

C. All orders with customer names.

D. An error because of incorrect subquery.

Solution

1. Step 1: Understand the EXISTS subquery condition

   The subquery checks if there is at least one order with customer_id matching the customer's id.

2. Step 2: Determine which customers are returned

   Only customers with matching orders exist, so only those customers' names are returned.

3. Final Answer:

   All customers who have placed at least one order. -> Option B

4. Quick Check:

   EXISTS filters customers with orders [OK]

Hint: EXISTS filters rows with matching related data [OK]

Common Mistakes:

• Thinking it returns all customers regardless of orders
• Confusing with JOIN that returns all orders
• Assuming syntax error due to subquery

4. Identify the error in this query:

SELECT * FROM Employees e WHERE EXISTS SELECT * FROM Salaries s WHERE s.emp_id = e.id;

medium

A. EXISTS cannot be used in WHERE clause.

B. Using SELECT * inside EXISTS is not allowed.

C. Missing parentheses around the subquery after EXISTS.

D. The alias 'e' is not defined.

Solution

1. Step 1: Check EXISTS syntax

   EXISTS requires the subquery to be enclosed in parentheses.

2. Step 2: Identify the missing parentheses

   The query lacks parentheses around the subquery after EXISTS, causing a syntax error.

3. Final Answer:

   Missing parentheses around the subquery after EXISTS. -> Option C

4. Quick Check:

   EXISTS (subquery) needs parentheses [OK]

Hint: Always put parentheses around subquery after EXISTS [OK]

Common Mistakes:

• Omitting parentheses around subquery
• Thinking SELECT * is invalid inside EXISTS
• Misunderstanding alias usage

5. You want to find all products that have never been ordered. Given tables Products(id, name) and Orders(id, product_id), which query correctly uses EXISTS to find these products?

hard

A. SELECT name FROM Products p WHERE NOT IN (SELECT product_id FROM Orders);

B. SELECT name FROM Products p WHERE EXISTS (SELECT 1 FROM Orders o WHERE o.product_id = p.id);

C. SELECT name FROM Products p WHERE EXISTS NOT (SELECT 1 FROM Orders o WHERE o.product_id = p.id);

D. SELECT name FROM Products p WHERE NOT EXISTS (SELECT 1 FROM Orders o WHERE o.product_id = p.id);

Solution

1. Step 1: Understand the goal

   We want products with no matching orders, so the subquery should check for orders and we want those where no such orders exist.

2. Step 2: Use NOT EXISTS correctly

   NOT EXISTS with a subquery checking orders for the product returns products never ordered.

3. Step 3: Evaluate other options

   SELECT name FROM Products p WHERE EXISTS (SELECT 1 FROM Orders o WHERE o.product_id = p.id); returns products with orders, C has invalid syntax, D uses NOT IN which can cause issues with NULLs.

4. Final Answer:

   SELECT name FROM Products p WHERE NOT EXISTS (SELECT 1 FROM Orders o WHERE o.product_id = p.id); -> Option D

5. Quick Check:

   NOT EXISTS finds missing related rows [OK]

Hint: Use NOT EXISTS to find items with no related rows [OK]

Common Mistakes:

• Using EXISTS instead of NOT EXISTS for missing data
• Incorrect syntax like EXISTS NOT
• Using NOT IN which may fail with NULLs