INNER JOIN with multiple conditions in SQL - Step-by-Step Execution

Concept Flow - INNER JOIN with multiple conditions

Start with Table A

↓

Start with Table B

↓

Check Condition 1: A.col1 = B.col1

Yes↓

Check Condition 2: A.col2 = B.col2

Yes↓

Combine rows from A and B

↓

Add combined row to result

↓

Repeat for all rows in A and B

↓

End with result set of matched rows

The INNER JOIN compares rows from two tables and includes only those where all specified conditions are true.

Execution Sample

SQL

SELECT *
FROM Employees E
INNER JOIN Departments D
ON E.DeptID = D.DeptID
AND E.Location = D.Location;

This query joins Employees and Departments where both DeptID and Location match.

Execution Table

Step	E.DeptID	E.Location	D.DeptID	D.Location	Condition 1 (E.DeptID = D.DeptID)	Condition 2 (E.Location = D.Location)	Row Included?
1	10	NY	10	NY	True	True	Yes
2	10	NY	10	LA	True	False	No
3	20	LA	20	LA	True	True	Yes
4	20	LA	30	LA	False	True	No
5	30	NY	30	NY	True	True	Yes
6	30	NY	10	NY	False	True	No
7	-	-	-	-	-	-	End of rows

💡 All rows compared; only rows where both conditions are true are included.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
Row Included?	N/A	Yes	No	Yes	No	Yes	No	Final result includes rows 1, 3, and 5

Key Moments - 2 Insights

Why are some rows with matching DeptID excluded?

Rows are excluded if the second condition (Location match) is false, as shown in execution_table rows 2 and 6.

Does INNER JOIN include rows if only one condition matches?

No, both conditions must be true for a row to be included, as seen in rows 2 and 4 where one condition fails.

Visual Quiz - 3 Questions

Look at the execution table, at step 4, why is the row not included?

A Because Condition 2 is false

B Because both conditions are true

C Because Condition 1 is false

D Because both conditions are false

💡 Hint

Check the values under Condition 1 and Condition 2 columns at step 4 in the execution_table.

At which steps are rows included in the result?

A Steps 1, 3, 5

B Steps 2, 4, 6

C Steps 1, 2, 3

D Steps 4, 5, 6

💡 Hint

Look at the 'Row Included?' column in the execution_table for 'Yes' values.

If the second condition (Location match) was removed, what would happen to the included rows?

A No change in included rows

B More rows would be included

C Fewer rows would be included

D Query would return no rows

💡 Hint

Removing a condition relaxes the join criteria, so check how many rows satisfy only the first condition.

Concept Snapshot

INNER JOIN with multiple conditions syntax:
SELECT * FROM A INNER JOIN B ON A.col1 = B.col1 AND A.col2 = B.col2;
Only rows matching all conditions appear in the result.
Conditions are combined with AND.
Useful to filter joined rows precisely.

Full Transcript

This visual execution shows how an INNER JOIN with multiple conditions works. We start with two tables, Employees and Departments. For each pair of rows, we check if the DeptID matches and if the Location matches. Only when both conditions are true do we include the combined row in the result. The execution table lists each comparison step, showing which rows pass both conditions and are included. The variable tracker highlights how the inclusion decision changes step by step. Key moments clarify why some rows are excluded despite partial matches. The quiz tests understanding by asking about specific steps and the effect of conditions. This helps beginners see how multiple conditions in INNER JOIN filter the results carefully.

Practice

1. What does an INNER JOIN with multiple conditions do in SQL?

easy

A. It returns rows where all join conditions are true.

B. It returns rows where any one of the join conditions is true.

C. It returns all rows from both tables regardless of conditions.

D. It returns rows only from the first table.

Solution

1. Step 1: Understand INNER JOIN basics

   INNER JOIN returns rows that have matching values in both tables based on join conditions.

2. Step 2: Analyze multiple conditions with AND

   When multiple conditions are combined with AND, all must be true for a row to be included.

3. Final Answer:

   It returns rows where all join conditions are true. -> Option A

4. Quick Check:

   INNER JOIN + AND = all conditions true [OK]

Hint: All join conditions must be true with AND in INNER JOIN [OK]

Common Mistakes:

• Thinking any one condition is enough
• Confusing INNER JOIN with OUTER JOIN
• Ignoring the AND operator effect

2. Which of the following is the correct syntax for an INNER JOIN with two conditions on tables Orders and Customers?

easy

A. SELECT * FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.ID AND Orders.Status = Customers.Status;

B. SELECT * FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.ID OR Orders.Status = Customers.Status;

C. SELECT * FROM Orders INNER JOIN Customers WHERE Orders.CustomerID = Customers.ID AND Orders.Status = Customers.Status;

D. SELECT * FROM Orders JOIN Customers ON Orders.CustomerID = Customers.ID, Orders.Status = Customers.Status;

Solution

1. Step 1: Identify correct JOIN syntax

   INNER JOIN requires ON keyword followed by conditions combined with AND for multiple matches.

2. Step 2: Check each option's syntax

   SELECT * FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.ID AND Orders.Status = Customers.Status; correctly uses ON with two conditions joined by AND. SELECT * FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.ID OR Orders.Status = Customers.Status; uses OR which is incorrect for multiple conditions requiring all true. SELECT * FROM Orders INNER JOIN Customers WHERE Orders.CustomerID = Customers.ID AND Orders.Status = Customers.Status; wrongly uses WHERE instead of ON. SELECT * FROM Orders JOIN Customers ON Orders.CustomerID = Customers.ID, Orders.Status = Customers.Status; has invalid syntax with comma.

3. Final Answer:

   SELECT * FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.ID AND Orders.Status = Customers.Status; -> Option A

4. Quick Check:

   INNER JOIN + ON + AND = correct syntax [OK]

Hint: Use ON with AND for multiple join conditions [OK]

Common Mistakes:

• Using OR instead of AND in ON clause
• Replacing ON with WHERE for join conditions
• Separating conditions with commas

3. Given tables Employees and Departments with columns Employees.DeptID, Departments.ID, and Departments.Location, what will this query return?

SELECT Employees.Name, Departments.Name FROM Employees INNER JOIN Departments ON Employees.DeptID = Departments.ID AND Departments.Location = 'NY';

medium

A. Syntax error due to multiple conditions in JOIN.

B. All employees with their department names regardless of location.

C. Departments located in NY with all employees listed multiple times.

D. Employees working only in departments located in NY with their department names.

Solution

1. Step 1: Understand the JOIN conditions

   The query joins Employees and Departments where DeptID matches ID and the department location is 'NY'.

2. Step 2: Analyze the result rows

   Only employees whose department is in NY will appear with their department names. Others are excluded.

3. Final Answer:

   Employees working only in departments located in NY with their department names. -> Option D

4. Quick Check:

   INNER JOIN + multiple conditions filters rows precisely [OK]

Hint: Multiple conditions filter rows strictly in INNER JOIN [OK]

Common Mistakes:

• Assuming all employees appear regardless of location
• Thinking multiple conditions cause syntax error
• Confusing join condition with WHERE clause

4. Identify the error in this SQL query:

SELECT * FROM Products INNER JOIN Suppliers ON Products.SupplierID = Suppliers.ID, Products.Category = Suppliers.Category;

medium

A. Using ON instead of WHERE for join conditions.

B. Missing WHERE clause for the second condition.

C. Using comma instead of AND to combine join conditions.

D. INNER JOIN cannot have multiple conditions.

Solution

1. Step 1: Review JOIN condition syntax

   Multiple join conditions must be combined with AND inside the ON clause.

2. Step 2: Identify the error in the query

   The query incorrectly uses a comma to separate conditions, which is invalid syntax.

3. Final Answer:

   Using comma instead of AND to combine join conditions. -> Option C

4. Quick Check:

   Multiple conditions joined by AND, not comma [OK]

Hint: Use AND, not comma, to join multiple ON conditions [OK]

Common Mistakes:

• Separating conditions with commas
• Confusing WHERE and ON clauses
• Believing INNER JOIN allows only one condition

5. You have two tables: Orders(OrderID, CustomerID, Status) and Customers(CustomerID, Country, Status). You want to find orders where the customer is from 'USA' and both order and customer have the same status. Which query correctly uses INNER JOIN with multiple conditions?

hard

A. SELECT Orders.OrderID FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID WHERE Customers.Country = 'USA' AND Orders.Status = Customers.Status;

B. SELECT Orders.OrderID FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID AND Customers.Country = 'USA' AND Orders.Status = Customers.Status;

C. SELECT Orders.OrderID FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID OR Customers.Country = 'USA' AND Orders.Status = Customers.Status;

D. SELECT Orders.OrderID FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID, Customers.Country = 'USA', Orders.Status = Customers.Status;

Solution

1. Step 1: Understand the filtering requirements

   We need to join on CustomerID and filter where customer is from USA and order status matches customer status.

2. Step 2: Check correct use of multiple conditions in INNER JOIN

   SELECT Orders.OrderID FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID AND Customers.Country = 'USA' AND Orders.Status = Customers.Status; correctly uses AND to combine all conditions inside ON clause. SELECT Orders.OrderID FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID OR Customers.Country = 'USA' AND Orders.Status = Customers.Status; wrongly uses OR which changes logic. SELECT Orders.OrderID FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID WHERE Customers.Country = 'USA' AND Orders.Status = Customers.Status; moves conditions to WHERE, which is valid but not the question's focus on multiple ON conditions. SELECT Orders.OrderID FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID, Customers.Country = 'USA', Orders.Status = Customers.Status; uses commas incorrectly.

3. Final Answer:

   SELECT Orders.OrderID FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID AND Customers.Country = 'USA' AND Orders.Status = Customers.Status; -> Option B

4. Quick Check:

   Multiple AND conditions inside ON for precise join [OK]

Hint: Combine all join filters with AND inside ON clause [OK]

Common Mistakes:

• Using OR instead of AND in join conditions
• Placing join filters in WHERE instead of ON
• Separating conditions with commas