Joining more than two tables in SQL - Step-by-Step Execution

Concept Flow - Joining more than two tables

Start with Table1

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Join Table2 on matching key

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Result: Combined Table1+Table2

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Join Table3 on matching key

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Final Result: Combined Table1+Table2+Table3

Start by joining the first two tables on a common column, then join the third table to the result using another matching column.

Execution Sample

SQL

SELECT A.id, A.name, B.order_id, C.product_name
FROM Customers A
JOIN Orders B ON A.id = B.customer_id
JOIN Products C ON B.product_id = C.id;

This query joins three tables: Customers, Orders, and Products to get customer info, their orders, and product names.

Execution Table

Step	Action	Tables Involved	Matching Condition	Result Rows
1	Start with Customers table	Customers	N/A	3 rows (Alice, Bob, Carol)
2	Join Orders on Customers.id = Orders.customer_id	Customers + Orders	Customers.id = Orders.customer_id	4 rows (matching orders)
3	Join Products on Orders.product_id = Products.id	Customers + Orders + Products	Orders.product_id = Products.id	4 rows (final combined)
4	Return final result with selected columns	All three tables	N/A	4 rows with customer, order, product info

💡 All matching rows combined; no more tables to join.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
Result Rows	3 (Customers)	4 (Customers + Orders)	4 (Customers + Orders + Products)	4 (Final combined rows)

Key Moments - 3 Insights

Why does the number of rows increase after the first join?

Because one customer can have multiple orders, the join duplicates customer rows for each matching order (see execution_table step 2).

Why do we join Products last?

We join Products last to add product details to each order. This step uses the product_id from Orders to match Products (execution_table step 3).

What happens if a customer has no orders?

In this INNER JOIN example, customers without orders are excluded because the join requires matching rows (not shown in this example).

Visual Quiz - 3 Questions

Look at the execution_table, how many rows are there after joining Customers and Orders?

A 3 rows

B 7 rows

C 4 rows

D 1 row

💡 Hint

Check the 'Result Rows' column in execution_table at Step 2.

At which step does the Products table get joined?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Action' column in execution_table for when Products is joined.

If a customer has no orders, what happens to that customer in this query?

A They appear with NULL order and product info

B They are excluded from the result

C They appear multiple times

D The query returns an error

💡 Hint

Recall the join type is INNER JOIN, which only includes matching rows (see key_moments).

Concept Snapshot

Joining more than two tables:
Use multiple JOIN clauses.
Each JOIN connects two tables on matching columns.
Result grows as tables combine.
INNER JOIN excludes non-matching rows.
Order of joins matters for clarity.

Full Transcript

This visual execution shows how to join more than two tables in SQL. We start with the Customers table, then join Orders using the customer ID, increasing rows when customers have multiple orders. Next, we join Products using the product ID from Orders to add product names. The final result combines all three tables' data. The execution table tracks each step, showing how rows increase and how tables combine. Key moments clarify why rows increase after joins and what happens to customers without orders. The quiz tests understanding of row counts and join order. Remember, INNER JOIN only includes rows with matches in all tables.