Multiple LEFT JOINs in one query in SQL - Step-by-Step Execution

Concept Flow - Multiple LEFT JOINs in one query

Start with main table

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LEFT JOIN first table

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Combine rows, keep all from main

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LEFT JOIN second table

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Combine rows, keep all from main

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Result with all rows from main and matching rows from joined tables

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END

Start with a main table, then add rows from other tables using LEFT JOINs one by one, keeping all rows from the main table even if no match is found.

Execution Sample

SQL

SELECT A.id, B.name, C.status
FROM A
LEFT JOIN B ON A.id = B.a_id
LEFT JOIN C ON A.id = C.a_id;

This query selects data from table A and adds matching data from tables B and C using LEFT JOINs.

Execution Table

Step	Action	Tables Involved	Rows Resulting	Explanation
1	Start with table A	A	3 rows	Initial rows from A: ids 1, 2, 3
2	LEFT JOIN B on A.id = B.a_id	A, B	3 rows	Rows from A kept; B data added where matches exist (id=1,2) else NULL
3	LEFT JOIN C on A.id = C.a_id	A, B, C	3 rows	Rows from previous step kept; C data added where matches exist (id=2,3) else NULL
4	Final result	A, B, C	3 rows	All rows from A with matching B and C data or NULLs where no match

💡 All rows from A are included; LEFT JOINs add matching rows from B and C or NULL if no match

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
Rows	3 rows from A	3 rows with B data or NULL	3 rows with B and C data or NULL	3 rows with combined data

Key Moments - 2 Insights

Why do we still have 3 rows after the LEFT JOINs even if some joined tables have no matching rows?

Because LEFT JOIN keeps all rows from the main table (A) regardless of matches in joined tables, as shown in execution_table rows 2 and 3.

What happens to columns from joined tables when there is no matching row?

Those columns show NULL values, as seen in execution_table row 2 for id=3 in table B and row 3 for id=1 in table C.

Visual Quiz - 3 Questions

Look at the execution table, how many rows are in the result after the first LEFT JOIN?

A 2 rows

B 3 rows

C 5 rows

D 0 rows

💡 Hint

Check execution_table row 2 under 'Rows Resulting'

At which step do columns from table C get added to the result?

A Step 1

B Step 2

C Step 3

D Final step only

💡 Hint

Look at execution_table row 3 'Action' column

If table B had no matching rows for any A.id, what would happen to the number of rows after the first LEFT JOIN?

A Number of rows would stay the same

B Number of rows would increase

C Number of rows would decrease

D Query would fail

💡 Hint

LEFT JOIN keeps all rows from the main table as shown in execution_table row 2

Concept Snapshot

Multiple LEFT JOINs combine one main table with several others.
Syntax: SELECT ... FROM main LEFT JOIN table1 ON ... LEFT JOIN table2 ON ...
All rows from main table stay in result.
Joined tables add matching data or NULL if no match.
Useful to gather related info without losing main rows.

Full Transcript

This visual execution shows how multiple LEFT JOINs work in SQL. We start with a main table A with 3 rows. Then we LEFT JOIN table B on matching ids, keeping all rows from A and adding B's data where it matches, else NULL. Next, we LEFT JOIN table C similarly. The final result has all rows from A with matching data from B and C or NULLs if no match. Key points: LEFT JOIN keeps all main rows; unmatched joined columns become NULL. This helps combine data from multiple tables without losing main table rows.