LEFT JOIN in MySQL - Step-by-Step Execution

Concept Flow - LEFT JOIN

Start with LEFT table

↓

For each row in LEFT table

↓

Find matching rows in RIGHT table

Yes No↓

Combine

↓

Add to result

↓

Next row

↓

End

LEFT JOIN takes all rows from the left table and matches rows from the right table. If no match, it fills with NULLs.

Execution Sample

MySQL

SELECT A.id, A.name, B.score
FROM Students A
LEFT JOIN Scores B ON A.id = B.student_id;

This query lists all students with their scores if available; if no score, shows NULL.

Execution Table

Step	Current LEFT row (A.id, A.name)	Matching RIGHT rows found?	Action	Output row
1	(1, 'Alice')	Yes (score=85)	Combine A and B row	(1, 'Alice', 85)
2	(2, 'Bob')	No	Combine A row with NULLs	(2, 'Bob', NULL)
3	(3, 'Charlie')	Yes (score=92)	Combine A and B row	(3, 'Charlie', 92)
4	(4, 'Diana')	No	Combine A row with NULLs	(4, 'Diana', NULL)
End	No more rows in LEFT table	-	-	Query complete

💡 All rows from LEFT table processed; unmatched RIGHT rows ignored.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
Current LEFT row	None	(1, 'Alice')	(2, 'Bob')	(3, 'Charlie')	(4, 'Diana')	None
Matching RIGHT rows	None	Yes (score=85)	No	Yes (score=92)	No	None
Output rows count	0	1	2	3	4	4

Key Moments - 2 Insights

Why do some output rows have NULL values?

When no matching row is found in the RIGHT table for a LEFT row, LEFT JOIN fills those columns with NULLs, as shown in steps 2 and 4 in the execution_table.

Are rows from the RIGHT table without matches included?

No, LEFT JOIN includes all LEFT table rows but excludes RIGHT table rows without matches, as seen in the exit_note and no extra rows beyond LEFT table count.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output row at step 3?

A (3, 'Charlie', NULL)

B (3, 'Charlie', 92)

C (3, 'Charlie', 85)

D (3, 'Charlie', 0)

💡 Hint

Check the 'Output row' column in row with Step '3' in execution_table.

At which step does the LEFT JOIN fill NULLs for the RIGHT table columns?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at 'Matching RIGHT rows found?' and 'Output row' columns for NULL values in execution_table.

If a new LEFT row has no matching RIGHT row, what happens to the output?

A It appears with NULLs for RIGHT table columns

B It is excluded from the result

C It causes an error

D It duplicates the last matched RIGHT row

💡 Hint

Refer to steps 2 and 4 in execution_table where no matches lead to NULLs.

Concept Snapshot

LEFT JOIN syntax:
SELECT columns
FROM left_table
LEFT JOIN right_table ON condition;

Behavior:
- Includes all rows from left_table
- Matches rows from right_table
- Fills NULLs if no match
- Ignores unmatched right_table rows

Full Transcript

LEFT JOIN takes every row from the left table and tries to find matching rows in the right table using the join condition. For each left row, if a matching right row exists, it combines them into one output row. If no match is found, it still outputs the left row but fills the right table columns with NULL values. Rows in the right table without matches are not included in the result. This ensures all left table data is preserved, with right table data added when available.