LEFT JOIN vs RIGHT JOIN decision in SQL - Visual Side-by-Side Comparison

Concept Flow - LEFT JOIN vs RIGHT JOIN decision

Start with LEFT JOIN

↓

Include all rows from Left Table

↓

Match rows from Right Table

↓

If no match, fill Right Table columns with NULL

↓

Result: All Left Table rows + matched Right Table rows

↓

Start with RIGHT JOIN

↓

Include all rows from Right Table

↓

Match rows from Left Table

↓

If no match, fill Left Table columns with NULL

↓

Result: All Right Table rows + matched Left Table rows

LEFT JOIN keeps all rows from the left table, matching right table rows or NULLs; RIGHT JOIN keeps all rows from the right table, matching left table rows or NULLs.

Execution Sample

SQL

SELECT A.id, B.value
FROM A
LEFT JOIN B ON A.id = B.id;

This query returns all rows from table A, and matching rows from B; if no match, B columns are NULL.

Execution Table

Step	Action	Left Table Row	Right Table Row	Result Row	Notes
1	Take first row from A	A.id=1	Find B.id=1	1, B.value=100	Match found, include both
2	Take second row from A	A.id=2	Find B.id=2	2, B.value=NULL	No match in B, fill NULL
3	Take third row from A	A.id=3	Find B.id=3	3, B.value=300	Match found, include both
4	No more rows in A				LEFT JOIN ends after all A rows processed

💡 All rows from left table A processed; right table B matched or NULL filled

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
Current A row	None	id=1	id=2	id=3	None
Matched B row	None	id=1,value=100	None	id=3,value=300	None
Result row	None	(1,100)	(2,NULL)	(3,300)	All rows processed

Key Moments - 2 Insights

Why does the second result row have NULL for B.value?

Because in execution_table row 2, A.id=2 has no matching B.id=2, so LEFT JOIN fills B columns with NULL.

What happens if we switch to RIGHT JOIN?

The join would include all rows from B, matching A rows or NULL if no match, reversing the role of tables as shown in concept_flow.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Result Row at Step 3?

A (3, NULL)

B (3, 300)

C (3, 100)

D (NULL, 300)

💡 Hint

Check execution_table row 3 under Result Row column

At which step does the LEFT JOIN fill NULL for the right table columns?

A Step 1

B Step 3

C Step 2

D No NULLs filled

💡 Hint

Look at execution_table row 2 Notes and Result Row

If we change the query to RIGHT JOIN, which table's rows will all appear in the result?

A Right table B

B Left table A

C Only matching rows

D No rows

💡 Hint

Refer to concept_flow description about RIGHT JOIN behavior

Concept Snapshot

LEFT JOIN returns all rows from the left table plus matched rows from the right table; unmatched right rows show NULL.
RIGHT JOIN returns all rows from the right table plus matched rows from the left table; unmatched left rows show NULL.
Use LEFT JOIN when you want to keep all left table data.
Use RIGHT JOIN when you want to keep all right table data.
Both joins match rows based on the ON condition.
If no match, columns from the other table are NULL.

Full Transcript

This visual execution shows how LEFT JOIN and RIGHT JOIN work in SQL. LEFT JOIN keeps all rows from the left table and matches rows from the right table. If no match is found, the right table columns are filled with NULL. The execution table traces each row from the left table, showing matches or NULLs. The variable tracker shows how the current rows and results change step-by-step. Key moments clarify why NULLs appear and what changes if we switch to RIGHT JOIN. The quiz tests understanding of the join results and behavior. The concept snapshot summarizes the key differences and when to use each join type.