The join operation starts by choosing one of three methods: nested loop, sort-merge, or hash join. Each method processes the tables differently to find matching rows and produce the joined result.
Execution Sample
DBMS Theory
for each row R in OuterTable:
for each row S in InnerTable:
if R.key == S.key:
output (R, S)
This code shows a nested loop join: for every row in the outer table, it checks every row in the inner table for matching keys and outputs the joined rows.
Analysis Table
Step
Outer Row (R)
Inner Row (S)
Condition R.key == S.key?
Action
Output
1
R1 (key=2)
S1 (key=1)
2 == 1? No
No output
2
R1 (key=2)
S2 (key=2)
2 == 2? Yes
Output (R1,S2)
(R1,S2)
3
R1 (key=2)
S3 (key=3)
2 == 3? No
No output
4
R2 (key=3)
S1 (key=1)
3 == 1? No
No output
5
R2 (key=3)
S2 (key=2)
3 == 2? No
No output
6
R2 (key=3)
S3 (key=3)
3 == 3? Yes
Output (R2,S3)
(R2,S3)
7
R3 (key=4)
S1 (key=1)
4 == 1? No
No output
8
R3 (key=4)
S2 (key=2)
4 == 2? No
No output
9
R3 (key=4)
S3 (key=3)
4 == 3? No
No output
10
End
All rows checked
💡 All rows in OuterTable and InnerTable have been compared; join operation ends.
State Tracker
Variable
Start
After Step 2
After Step 6
After Step 9
Final
R (Outer Row)
None
R1 (key=2)
R2 (key=3)
R3 (key=4)
End
S (Inner Row)
None
S2 (key=2)
S3 (key=3)
S3 (key=3)
End
Output
Empty
(R1,S2)
(R1,S2),(R2,S3)
(R1,S2),(R2,S3)
(R1,S2),(R2,S3)
Key Insights - 3 Insights
Why does the nested loop join check every row in the inner table for each outer row?
Because nested loop join compares each outer row with all inner rows to find matches, as shown in execution_table rows 1-9 where every combination is checked.
How does the join know when to stop?
It stops after all outer rows have been checked against all inner rows, as indicated in execution_table step 10 where the process ends.
Why are some comparisons resulting in no output?
Only when the keys match (condition true) does the join output a row; otherwise, no output is produced, as seen in execution_table steps with 'No output'.
Visual Quiz - 3 Questions
Test your understanding
Look at the execution_table at step 2. What is the output?
ANo output
B(R1,S2)
C(R1,S1)
D(R2,S3)
💡 Hint
Check the 'Output' column at step 2 in the execution_table.
At which step does the join first output a matching pair?
AStep 6
BStep 1
CStep 2
DStep 10
💡 Hint
Look for the first 'Yes' in the 'Condition' column and corresponding output in execution_table.
If OuterTable had one more row with key=2, how would the output change?
AMore outputs with pairs having key=2
BNo change in output
COutputs with key=3 would increase
DJoin would stop earlier
💡 Hint
Refer to variable_tracker and execution_table showing outputs depend on matching keys.
Concept Snapshot
Join algorithms combine rows from two tables based on matching keys.
Nested Loop: checks every pair of rows.
Sort-Merge: sorts tables then merges matching keys.
Hash Join: builds a hash table on one table, probes with the other.
Each method balances speed and resource use differently.
Full Transcript
Join algorithms are methods to combine rows from two tables based on matching keys. The nested loop join checks every row in the outer table against every row in the inner table, outputting pairs when keys match. The process continues until all rows are checked. This method is simple but can be slow for large tables. Sort-merge join sorts both tables and then merges them by scanning in order, which is faster if tables are sorted. Hash join builds a hash table from one table's keys and then quickly finds matches by probing with the other table's rows. Each join method has steps to process data and produce the joined result efficiently depending on the data size and indexes.