Writing Debug Tables: Your Logic Verifier

Core Concept

Example: LLM says 4 users, manual trace shows 1 - debug tables catch semantic errors!

Debug tables verify your SQL logic - essential for logic debugging and whiteboard interviews.

Why This Matters

LLMs and Semantics

Syntax: 95%+ accurate
Semantics: 16-82% accurate per BIRD benchmarks
See LLM Debug

Whiteboard Interviews and Tests

Filter out expensive LLM cut-and-pasters
"Is this query correct?" - common question
Logic = what gets you hired
See Learning Outcomes

Actually Faster

Avoid reprompting loop: 30 minutes
Drawing one debug table: 5 minutes
Build intuition

Example 1: Users with Multiple Genres

-- Find users who listen to 2+ songs per genre
SELECT user_id, genre, COUNT(*) AS song_count
FROM Listens l
JOIN Songs s ON l.song_id = s.song_id  
WHERE genre IS NOT NULL
GROUP BY user_id, genre
HAVING COUNT(*) >= 2

Step-by-Step Debug Table:

Step 1: FROM + JOIN (All listen records with song info)

user_id	name	song_id	genre	rating
1	Mickey	1	Pop	4.5
1	Mickey	2	Pop	4.2
1	Mickey	6	Classic	3.9
2	Minnie	2	Pop	4.7
2	Minnie	7	Classic	4.6
2	Minnie	8	Classic	3.9
3	Daffy	1	Pop	2.9
3	Daffy	2	Pop	4.9
3	Daffy	6	Classic	NULL

Step 2: GROUP BY user_id, genre (Create groups)

user_id	genre	Notes: songs_in_group	Notes
1	Pop	2 rows	✓ Keep (≥2)
1	Classic	1 row	✗ Filter out
2	Pop	1 row	✗ Filter out
2	Classic	2 rows	✓ Keep (≥2)
3	Pop	2 rows	✓ Keep (≥2)
3	Classic	1 row	✗ Filter out

Step 3: HAVING COUNT(*) >= 2 (Final output)

user_id	genre	song_count
1	Pop	2
2	Classic	2
3	Pop	2

Shortcuts for Big Data

Nobody Traces 100,000 Rows!

Instead of This:

user_id | genre | rating
1       | Rock  | 4.5
1       | Rock  | 4.2
1       | Rock  | 3.9
... (497 more Rock rows)
1       | Pop   | 4.8
1       | Pop   | 4.1
... (300 more Pop rows)

Use This Shorthand:

GROUP BY Summary:
User 1, Rock: ~500 rows, AVG ≈ 4.2
User 1, Pop: ~300 rows, AVG ≈ 4.5
User 2, Rock: ~200 rows, AVG ≈ 3.8
...

Edge cases checked:
- MAX(rating) = 5.0 ✓
- MIN(rating) = 1.0 ✓  
- NULL ratings: 42 rows ✓

Simple Notation Tricks

Scenario	Full Trace	Smart Shorthand
1000 rows after JOIN	Show all 1000	"First 3... middle ties... last 2"
GROUP BY with counts	List every row	"Rock: ~500, Pop: ~300"
Window function ranks	Show all ranks	"Top 3, any ties, bottom 2"
Complex aggregation	Calculate exactly	"AVG ≈ 4.2" (close enough!)

Quick Verification Checklist

JOINs: Expected row count? (not accidentally cartesian? INNER? LEFT? OUTER?)
NULLs: Handled correctly? (IS NULL vs = NULL?)
Groups: Right granularity? (user vs user+genre?)
Aggregates: Makes sense? (COUNT(*) vs COUNT(column)?)
Windows: Partitions correct? (RANK vs ROW_NUMBER?)

Remember: It's YOUR Tool

Tool for correctness, not busy work - skip obvious parts, focus on confusing bits
Like multiplication tables in school - use them often early to build intuition
You'll naturally start skipping easy steps as your SQL brain develops
Use simple conventions that interviewers and graders can follow (for partial credit!)
Approximate liberally - "~500 rows" is perfectly fine

For Interviews, PSETs, Tests

❌ Cryptic Hieroglyphs

𓀀 → 𓂋 𓊖 = 2 𓆣
𓁶 → 𓂋 𓄿 = 1 ✗
𓅱 → 𓂋 𓊖 = 2 𓆣
∴ 𓆣 = {𓀀𓊖, 𓅱𓊖}

"Trust me, 𓀀 means Mickey and 𓆣 means keep..."

✓ Clear Convention

User 1, Pop: 2 songs → Keep ✓
User 2, Pop: 1 song → Filter ✗  
User 3, Pop: 2 songs → Keep ✓
Final: {(1,Pop,2), (3,Pop,2)}

Interviewer: "I can follow this!" (+partial credit)