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An INNER JOIN only returns matching rows. Use LEFT JOIN to include ALL artists, even those without any albums.
Write a query showing all artists with their album counts, including artists with zero albums.
| Column | Type |
|---|---|
| ArtistId | INTEGER (Primary Key) |
| Name | TEXT |
| Column | Type |
|---|---|
| AlbumId | INTEGER (Primary Key) |
| Title | TEXT |
| ArtistId | INTEGER (Foreign Key → Artist.ArtistId) |
Returns all rows from the left table, plus matching rows from the right. Non-matching right-side rows are NULL.
artistname | albumcount ----------------------- Academy of St. Martin in the Fields, Sir Neville Marriner & William Bennett | 0 A Cor Do Som | 0 Aerosmith & Sierra Leone's Refugee Allstars | 0 Avril Lavigne | 0 Azymuth | 0 Baby Consuelo | 0 Banda Black Rio | 0 Barão Vermelho | 0 ... (20 rows total)
Using `NOT IN (SELECT nullable_col FROM ...)` when the subquery contains NULLs. In SQL's three-valued logic, `x NOT IN (1, 2, NULL)` evaluates to `UNKNOWN` for all rows, causing the entire query to return zero results. Use `NOT EXISTS` or `WHERE nullable_col IS NOT NULL`.
Interviewers evaluate whether you are aware of SQL three-valued logic (TRUE, FALSE, UNKNOWN), safe NULL coercion with COALESCE, and anti-join optimization.
Construct the solution logically from first principles to avoid typical edge case pitfalls.
Identify columns with NULL values and choose safe operators (IS NULL, COALESCE, or NOT EXISTS).
WHERE column_name IS NULL OR COALESCE(amount, 0) > 0
Use LEFT JOIN ... WHERE right.id IS NULL or NOT EXISTS to find non-matching records without falling into the NOT IN trap.
LEFT JOIN related_table rt ON pt.id = rt.ref_id WHERE rt.ref_id IS NULL
Provide user-friendly fallbacks for empty fields using COALESCE(col, 'Unknown').
SELECT id, COALESCE(title, 'Untitled') AS title FROM table_name;
SELECT Artist.Name AS ArtistName, COUNT(Album.AlbumId) AS AlbumCount FROM Artist LEFT JOIN Album ON Artist.ArtistId = Album.ArtistId GROUP BY Artist.ArtistId, Artist.Name ORDER BY AlbumCount ASC, ArtistName ASC, Artist.ArtistId ASC LIMIT 20;
Real code patterns candidates submit that fail the grading suite.
SELECT * FROM Customer WHERE Company = NULL;
Three recurring syntax and semantic traps relevant to this problem domain.
If any subquery row returns NULL, NOT IN evaluates to UNKNOWN and discards all records.
SELECT * FROM customers WHERE id NOT IN (SELECT customer_id FROM inactive_list); -- ❌ Fails if NULL exists
SELECT * FROM customers c WHERE NOT EXISTS (SELECT 1 FROM inactive_list i WHERE i.customer_id = c.id); -- ✅ Safe
Writing `col = NULL` or `col != NULL` always returns UNKNOWN. Always use `IS NULL` or `IS NOT NULL`.
SELECT * FROM tracks WHERE composer = NULL; -- ❌ Returns 0 rows always
SELECT * FROM tracks WHERE composer IS NULL; -- ✅ Proper NULL check
NULL values in numerical addition yield NULL. Use COALESCE(col, 0) before performing arithmetic or string concatenation.
SELECT salary + bonus AS total_comp FROM staff; -- ❌ total_comp is NULL if bonus is NULL
SELECT salary + COALESCE(bonus, 0) AS total_comp FROM staff; -- ✅ Robust arithmetic
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