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The customer success team wants a quick contact list of customers from North America, showing each as a single readable row with sensible defaults for missing company info.
| Column | Type |
|---|---|
| CustomerId | INTEGER (Primary Key) |
| FirstName | TEXT NOT NULL |
| LastName | TEXT NOT NULL |
| Company | TEXT |
| Country | TEXT |
| TEXT NOT NULL |
Country is one of: USA, CanadaCustomerName (FirstName + space + LastName), Region (Country in UPPERCASE), Org (Company; if NULL, use the string 'Individual')Your query should return 10 rows with 3 columns: | customername | region | org | |--------------------|--------|------------| | Julia Barnett | USA | Individual | | Michelle Brooks | USA | Individual | | Robert Brown | CANADA | Individual | | Kathy Chase | USA | Individual | | Richard Cunningham | USA | Individual | | ... | ... | ... |
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 FirstName || ' ' || LastName AS CustomerName, UPPER(Country) AS Region, COALESCE(Company, 'Individual') AS Org FROM Customer WHERE Country IN ('USA', 'Canada') ORDER BY LastName ASC LIMIT 10;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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