These 5 Queries Are Destroying Your Database

Five common SQL query patterns that cause silent failures, performance degradation, or data loss in production are examined with fixes. The patterns covered are: (1) NOT IN with nullable subquery columns returning zero rows due to SQL's NULL semantics — fix with LEFT JOIN or NOT EXISTS; (2) wrapping indexed columns in functions like YEAR() causing full table scans — fix by comparing raw column values against date literals; (3) SELECT * inside subqueries fetching unnecessary data and blocking predicate pushdown optimizations; (4) UPDATE statements without a WHERE clause overwriting entire tables — mitigated by running a SELECT first and wrapping in a transaction; (5) joining multiple tables before aggregating causing row multiplication and inflated counts/sums — fix by pre-aggregating each table in its own subquery before joining.