What common mistakes do you encounter in SQL queries?

Typical errors in SQL queries I have encountered:

• Syntax errors: missing commas, incorrect use of keywords.
• JOIN errors: incorrect join conditions, leading to redundant or missing data.
• Incorrect use of aggregate functions without GROUP BY.
• Data type errors, such as comparing strings with numbers.
• Subquery issues, when they return multiple rows instead of one.
• Lack of indexes, resulting in slow query performance.

To prevent such errors, it is important to carefully check queries, use profiling tools, and write tests to verify data correctness.