SQL Server Interview Topic 25: Common SQL Mistakes

This topic helps you speak honestly about mistakes developers must avoid. Interviewers like candidates who know safe habits.

🎯 Interview Goal

You should be able to identify common SQL mistakes and explain how to avoid them.

Q160. Why is missing WHERE dangerous in UPDATE or DELETE?

Quick interview answer:

Without WHERE, UPDATE or DELETE can affect every row in the table. This is one of the most dangerous beginner mistakes. Always run a SELECT first with the same condition to verify affected rows.

Study in detail: Update and Delete - This lesson explains safe update and delete.

Q161. Why can the wrong JOIN give wrong results?

Quick interview answer:

The wrong join can remove records, duplicate records, or show incorrect totals. For example, using INNER JOIN can hide students who do not yet have marks. Choose the join based on the required output.

Study in detail: Joins and Reports - This topic revises join choices.

Q162. Why is NULL confusing for beginners?

Quick interview answer:

NULL means unknown or missing, not zero or empty text. Normal comparisons like = NULL do not work. We should use IS NULL or IS NOT NULL.

Study in detail: NULL Handling - This article explains missing values clearly.

Q163. Why should we avoid SELECT * in application queries?

Quick interview answer:

SELECT * returns all columns, including columns the screen may not need. It can slow performance, expose sensitive data, and break code if table structure changes. Select only required columns.

Study in detail: Select and Where and Security - These lessons explain safer query habits.

Q164. Why is copying production data risky?

Quick interview answer:

Production data may contain sensitive information. Copying it to development or sharing it without masking can create privacy and security issues. Teams should follow approved data handling rules.

Study in detail: Security and Compliance and Audit - These lessons explain safe data handling.

💡 Interview Tip

When asked about mistakes, say what you learned and what habit prevents it next time.