Common SQL Mistakes QA Engineers Should Avoid

Mistake 1: Forgetting WHERE

Be careful with:

UPDATE users
SET status = 'Inactive';

This can update every record.

Instead:

UPDATE users
SET status = 'Inactive'
WHERE id = 10;

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Mistake 2: Using = NULL

Incorrect:

WHERE email = NULL;

Correct:

WHERE email IS NULL;

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Mistake 3: Not Understanding JOINs

Many QA Engineers know basic SELECT queries but struggle with multiple tables.

Spend time practicing:

• INNER JOIN
• LEFT JOIN
• RIGHT JOIN
• Multiple-table JOINs

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Mistake 4: Testing Only the UI

A successful UI message does not always guarantee correct database behavior.

Whenever database access is available, validate important transactions at the database level.

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SQL Interview Questions for QA Engineers

Here are some common questions you should prepare.

1. What is SQL?

SQL is a language used to communicate with and manage data in relational databases.

2. What is the difference between DELETE and TRUNCATE?

DELETE removes rows and can be used with a WHERE condition.

TRUNCATE removes all rows from a table and generally operates differently from row-by-row DELETE in terms of logging and transaction behavior, depending on the database.

3. What is a JOIN?

A JOIN combines data from multiple tables using a related column.

4. What is the difference between WHERE and HAVING?

WHERE filters rows before grouping.

HAVING filters grouped results.

5. What is a primary key?

A primary key uniquely identifies a record in a table.

6. What is a foreign key?

A foreign key creates a relationship between tables by referencing a key in another table.

7. How do you find duplicate records?

For example:

SELECT email, COUNT(*)
FROM users
GROUP BY email
HAVING COUNT(*) > 1;

8. How do you find NULL values?

SELECT *
FROM users
WHERE email IS NULL;

9. What is the difference between INNER JOIN and LEFT JOIN?

INNER JOIN returns matching records from both tables.

LEFT JOIN returns all records from the left table and matching records from the right table.

10. Why should a QA Engineer learn SQL?

SQL helps testers validate backend data, investigate defects, verify business rules, and perform database testing.

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