Are You Making The Most Common SQL Interview Mistakes?
The three most common SQL interview mistakes that business analysts to data scientists make again and again.
How do you prepare for your SQL interviews? You might think doing some research online and visiting sites like Glassdoor will provide you with ample resources to prepare before your interview, but don’t stop there.
On average 2 out of 3 candidates fail to make it past the technical interview round, and for good reason too.
We’ve spoken to recruiters and candidates to identify the 3 most common SQL interview mistakes that you can avoid in your next SQL interview.
So how does someone like yourself ace your SQL interview and not fall into the majority of candidates failing to pass?
“You start by understanding where the most common SQL interview pitfalls are and how to overcome them.”
Number #1: Syntax errors.
SQL has very few keywords and they’re all fairly easy to memorize. If you claim to have written hundreds of queries but don’t remember the right way to format your query, it’ll be a red flag to your interviewer that you’re misrepresenting your SQL skills.
What does a SQL syntax mean? Essentially, it means a set arrangement of words and commands. If you use improper syntax, your query gets thrown off and the table does not know what you’re trying to tell it.
To better comprehend how SQL syntax operates, think of it like a spoken language. Imagine I walked up to you and said, “Nice to met you!” When I actually mean “Nice to meet you!”.
So when it comes time to run your query in your SQL interview, always remember to check your syntax.
The most common SQL syntax mistakes are:
- Misspelling Commands
WHERE TREE = 'Ready';
- Forgetting Brackets, Parentheses, or Quotes
WHERE first_name = 'Larry' and last_name = 'Smith' or last_name = 'Edible';
- Specifying an Invalid Statement Order
WHERE name = 'ready'
GROUP BY name
HAVING count(*) = 1
ORDER BY name;
- Omitting Table Aliases
SELECT ap.* , app.name
JOIN apple ON (ap.id = app.previous_id);
- Using Case-Sensitive Names
SELECT * FROM
WHERE cust_name = 'Mijona';
There you have it, those are the 5 most common SQL syntax errors. If you do find yourself making this mistake during an interview, make sure to learn from it and brush it off. At the end of the day, the interviewer will weigh this mistake against your logic.
Number #2: Conceptual errors.
Have you confused a WHERE clause with a HAVING clause? Or maybe used a UNION clause when you should’ve used a UNION ALL?
Misusing a UNION and UNION ALL is one of the most common mistakes we see during SQL interviews. UNION ALL and UNION both concatenate the query output from numerous SQL queries, but UNION ALL keeps the duplicate output when concatenating results.
A lot of candidates make mistakes like this when taking a SQL assessment and it’s crucial that you avoid these conceptual mistakes.
In fact, the hiring managers we’ve talked to here at Big Tech Interviews (BTI) have indicated this is the single most important mistake to watch out for.
Sure, syntax errors don’t look great, but if you’re misusing SQL concepts in an attempt to solve a problem then you’re going to throw a giant red flag to the interviewer.
The most common SQL interview concepts are:
- Question: What is a DISTINCT clause?
- Answer: Used to return only unique (different) values
- Question: What is the difference between a UNION and UNION ALL?
- Answer: Union extracts the rows that are being specified in the query while Union All extracts all the rows including the duplicates (repeated values) from both the queries.
- Question: How do Window Functions work?
- Answer: Window Functions execute calculations at the row level with a related set of values. Compared to using aggregate functions like SUM(), window functions don’t collapse the output of the rows into a single value. Rather, the output is returned for every row.
- Question: What is the difference between a primary and a foreign key?
- Answer: Primary keys are either a column or a set of columns in a table that hold uniquely identifiable rows in the table. While a foreign key could be a column or a set of columns in a table whose values correspond to the values of the primary key in another table.
- Question: What is the difference between LEAD and LAG?
- Answer: LEAD will give you the rows AFTER the row you are finding a value for. LAG will give you the rows BEFORE the row you are finding a value for.
If you want to increase your odds of passing your SQL interview then I’d recommend checking out the 4 most common SQL interview questions and answers from H1 2022.
Number #3: Practice made perfect.
Alright, I know what you’re thinking.
“Leslie, I know practice is important! That’s why I’m here reading your article!!”
So let me explain.
Back when I was first learning to program, I had a mentor pull me aside and tell me the hard truth. I wasn’t going to break into data science with my current work.
In fact, he said, “Practice will not make you the perfect contender” Rather, you’ll only get there from “Practicing ruthless perfection”
Do you see where I’m going with this?
“It’s not about how many SQL questions you’ve solved, but how you solved them. “
You’ve got to remember why you started and become obsessed with the process. Instead of solving all the questions, you come across between now and your next SQL interview, pick 5 questions like these ones until you have perfected your logic and solution.
Remember, here at BTI we love to give feedback so if you have any questions or need help with a problem just reach out!
Alright, so I hope this article was helpful and you feel more confident about how to write better SQL queries. Remember, first learn the syntax, then master the concepts, and finally ruthlessly refine your SQL skills until you’ve perfected them.