50 Must-Know SQL Interview Questions For Data Engineers

You Should Know:

To help you prepare for SQL interviews, here are some essential SQL commands and concepts that every Data Engineer should be familiar with:

1. Basic SQL Commands:

SELECT: Retrieves data from a database.
```
SELECT * FROM employees;
```

WHERE: Filters records based on a condition.

SELECT * FROM employees WHERE department = 'Engineering';

JOIN: Combines rows from two or more tables.

SELECT employees.name, departments.department_name 
FROM employees 
JOIN departments ON employees.department_id = departments.id;

2. Aggregate Functions:

COUNT: Counts the number of rows.
```
SELECT COUNT(*) FROM employees;
```
SUM: Calculates the total sum of a numeric column.
```
SELECT SUM(salary) FROM employees;
```
AVG: Calculates the average value of a numeric column.
```
SELECT AVG(salary) FROM employees;
```

3. Grouping Data:

GROUP BY: Groups rows that have the same values into summary rows.

SELECT department, COUNT(*) 
FROM employees 
GROUP BY department;

HAVING: Filters groups based on a condition.

SELECT department, COUNT(<em>) 
FROM employees 
GROUP BY department 
HAVING COUNT(</em>) > 10;

4. Subqueries:

A subquery is a query nested inside another query.

SELECT name 
FROM employees 
WHERE department_id = (SELECT id FROM departments WHERE department_name = 'Engineering');

5. Window Functions:

ROW_NUMBER(): Assigns a unique number to each row.

SELECT name, salary, ROW_NUMBER() OVER (ORDER BY salary DESC) as rank 
FROM employees;

RANK(): Assigns a rank to each row within a partition.

SELECT name, salary, RANK() OVER (ORDER BY salary DESC) as rank 
FROM employees;

6. Indexing:

Creating an index can speed up data retrieval.

CREATE INDEX idx_employee_name ON employees(name);

7. Transactions:

BEGIN TRANSACTION: Starts a transaction.
```
BEGIN TRANSACTION;
```
COMMIT: Saves the changes made during the transaction.
```
COMMIT;
```
ROLLBACK: Undoes the changes made during the transaction.
```
ROLLBACK;
```

What Undercode Say:

Mastering SQL is crucial for any Data Engineer, as it forms the backbone of data manipulation and retrieval in relational databases. The commands and concepts listed above are just the tip of the iceberg. To truly excel, you should also be familiar with advanced topics like query optimization, database design, and working with NoSQL databases. Practice these commands in a real-world scenario, and you’ll be well-prepared for any SQL-related interview questions.

For further reading, consider exploring the following resources:

Remember, the key to mastering SQL is consistent practice and real-world application. Good luck!

References:

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