Common Interview Questions on SELECT Queries

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Q1. What is the basic syntax of a SELECT query?

The most basic SELECT query retrieves data from a table:

SELECT column1, column2 
FROM table_name;

Key components:

• SELECT specifies columns to retrieve
• FROM specifies the source table
• Semicolon (;) terminates the statement (in most SQL dialects)

To select all columns:

SELECT * FROM table_name;

Q2. How do you filter results using WHERE?

The WHERE clause filters rows based on conditions:

SELECT product_name, price
FROM products
WHERE price > 100 AND category = 'Electronics';

Common operators:

Operator	Description	Example
=	Equal	WHERE id = 5
>	Greater than	WHERE age > 30
LIKE	Pattern matching	WHERE name LIKE 'A%'
BETWEEN	Range inclusive	WHERE price BETWEEN 50 AND 100

Q3. What's the difference between INNER JOIN and LEFT JOIN?

INNER JOIN: Returns only matching rows from both tables

SELECT a.name, b.order_date
FROM customers a
INNER JOIN orders b ON a.id = b.customer_id;

LEFT JOIN: Returns all rows from left table + matches from right (NULL if no match)

SELECT a.name, b.order_date
FROM customers a
LEFT JOIN orders b ON a.id = b.customer_id;

Visualization:
Table A: [1, 2, 3]
Table B: [2, 3, 4]
INNER JOIN: [2, 3]
LEFT JOIN: [1, 2, 3] (with NULL for 1's match)

Q4. How do GROUP BY and HAVING work together?

GROUP BY aggregates data by specified columns, while HAVING filters the grouped results:

SELECT department, COUNT(*) as emp_count, AVG(salary) as avg_salary
FROM employees
GROUP BY department
HAVING COUNT(*) > 5 AND AVG(salary) > 50000;

Key differences from WHERE:

• WHERE filters rows before grouping
• HAVING filters groups after aggregation
• HAVING can use aggregate functions (COUNT, AVG, etc.)

Q5. What are subqueries and when would you use them?

Subqueries (nested queries) are SELECT statements within other queries:

Example 1: In WHERE clause

SELECT product_name
FROM products
WHERE price > (SELECT AVG(price) FROM products);

Example 2: In FROM clause (derived table)

SELECT dept, avg_salary
FROM (
  SELECT department as dept, AVG(salary) as avg_salary
  FROM employees
  GROUP BY department
) AS dept_stats
WHERE avg_salary > 75000;

Common use cases:

• Comparing values against aggregated results
• Creating temporary result sets for complex operations
• Exists/Not Exists conditions

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