"Ace Your Next Interview with These Efficient SQL Query Techniques"

"Failure is an amazing data point that tells you which direction not to go." — Payal Kadakia

In SQL interviews, it’s not just about getting the right result — it’s about how efficiently and cleanly you get there. Whether you're writing queries for analytics, reporting, or application logic, understanding how to structure them efficiently can set you apart from other candidates.

In this blog, we’ll walk through key SQL techniques that can boost your performance in interviews — from correctly using WHERE vs HAVING, to optimizing queries with LIMIT, handling NULL values properly, and using CASE expressions effectively. We’ll also cover string handling tricks and best practices for making your output reporting-friendly.

Here’s a preview of what you’ll learn:

• How to apply different conditions on the same column

•  Aggregation belongs in HAVING, not WHERE

• The importance of execution order in CASE statements

• How to use LIMIT to improve query efficiency

• Techniques to handle NULL values in reports using COALESCE and IFNULL

• Best practices for string manipulation and formatting

  
  

1) Different Conditions on the Same Column

The IN condition in SQL is used to filter rows based on whether a column's value matches any value in a list. It’s a cleaner and often more efficient alternative to using multiple OR conditions.

2) Aggregation Belongs in HAVING, Not WHERE

When a query includes aggregate functions (like SUM, COUNT, AVG), the filtering condition must use HAVING, not WHERE.

In that case,  group by should be used:

Having  and where Condition are used to filter the query.

Having works with aggregation while where condition works with raw data

   Note: WHERE must always appear before GROUP BY in query order

3) Importance of Execution Order in CASE Statements

For example, to replace NULL values in the email column with "9999":

CASE statements execute in order. The first matched condition is applied, so priority matters.

4) How to Use LIMIT to Improve Query Efficiency

The LIMIT clause helps optimize performance and improve usability.

Benefits of LIMIT:

• Performance – Reduces processing time and memory usage.

• Data Exploration – Helps preview small samples safely.

• Top-N Queries – Easily fetch top records using ORDER BY.

• Query Safety – Limits the effect of testing queries (e.g., in deletes/updates).

5) Handling NULL Values in Reports with COALESCE and IFNULL

NULL values are common in raw data but should not appear in reports. Replace them with meaningful values like 0.00 or 'N/A' using:

In this , there are two values with null values but the response is ZERO. This is because we are looking for a string values of phone as NULL, not the phone numbers with null values.

In this case, ISNULL condition should be used.

NULL as a string and as a value are two different things. Better understanding of the requirement and understanding of the data set can avoid this error.

Null values are not appropriate way to report ,In the report , always replace them with a clear values.

To handle these values IFNULL or COALESCE condition can be used as below.

6) Best practices for string manipulation and formatting

LENGTH() / CHAR_LENGTH() – Get string length

CONCAT() – Combine strings

SUBSTRING() / SUBSTR() – Extract part of a string

TRIM() / LTRIM() / RTRIM() – Remove spaces

REPLACE() – Replace part of a string

POSITION() / INSTR() – Find position of substring

Mastering efficient SQL techniques not only helps you write clean, optimized queries, but also shows interviewers that you understand how SQL actually works under the hood. Focus on clarity, correctness, and performance — and you’ll stand out in any SQL interview