Raul Junco

An Interview question every developer should know.

What's the difference between Tokens and API keys?


We use API keys and tokens for authentication and authorization.

But they serve different purposes and have distinct characteristics.


𝗧𝗼𝗸𝗲𝗻𝘀 (𝗹𝗶𝗸𝗲 𝗝𝗪𝗧 - 𝗝𝗦𝗢𝗡 𝗪𝗲𝗯 𝗧𝗼𝗸𝗲𝗻𝘀):

Carries user context and permissions for authentication and authorization.

Encoded with a user ID, permissions, and expiration time, often in JWT format.

Critical for user-specific access, like accessing a user's profile data in an e-commerce platform.

It is issued by an authentication server after user login and contains user-specific information.


𝗔𝗣𝗜 𝗞𝗲𝘆:

Primarily for identifying the application or the consumer making the API call.

They are long strings we pass in the header or as a query parameter in the API request.

You use API keys when access does not involve user context. For example, accessing a public API or service-to-service communication.

They are long-lived and created through the API provider's platform or admin console.


𝗜𝗻 𝘀𝗶𝗺𝗽𝗹𝗲 𝘁𝗲𝗿𝗺𝘀:

-API keys are for identifying applications.
-Tokens are for managing user sessions, permissions, and context.

Which one have you used the most?

90% of the candidates will fail this SQL question.

What's wrong with this query?


The initial query is functionally correct; it will compile and even work in a small set, but as soon as your data grows, it will let you down.

Using the YEAR() function in the WHERE clause can have two significant drawbacks:

𝟭. 𝗣𝗲𝗿𝗳𝗼𝗿𝗺𝗮𝗻𝗰𝗲 𝗜𝗺𝗽𝗮𝗰𝘁 𝗗𝘂𝗲 𝘁𝗼 𝗙𝘂𝗹𝗹 𝗧𝗮𝗯𝗹𝗲 𝗦𝗰𝗮𝗻

When the YEAR() function is applied to s.SaleDate, SQL needs to evaluate this function for every row in the table.

This often results in a full table scan, where every row is checked against the condition. That will hurt, especially for tables with millions of rows.

𝟮. 𝗜𝗻𝗲𝗳𝗳𝗲𝗰𝘁𝗶𝘃𝗲 𝗜𝗻𝗱𝗲𝘅 𝗨𝘀𝗲

The function YEAR(s.SaleDate) prevents the query optimizer from using any indexes on the SaleDate column.

Indexes help the database quickly locate and filter the relevant rows, but they cannot be used when the column is wrapped inside a function.

As a result, the query's performance suffers because the database is unable to take advantage of the available indexing.

𝗔 𝗕𝗲𝘁𝘁𝗲𝗿 𝗔𝗽𝗽𝗿𝗼𝗮𝗰𝗵

Use a range-based query that allows the database to utilize indexes effectively.

The BETWEEN clause filters records by defining a range of dates (2024-01-01 to 2024-12-31).

The query optimizer will use any index on the SaleDate column to locate the relevant rows without scanning the entire table.


You may make your query/code more complex since you must generate those dates. This is a trade-off, and I'm willing to pay any day for good performance.

Using functions in the WHERE clause is a performance killer!