Find Books with No Available Copies

Find Books with No Available Copies

What is this problem about?

The Find Books with No Available Copies coding problem is a database task involving two tables: Books and Orders. You need to identify all books that have at least one copy in the library system but currently have zero "available" copies because all of them are checked out or reserved.

Why is this asked in interviews?

Meta uses this "Easy" database question to test proficiency with the SQL interview pattern involving aggregation and filtering. It evaluation your ability to perform a join between tables and calculate differences or sums across groups. It’s a foundational skill for any backend or data engineering role.

Algorithmic pattern used

This is solved using SQL JOIN and Aggregation.

1. Join the Books table with the Orders (or Availability) table.
2. Group the records by book ID or title.
3. Use the HAVING clause to filter for books where the sum of available copies is exactly 0.
   • Alternatively, use a NOT IN or NOT EXISTS subquery if the schema allows for identifying "un-ordered" books.

Example explanation

Table Books: [ID: 1, Title: "Code"], [ID: 2, Title: "Design"]. Table Copies: [BookID: 1, Status: "Borrowed"], [BookID: 1, Status: "Borrowed"], [BookID: 2, Status: "Available"].

1. Group by BookID.
2. Book 1: 2 total copies, 0 are "Available". Match!
3. Book 2: 1 total copy, 1 is "Available". No match. Result: "Code".

Common mistakes candidates make

• Ignoring books with zero copies: Including books that aren't even in the catalog.
• Incorrect filtering: Using WHERE instead of HAVING for aggregated counts.
• NULL handling: Not accounting for books that have no entries in the orders table (which usually implies they are available).

Interview preparation tip

Practice using LEFT JOIN and checking for NULL values. In many real-world scenarios, "no available copies" is represented by the absence of a record in an availability table or a specific status flag.