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Databases SQL Basics Finding the Data You Want Review & Practice with SQL Playgrounds

Posted by Kristian Woods
Kristian Woods
23,414 Points

SQL relationships between tables?

could somebody please explain what this SQL query means? 

SELECT * FROM loans, books
WHERE loans.book_id = books.id;

thanks in advance

1 Answer

Steven Parker
Steven Parker
229,744 Points
Steven Parker
Steven Parker
229,744 Points

This is an implicit INNER JOIN of two tables.

Let's break it down:

  • SELECT * :point_left: Show ALL columns...
  • FROM loans, books :point_left: ...from both the loans table and the books table...
  • WHERE loans.book_id = books.id; :point_left: ...where you can match book_id in loans with id in books

Note that any row of either table that cannot be matched with the other table will not be shown.

Another way to write the exact same query using an explicit join would be:

SELECT * FROM loans INNER JOIN books ON loans.book_id = books.id;