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Why do you need to combine tables?

SQLJunior~1 min read
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Tables get combined to pull together related data that's stored separately.

In real databases, data rarely lives in a single table - to avoid duplication and errors, data gets normalized (spread across different tables). JOIN is what "reassembles" these pieces during analysis.

Example 1: separated entities

  • The users table stores user names and ids.
  • The orders table stores orders, which only have user_id.

To see who placed which order, you need to join them:

sql
SELECT users.name, orders.amount FROM users JOIN orders ON users.id = orders.user_id;

Without JOIN, you can't link a person's name to their order.

Example 2: saving space and cleaner logic

Instead of storing the customer's name in every order row, the orders table only holds user_id. This makes the database faster, more compact, and safer, and you don't need to duplicate data.

Example 3: multi-layer data

Sometimes data is related as a chain: users -> orders -> products -> categories. JOIN lets you walk those relations and roll everything into one report:

sql
SELECT users.name, products.title, categories.name FROM users JOIN orders ON users.id = orders.user_id JOIN products ON orders.product_id = products.id JOIN categories ON products.category_id = categories.id;

You get the full picture: who bought what, and from which category.

Summary: Tables are combined to:

  • relate data from different sources (e.g. a user <-> an order);
  • avoid duplicating information;
  • build reports, analytics, and selections from several tables at once.

In short, JOIN turns scattered tables into a single logical picture of the data.

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