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1. What Is SQL and Why Analysts Need It

1.3 Tables, Rows, Columns and Keys Explained for Beginners

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Why. Wrong grain (what one row means) breaks every total later.

What. A table has columns (fields) and rows (records). A primary key uniquely names a row. A foreign key points to a key in another table.

How. In raja_coffee: customers.customer_id is a key. orders.customer_id points to it.

Steps

  1. Open customers — one row per person.
  2. Open orders — one row per order.
  3. Notice orders.customer_id links to customers.

Examples

Example 1 · dataset: raja_coffee

SELECT customer_id, name, city FROM customers ORDER BY customer_id LIMIT 4;

Output

customer_idnamecity
1Asha PatilPune
2Omar KhanMumbai
3Sofia MendesGoa
4James OkonkwoPune

Meaning. Each customer_id appears once — that is the key idea.

Ravindra Bagale's Tip

Before writing SQL, say out loud: One row in this table means ___.