Databases and Your First SQL Query

What is a relational database?

A relational database stores related information in tables.

Example relationship:

• customers (customer_id, customer_name, city)
• orders (order_id, customer_id, order_date, total_amount)
• One customer → many orders (a one-to-many relationship)

Example

To retrieve customer names and cities:

SELECT customer_name, city
FROM customers;

How it works

• SQL keywords are commonly written in uppercase for readability. Most systems don't require it.
• You can rename output columns with AS: SELECT customer_name AS name FROM customers;
• SELECT DISTINCT city FROM customers; returns each city once.

Common mistakes

• Misspelled table or column names – check the schema first.
• Using SELECT * in real analysis when you need only a few fields:
  • Slows queries on large tables
  • Can be expensive on pay-per-query platforms such as BigQuery
  • May expose sensitive data you don't need
• Forgetting commas between column names.
• Using double quotes for text values. Many systems treat double quotes as identifiers. Use single quotes for text.

Try it: Write a query that returns order_id, order_date, and total_amount from a table named orders.