D8LooP - Practise Data Engineering

Orientation

What You'll Master Here

SQL Essentials builds the reliable mental model that every later chapter depends on: what a table is, what one row means, how a query is shaped, and how to read a result like an engineer instead of just trusting it.

Everything is taught on one small marketplace dataset, with familiar customers, orders, and order items on purpose, so your attention stays on SQL reasoning rather than on a complicated business domain.

By the end you should be able to inspect an unfamiliar table, select and name columns deliberately, filter without distorting the data, handle types and NULLs safely, and explain in plain language what your query actually returns.

Why data engineers care

Data engineers spend most of their query time reading unfamiliar tables, validating assumptions, and turning raw data into trusted tables. First-query literacy is the base layer for joins, windows, modeling, and pipelines.

Core mental model

A good first query answers three questions before anything fancy: which table, which rows, and which columns?

A safe first inspection query

select
  order_id,
  buyer_id,
  status,
  created_at
from orders
order by created_at desc
limit 5;

Query result5 rows

order_id	buyer_id	status	created_at
1044	9	refunded	2026-01-31 18:04
1043	7	paid	2026-01-31 09:22
1042	7	paid	2026-01-30 21:10
1041	4	pending	2026-01-30 12:48
1040	2	paid	2026-01-29 15:31

Start narrow and ordered: a handful of recent rows tells you the shape of the table before you commit to anything.

customers

one row per customer

key: customer_id

orders

one row per order

key: order_id

order_items

one row per item in an order

key: order_id + sku

A customer places many orders; each order contains many items. Notice the grain gets finer left to right.

Key terms

row: One record in a table. What it represents in the real world is the table’s grain.
column: A single attribute shared by every row, with one data type.
grain: The real-world thing one row stands for, e.g. one order or one item.
result set: The rows and columns a query returns, which you should always inspect.

Common mistake

Writing a clever query before inspecting the table.

You can produce a polished answer built on the wrong row set or a misread column.

Better habit

Inspect a small, ordered sample first.
Say the table grain out loud before querying.
Keep the first query readable before making it fancy.

Interview note

A strong beginner SQL answer is not fancy. It is clear about the table, the filter, the selected columns, and the assumptions being made.

Study tip

Use the topic menu on the left as a checklist. Each topic is a habit you should be able to demonstrate, not just recognize.

Remember this

Before optimizing syntax, be able to explain which table, which rows, and which columns your query touches.