lesson 01 SQL foundations · beginner

Meet your first SELECT

Choose columns and read rows from a table.

the idea

A table is a set of rows with named columns. SELECT lists the columns you want; FROM names the table that holds them. In Retail, every row of customers is one identified wholesale customer and every row of products is one stock code. A query reads rows; it never changes them.

Dataset: Retail

The tables the challenges use, with their first 6 rows. Under each column name: the kind of value it holds, and which table it links to.

worked example

Run these steps in order to see the idea before you write your own query. The step marked “the pattern” is what the challenges ask for. Running the example does not count toward progress.

1. 1

   See a few customers. Run this first. Each row is one customer. You get every column so you can see what the table holds before you pick any.

   step-1.sql Run

   sql›

   SELECT *
   FROM customers
   LIMIT 10;

2. 2

   Keep only the columns you need. Same customers, but SELECT now lists just two columns—the ones you ask for.the pattern

   example.sql Run

   sql›

   SELECT customer_id, first_order_at
   FROM customers;

common mistake

SELECT * returns every column, including ones the task did not ask for. A checked answer must return exactly the requested columns, in the requested order.