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SQL for Beginners: Ask Your Data Straight Questions
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SQL for Beginners: Ask Your Data Straight Questions

Check Yourself: Before You Write a Query

Five scenario questions on tables, keys, what SQL is and how psql behaves. If you miss more than one, reread the first two lessons.

5 min SQLDatabases Beginner
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1. A shop keeps each customer's city on every order row. A customer moves, and someone updates only one of her three orders. What is the real problem?
The orders table should have used a faster type for the city column. UPDATE cannot change one row of a table without changing all of them. The same fact is stored in several places, so the data can now disagree with itself. The database refuses to store two different cities for one name.
2. Which sentence best describes how SQL works?
You write the steps the database must follow to loop over each row. You write a program that the database compiles and runs once a night. You describe the result you want, and the database decides how to get it. You edit the table file directly and the database reads your changes.
3. You type SELECT name FROM customers and press Enter in psql. The prompt changes to postgres-# and nothing else happens. What is the most likely reason?
The server crashed and psql is reconnecting. psql only runs one statement per session. The customers table is empty, so psql has no rows to show. The statement has no semicolon yet, so psql is waiting for the rest of it.
4. Why does the course run PostgreSQL in a throwaway Docker container instead of on a database you already use for real work?
A mistake costs nothing, because you can delete the container and start again. psql only works inside Docker. A container makes queries run faster than an installed server. SQL only works when the server runs in a container.
5. Which of these lines is a psql command and not an SQL statement?
CREATE DATABASE shop; \dt DROP TABLE orders; SELECT 1;

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Five scenario questions on tables, keys, what SQL is and how psql behaves. If you miss more than one, reread the first two lessons.

Duration
5 min
Last update
10/02/2026
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