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SQL for Beginners: Ask Your Data Straight Questions
Check Yourself: Asking for Rows
Five scenario questions on AND and OR, aliases, quotes and NULL. If you miss more than one, reread the lessons in this section.
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Before You Write a Query
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What a Relational Database Is and Why SQL Exists
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Set Up PostgreSQL and Run Your First Query
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Asking for Rows
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SELECT, WHERE, ORDER BY and LIMIT
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NULL Is Not a Value
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Building and Changing Data
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CREATE TABLE: Types and Constraints That Matter
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INSERT, UPDATE and DELETE Without Regret
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Summaries
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COUNT, SUM and AVG
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GROUP BY and HAVING
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Combining Tables
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Primary Keys, Foreign Keys and Why Tables Are Split
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INNER JOIN and LEFT JOIN
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When a Join Gives Too Many Rows or Too Few
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- Final Check: SQL for Beginners
1.
You run SELECT name, category, price FROM products WHERE category = 'bags' OR category = 'home' AND price < 10. Which rows come back?
2.
SELECT name AS customer FROM customers WHERE customer = 'Ana Reyes' fails with column customer does not exist. Why?
3.
SELECT name FROM customers WHERE city = "Cebu" fails with column Cebu does not exist. What is the fix?
4.
You know two customers have no city, but WHERE city = NULL returns no rows and no error. What should the condition be?
5.
Three customers live in Manila and two have no city. WHERE city <> 'Manila' returns only the customers with another city. Why are the two with no city missing?
About
Five scenario questions on AND and OR, aliases, quotes and NULL. If you miss more than one, reread the lessons in this section.
- Duration
- 5 min
- Last update
- 09/29/2026
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