Database Concepts

A common exam question gives a table and a SELECT query and asks which records the query returns. The approach:

1. Look at the FROM clause to find the table.
2. Look at the WHERE clause and identify the condition each record must satisfy.
3. Check each record in the table against the condition.
4. Keep only the records where every condition is true.
5. Look at the SELECT clause and write down only the columns asked for.

Walkthrough: a Dublin or Osaka query

Using the Customers table from section 6:

SELECT *
FROM Customers
WHERE City = 'Dublin' OR City = 'Osaka';

Step by step:

• Maya Hassan: City = 'Toronto' → does not match either → excluded.
• Liam O'Brien: City = 'Dublin' → matches → included.
• Hiroshi Tanaka: City = 'Osaka' → matches → included.
• Priya Gupta: City = 'Mumbai' → does not match either → excluded.

Result:

Walkthrough: an animal breeding query

Consider the tbl_zoo table:

Example — Write a SQL query that returns the names of every species that is currently in captivity AND requires more than 10 kg of feed per day.

SELECT Species
FROM tbl_zoo
WHERE InCaptivity = 'Yes' AND DailyFeedKg > 10;

Result:

(Cheetah, Tortoise, Macaw, Pelican and Komodo Dragon are excluded because their feed requirement is at or below 10 kg. Anaconda and Tarantula are excluded because InCaptivity = 'No'.)