Course Content
Introduction to SQL
1. Retrieving Data
Retrieving Individual ColumnsRetrieving Multiple Columns Retrieving All ColumnsRetrieving Distinct RowsLimiting ResultsChallenge: Find the Population of the CountriesChallenge: Find All CountriesChallenge: Find All Countries With Their IDsChallenge: Find Country CapitalsChallenge: Find the Regions in Which All Countries Are Located
2. Sorting Retrieved Data
Sorting DataChallengeSorting by Multiple ColumnsChallengeSpecifying Sort DirectionChallenge: Sort the Countries by Region and CapitalChallenge: Sort Capitals in Descending OrderChallenge: Sort Countries in Ascending OrderChallenge: Find Countries, Their IDs, and Their PopulationsChallenge: Find Countries, IDs, Populations, Regions, and Sort ThemChallenge: Find All Continents and Sort Them in Ascending Order
4. Advanced Data Filtering
Introduction to SQL
Checking for NO Value
When a column doesn't have any value, it's regarded as having a NULL
value. To identify whether a value is NULL
, we check if it's equal to NULL
. Alternatively, the SELECT
statement provides a WHERE
clause and an IS NULL
clause, which is used to examine columns with NULL
values. Let's illustrate this with an example:
Task
Write an SQL query that returns the name
and capital
columns with no population.
Here's a short example of the country
table:
id | name | continent | region | surfacearea | capital | population |
1 | Japan | Asia | Eastern Asia | 377829 | Tokyo | 126714000 |
2 | Latvia | Europe | NULL | 64589 | Riga | 2424200 |
3 | Mexico | North America | Central America | 1958201 | Mexico City | 98881000 |
... | ... | ... | ... | ... | ... | ... |
15 | Malta | Europe | Southern Europe | 316 | Valletta | 380200 |
Everything was clear?
Section 3. Chapter 6