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Introduction to SQL

Limiting Results

We know that the SELECT operator can retrieve all rows from a table of all or the specific columns. However, what if we only need to fetch a specific number of rows?

Note

This can be achieved using different syntax in various database management systems. In our case, since we are using PostgreSQL, we should utilize the LIMIT clause.

In the example below, we extract the first 7 rows (in our case, the capitals of the countries) from the column.

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
4 Netherlands Europe Western Europe 41526 Amsterdam 15864000
... ... ... ... ... ... ...
15 Malta Europe Southern Europe 316 Valletta 380200

Task

From the ' country ' table, let's retrieve 3 distinct (unique) continent values.

Everything was clear?

Section 1. Chapter 5
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course content

Course Content

Introduction to SQL

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 DataChallenge: Population SortSorting by Multiple ColumnsChallenge: Dual-Sort QuerySpecifying 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

3. Filtering Data

Using the WHERE ClauseChallenge: North America Population SortThe WHERE Clause Operators Challenge: Population Threshold QueryChecking for a Range of ValuesChecking for NO Value

4. Advanced Data Filtering

Using the AND OperatorChallenge: Europe Region SortUsing the OR operatorChallenge: America Regions FilterUsing the NOT OperatorChallenge: Non-Europe Capitals Sort

5. Aggregate Functions

The AVG() FunctionChallenge: Large Surface Area AverageThe COUNT() FunctionChallenge: Continental CountThe MAX() FunctionThe MIN() FunctionChallenge: Surface Area SelectionThe SUM() Function

Limiting Results

We know that the SELECT operator can retrieve all rows from a table of all or the specific columns. However, what if we only need to fetch a specific number of rows?

Note

This can be achieved using different syntax in various database management systems. In our case, since we are using PostgreSQL, we should utilize the LIMIT clause.

In the example below, we extract the first 7 rows (in our case, the capitals of the countries) from the column.

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
4 Netherlands Europe Western Europe 41526 Amsterdam 15864000
... ... ... ... ... ... ...
15 Malta Europe Southern Europe 316 Valletta 380200

Task

From the ' country ' table, let's retrieve 3 distinct (unique) continent values.

Everything was clear?

Section 1. Chapter 5
toggle bottom row
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