004 SQL SELECT DISTINCT

The current webpage explains the SQL SELECT DISTINCT statement:

Purpose: SELECT DISTINCT is used to retrieve only unique (distinct) values from a column in a SQL table.

Basic Syntax:

SELECT DISTINCT column1, column2, ...
FROM table_name;

Example: Selecting all unique countries from a "Customers" table:
```
SELECT DISTINCT Country FROM Customers;
```
Without DISTINCT: If you use just SELECT Country FROM Customers;, you get all values including duplicates.
Counting Unique Values: You can count the number of unique items with:
```
SELECT COUNT(DISTINCT Country) FROM Customers;
```
Note: Microsoft Access does not support COUNT(DISTINCT column_name). As a workaround, you can use:
```
SELECT Count(*) AS DistinctCountries
FROM (SELECT DISTINCT Country FROM Customers);
```

The page includes syntax explanations, example tables, interactive SQL exercises, and a note about differences in Microsoft Access.

SQL SELECT DISTINCT Statement

The SELECT DISTINCT statement is used to return only distinct (different) values.

Select all the different countries from the "Customers" table:

SELECT DISTINCT Country FROM Customers;

Inside a table, a column often contains many duplicate values; and sometimes you only want to list the different (distinct) values.

SELECT DISTINCT column1, column2, ...
FROM table_name;

Below is a selection from the Customers table used in the examples:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

If you omit the DISTINCT keyword, the SQL statement returns the "Country" value from all the records of the "Customers" table:

SELECT Country FROM Customers;

By using the DISTINCT keyword in a function called COUNT, we can return the number of different countries.

SELECT COUNT(DISTINCT Country) FROM Customers;

Note: The COUNT(DISTINCT column_name) is not supported in Microsoft Access databases.

Here is a workaround for MS Access:

SELECT Count(*) AS DistinctCountries  
FROM (SELECT DISTINCT Country FROM Customers);

You will learn about the COUNT function later in this tutorial.