SQL DISTINCT Command

SQL DISTINCT COMMAND

The SQL DISTINCT clause works in conjunction with the SQL SELECT clause and selects only distinct (unique) data from a database table(s). Here is an example of SQL DISTINCT clause:

 SELECT DISTINCT Column1 FROM Table1

As you can see the DISTINCT keyword goes immediately after the SELECT clause and is then followed by a list of one or more column names. I'll give you an example why you might need to use the DISTINCT SQL clause. I'll use the Weather table from the SQL WHERE tutorial to demonstrate the SQL DISTINCT application:

City	AverageTemperature	Date
New York	22 C	10/10/2005
Seattle	21 C	10/10/2005
Washington	20 C	10/10/2005
New York	18 C	10/09/2005
Seattle	20 C	10/09/2005
Washington	17 C	10/09/2005

Consider the following SQL statement utilizing SQL DISTINCT:

 SELECT DISTINCT City  FROM Weather 

This SQL DISTINCT expression will return a list with all cities found in the City column of the Weather table, but it will remove the duplicates and leave only a single entry for each city:

City

New York

Seattle

Washington

You can use the SQL DISTINCT with any table column for example with the AverageTemperature:

 SELECT DISTINCT AverageTemperature  FROM Weather 

The result of this SQL DISTINCT will be:

AverageTemperature

22 C

21 C

20 C

18 C

17 C

You can use the SQL DISTINCT with more than one column and if you do that, the result will have all distinct combinations of values for all columns. For example if our Weather table has the following entries:

City	AverageTemperature	Date
New York	22 C	10/10/2005
New York	22 C	10/09/2005
New York	20 C	10/08/2005
New York	20 C	10/07/2005
New York	18 C	10/06/2005

And we run the following SQL DISTINCT statement:

 SELECT DISTINCT City, AverageTemperature  FROM Weather 

The result will be:

City	AverageTemperature
New York	22 C
New York	20 C
New York	18 C