How Can I Retrieve Only Unique Values from a Database Column Using SQL?

Selecting Unique Values from a Column

In database management, the ability to retrieve distinct values from a column is a common task. Consider a scenario where you have a table with a column containing non-unique values, and you want to display only the distinct values. In this article, we will discuss a method to achieve this using the DISTINCT operator.

Problem Statement

Imagine you have a MySQL table with two columns, 'Date' and 'product'. The 'Date' column contains duplicate values, as shown in the sample data below:

Date	product
2011-12-12	azd
2011-12-12	yxm
2011-12-10	sdx
2011-12-10	ssdd

A simple PHP script can be used to extract data from this table and display the dates:

<?php $con = mysql_connect("localhost","username","password");
if (!$con)
  {
  die('Could not connect: ' . mysql_error());
  }
mysql_select_db("db", $con);
$sql=mysql_query("SELECT * FROM buy ORDER BY Date");
while($row = mysql_fetch_array($sql))
{

 echo "<li><a href="http://www.website/%22%20.%20%24row%5B" date>buy ". date("j, M Y", strtotime($row["Date"]))."</a>";

    }
    mysql_close($con);
?>

The output of this script would be a list of dates, including duplicates:

12.dec 2011
12.dec.2011
10.dec.2011
10.dec.2011

Solution: DISTINCT Operator

To obtain only the distinct values in the 'Date' column, we can utilize the DISTINCT operator in the SQL query:

SELECT DISTINCT(Date) AS Date FROM buy ORDER BY Date DESC;

The DISTINCT operator ensures that only unique values are returned, eliminating the duplicates. The resulting output would be:

12.dec.2011
10.dec.2011

By incorporating the DISTINCT operator, you can effectively retrieve and display only the distinct values in your database table. This technique is applicable in various scenarios where eliminating duplicates is required.

The above is the detailed content of How Can I Retrieve Only Unique Values from a Database Column Using SQL?. For more information, please follow other related articles on the PHP Chinese website!