DISTINCT clause

In SQL, the DISTINCT operator is used to remove duplicate rows from the result set of a query. It is often used with the SELECT statement to retrieve unique or distinct values from a specific column or combination of columns.

The DISTINCT operator works by comparing the values in the specified columns and removing any duplicate rows from the query result, so that each row in the result set is unique.

The basic syntax of using DISTINCT with the SELECT statement is as follows:

Here's an example to illustrate its usage:

Suppose we have a table named "Orders" with a column named CustomerID. We want to retrieve a list of distinct customer IDs from the table.

In this example, the DISTINCT operator is used to retrieve unique customer IDs from the "Orders" table. It will remove any duplicate occurrences of the same CustomerID in the result set.

The DISTINCT operator can be used with one or more columns. For instance, if you want to retrieve distinct combinations of two columns:

Here's an example using the "Employees" table, retrieving distinct combinations of FirstName and LastName:

The DISTINCT operator is a useful tool when you want to eliminate duplicate rows and focus on unique values in your query results. However, it is important to use it judiciously, as it may impact query performance when dealing with large datasets. In some cases, using GROUP BY and aggregate functions like COUNT, SUM, or AVG might be more efficient if you need to group data and also eliminate duplicates.