Understanding SQL Server Select Distinct for Dev

Hi Dev, welcome to our guide on understanding SQL Server Select Distinct. This article is designed to help you understand the fundamentals of using the Select Distinct statement in SQL Server. We will cover the basics of what Select Distinct is, how it works, and provide some practical examples of how it can be used in your SQL queries.

What is Select Distinct?

SQL Server Select Distinct is a statement that allows you to retrieve unique values from a column in a database table. This means that if your table has multiple rows with the same value in a specific column, Select Distinct will only return one instance of that value.

For example, let’s say you have a table called “Sales” and one of the columns in the table is “Region.” If you run a Select Distinct statement on the “Region” column, it will only return one instance of each unique region in the table, even if there are multiple rows with the same region.

How does Select Distinct work?

When you use Select Distinct, SQL Server will examine each row in the column you are selecting from and remove any duplicate values. It does this by comparing the values in each row and only returning one instance of each unique value.

It’s important to note that Select Distinct only applies to the column you are selecting from. If there are duplicates in other columns in the same row, they will still be returned in the query results.

Using Select Distinct in SQL Queries

Now that you understand what Select Distinct does and how it works, let’s take a look at some practical examples of how you can use it in your SQL queries.

Example 1: Select Distinct with One Column

One of the most common uses for Select Distinct is to retrieve unique values from a single column in a table. Let’s take a look at an example:

ProductID	ProductName	CategoryID
1	Apple	1
2	Orange	1
3	Apple	2
4	Banana	2
5	Mango	3

If we want to retrieve a list of all the unique values in the “ProductName” column, we can use the following SQL query:

SELECT DISTINCT ProductName FROM Sales

This query will return the following result:

ProductName
Apple
Orange
Banana
Mango

Frequently Asked Questions

What is the difference between Select and Select Distinct?

Select retrieves all rows from a table, while Select Distinct retrieves only unique values from a column in a table.

Can I use Select Distinct with multiple columns?

Yes, you can use Select Distinct with multiple columns. However, it will only retrieve unique combinations of values in those columns.

What happens if I use Select Distinct with a column that has a null value?

If a column has a null value, it will be considered unique and will be included in the results of the Select Distinct query.

Will using Select Distinct affect the performance of my SQL query?

Using Select Distinct can have an impact on the performance of your SQL query, especially if you are working with large tables. It’s important to use it sparingly and only when necessary.

Can I combine Select Distinct with other SQL statements like Count or Order By?

Yes, you can combine Select Distinct with other SQL statements like Count or Order By to further refine your query results.