Mastering SQL Server Distinct for Devs

Hey there, Dev! Are you looking to improve your SQL Server skills? One thing you’ll definitely want to master is the DISTINCT keyword. It’s one of the most powerful tools in your SQL Server toolbox, and in this article, we’ll take a deep dive into how it works and how you can use it to your advantage. So buckle up and let’s get started!

Understanding Distinct

Before we dive into the specifics of how DISTINCT works, let’s take a step back and look at the big picture. In SQL Server, you use SELECT statements to retrieve data from tables. But what happens when you run a SELECT statement that returns duplicate rows?

That’s where DISTINCT comes in. When you add the keyword DISTINCT to your SELECT statement, SQL Server will only return the unique rows that match your criteria. Let’s look at an example:

ProductID	ProductName	SupplierID
1	Chai	1
2	Chang	1
3	Aniseed Syrup	2
4	Chef Anton’s Cajun Seasoning	2
5	Chef Anton’s Gumbo Mix	2

If we want to retrieve a list of all suppliers from this table, we might write a SELECT statement like this:

SELECT SupplierIDFROM Products

However, this would return duplicate SupplierID values, since some suppliers have multiple products. To get a list of unique suppliers, we can add DISTINCT to our SELECT statement:

SELECT DISTINCT SupplierIDFROM Products

Now we’ll get a list of only the unique supplier IDs:

SupplierID
1
2

The Syntax of DISTINCT

The syntax for using DISTINCT is very simple. You just need to add it after the SELECT keyword, like this:

SELECT DISTINCT column1, column2, ...FROM table_name

Here column1, column2, … are the columns you want to retrieve unique values for, and table_name is the name of the table you’re querying.

Using DISTINCT with Multiple Columns

In our previous example, we used DISTINCT with a single column. But what if we want to retrieve unique values based on combinations of columns?

For example, let’s say we have a table of orders:

OrderID	CustomerID	ProductID	OrderDate
1	ALFKI	1	2022-01-01
2	ALFKI	2	2022-01-02
3	BONAP	1	2022-01-03
4	BONAP	3	2022-01-04
5	BONAP	3	2022-01-05

If we want to retrieve a list of unique combinations of customer and product, we can use the following SELECT statement:

SELECT DISTINCT CustomerID, ProductIDFROM Orders

This will give us the following results:

CustomerID	ProductID
ALFKI	1
ALFKI	2
BONAP	1
BONAP	3

Sorting and Filtering with DISTINCT

Now that we understand how DISTINCT works, let’s look at some ways we can use it to sort and filter our data.

Sorting with DISTINCT

If you want to sort the results of a DISTINCT query, you can simply add an ORDER BY clause to your SELECT statement. For example:

SELECT DISTINCT column1, column2, ...FROM table_nameORDER BY column1

This will return the unique values of column1, sorted in ascending order.

Filtering with DISTINCT

You can also use WHERE clauses to filter the results of a DISTINCT query. For example:

SELECT DISTINCT column1, column2, ...FROM table_nameWHERE condition

This will return the unique values of the selected columns that meet the specified condition.

FAQs About SQL Server Distinct

Q: Can I use DISTINCT with NULL values?

A: Yes, DISTINCT will work with NULL values, but be aware that NULL is considered a unique value. So if you have multiple rows with NULL in the selected column, they will all be included in the results.

Q: Can I use DISTINCT with aggregate functions?

A: Yes, you can use DISTINCT with aggregate functions like COUNT, SUM, AVG, etc. This can be useful if you want to retrieve the unique values of a column and also perform some type of calculation on those values.

Q: Can I use DISTINCT with multiple tables?

A: Yes, you can use DISTINCT with JOIN statements to retrieve unique values from multiple tables. However, you’ll need to be careful to ensure that the columns you’re selecting are unique across all tables, or you may end up with unexpected results.

Q: Is DISTINCT case-sensitive?

A: Yes, DISTINCT is case-sensitive, so it will treat “foo” and “FOO” as distinct values. If you want to perform a case-insensitive DISTINCT query, you’ll need to use a function like LOWER() or UPPER() to convert the values to a consistent case before selecting them.

Q: Are there any performance considerations when using DISTINCT?

A: Yes, DISTINCT queries can be slower than regular SELECT queries, because SQL Server needs to perform additional processing to remove duplicate rows. If you’re working with a large dataset, it’s important to test the performance of your DISTINCT queries and optimize them as needed.