The SQL SELECT DISTINCT Statement

4 min read · Mar 2, 2023

The SQL SELECT DISTINCT statement is a powerful tool for querying databases that allows you to retrieve unique values from a specific column or set of columns. This statement is used to filter out duplicate data and ensure that the output of a query is concise, organized and easy to interpret.

In this article, we will take a closer look at the SQL SELECT DISTINCT statement, its syntax, and practical examples of how it can be used to retrieve unique data from databases.

Basic Syntax

The basic syntax for using the SELECT DISTINCT statement in SQL is as follows:

SELECT DISTINCT column1, column2, ...
FROM table_name;

The SELECT DISTINCT statement retrieves unique values from the specified columns in the table. If multiple columns are specified, the DISTINCT keyword applies to all of them together. The table name refers to the name of the table from which data is being queried.

Examples of SELECT DISTINCT

To better understand how the SELECT DISTINCT statement works in SQL, let’s look at some examples:

Example 1: Retrieve unique values from a single column

Suppose we have a table named “Customers” with the following data:

To retrieve a list of unique cities where customers are located, we can use the following SQL statement:

SELECT DISTINCT City
FROM Customers;

The output of this query will be:

Example 2: Retrieve unique values from multiple columns

Suppose we have another table named “Orders” with the following data:

To retrieve a list of unique customer IDs and their associated order dates, we can use the following SQL statement:

SELECT DISTINCT CustomerID, OrderDate
FROM Orders;

The output of this query will be:

Uses of SELECT DISTINCT

The SELECT DISTINCT statement is a useful tool for several reasons:

1. Filtering out duplicates: The primary use of the SELECT DISTINCT statement is to filter out duplicate data from query results. This can be especially useful when working with large datasets, where duplicate data can make it difficult to identify unique patterns and trends.
2. Simplifying query results: The SELECT DISTINCT statement can also be used to simplify query results by reducing the amount of data returned. This can make it easier to read and interpret query results, especially when dealing with complex datasets.
3. Identifying unique values: Another use of the SELECT DISTINCT statement is to identify unique values within a dataset. This can be useful when trying to understand patterns and trends within the data, or when looking for outliers
4. Grouping data: The SELECT DISTINCT statement can also be used to group data based on unique values in one or more columns. This can be useful when analyzing data and looking for patterns based on specific criteria.
5. Joining tables: The SELECT DISTINCT statement can be used to join tables on specific columns and retrieve unique values from those columns. This can be useful when working with related tables in a database and trying to retrieve data from multiple tables.
6. Data cleansing: The SELECT DISTINCT statement can be used to clean data by removing duplicate entries in a database. This can be especially useful when dealing with messy or incomplete data, as it can help to ensure that the data is accurate and up-to-date.

Limitations of SELECT DISTINCT

While the SELECT DISTINCT statement is a powerful tool for querying databases, it does have some limitations:

1. Performance: Using the SELECT DISTINCT statement can be resource-intensive, especially when working with large datasets. This is because the database must compare each record in the table to determine unique values, which can take a significant amount of time and processing power.
2. Multiple columns: When using the SELECT DISTINCT statement with multiple columns, it can be difficult to determine which columns are being compared for uniqueness. This can lead to unexpected results and make it difficult to interpret query results.
3. Null values: The SELECT DISTINCT statement does not work well with null values, as null values are considered unique and will not be filtered out. This can lead to unexpected results and may require additional filtering or cleaning of the data.

Conclusion

The SELECT DISTINCT statement is a powerful tool for querying databases and retrieving unique values from specific columns or sets of columns. It can be used to filter out duplicate data, simplify query results, identify unique values, group data, join tables, and clean data. However, it does have some limitations, including performance issues, difficulties with multiple columns, and issues with null values. Overall, the SELECT DISTINCT statement is a valuable tool for working with databases and analyzing data, and can help to ensure that data is accurate and up-to-date.