SQL stands for Structured Query Language. It is used to store, manipulate, and retrieve data present in a database server. We use the DISTINCT keyword in SQL with a SELECT statement to remove the duplicate records and return unique records. In this article, we will explore and understand the DISTINCT keyword.
The DISTINCT
keyword in SQL is a powerful tool for keeping your data tidy and focused. Want to avoid seeing the same record twice? DISTINCT
is your answer.
The Power of Uniqueness:
- Removes Duplicates: When used with the
SELECT
statement,DISTINCT
filters out duplicate rows based on the specified columns. Imagine a customer table with multiple entries for the same person. UsingDISTINCT
on thecustomer_name
column ensures you only see unique customers. - Counts Uniques: The
COUNT
function combined withDISTINCT
returns the count of unique values in a column. Need to know how many different countries your customers come from? UseCOUNT(DISTINCT country)
. - Multiple Columns: You can specify multiple columns in the
DISTINCT
clause. This ensures only rows with unique combinations of those columns appear. Find all cities with distinct combinations ofstate
andpopulation
.
But It's Not Magic:
- Performance Impact: Removing duplicates requires comparing rows, which can be resource-intensive, especially on large datasets. Use
DISTINCT
judiciously, considering alternative methods like grouping and aggregation when possible. - Hidden Details:
DISTINCT
by default considers all columns in theSELECT
clause for uniqueness. Be mindful of unintended consequences, especially with functions or expressions in your selections. - Not for Sorting:
DISTINCT
doesn't guarantee any specific order in the results. If order matters, useORDER BY
separately.
Beyond the Basics:
- Using Subqueries: Sometimes, you might need
DISTINCT
within a subquery to ensure unique results before joining or manipulating data. - Combining with Aggregation:
DISTINCT
can be used with aggregate functions likeCOUNT
,SUM
, andAVG
to get unique summaries based on specific criteria.
Remember:
- Use
DISTINCT
when you absolutely need unique data. - Consider performance implications, especially on large datasets.
- Understand how multiple columns and functions interact with
DISTINCT
. - Explore alternative methods like grouping and aggregation when suitable.
With these insights, you can harness the power of DISTINCT
to keep your SQL queries clean, efficient, and focused on the unique information you need.