SELECT DISTINCT Statement
DISTINCT clause is used to remove the duplicate records from result set, The DISTINCT clause is used only with SELECT statement.
Syntax
SELECT DISTINCT [ColumnNames]
FROM TableName
We will start with an example, we would like to display the column CardType on table CreditCard, This will proceed as:
SELECT [CardType] FROM [Sales].[CreditCard]
This query will return 19118 rows.
You’ve noticed that there are duplicates in the result.
Using DISTINCT on single column
DISTINCT clause allows us to eliminate duplicates in the result set.
SELECT DISTINCT [CardType] FROM [Sales].[CreditCard]
This query will return 4 rows.
Using DISTINCT on multiple columns
In the combination of multiple columns, DISTINCT eliminates the rows where all the selected rows are identical.
SELECT DISTINCT [CardType], [ModifiedDate] FROM [Sales].[CreditCard]
Note:
- In SQL Server the DISTINCT clause doesn’t ignore NULL values. So when using the DISTINCT clause use NOT NULL functions to remove NULLS.
- If possible, avoid using DISTINCT because DISTINCT impacts the performances.
Reference: [www.SQLServerLog.com]
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