Using DISTINCT Keyword to Retrieve Unique Values in SQL

One of the main skills an SQL programmer should possess is the ability to filter out duplicate values from the results of an SQL query. This feature is performed using the DISTINCT keyword. According to the question given, the correct command to retrieve the unique values from the 'EmployeeName' column in the 'Employees' table is:

SELECT DISTINCT EmployeeName FROM Employees

Understanding the DISTINCT Keyword

The DISTINCT keyword in SQL is used to return only distinct (different) values within a column. It filters out all duplicate values in the specified column(s).

In this context, when dealing with an 'Employees' table in a database, there could be several instances where employees have the same name. If we want to retrieve a list where each employee name appears only once, we would use the DISTINCT keyword:

SELECT DISTINCT EmployeeName FROM Employees

This SQL statement will yield a result set where each name is unique.

Incorrect Usage of DISTINCT

It's important to understand that the position of the DISTINCT keyword is critical. For instance, placing it at the end of the query as shown in option 4 in the question:

SELECT EmployeeName FROM Employees DISTINCT

This would lead to a syntax error. The DISTINCT keyword must be placed right after the SELECT keyword.

Similarly, using the UNIQUE keyword as suggested in option 2 is incorrect:

SELECT UNIQUE EmployeeName FROM Employees

The UNIQUE keyword in SQL is used in a different context, mainly when defining the structure of the database, to ensure the values in a column are unique.

Further Insights and Best Practices

• DISTINCT can be used with multiple columns. In this case, the combination of values in these columns are used to determine the uniqueness of the record.
• Be cautious when using DISTINCT, as eliminating duplicates requires additional processing resources which could slow down the query performance, especially when working with large data sets.
• Also, remember to only use DISTINCT when it is necessary to prevent unnecessary performance loss. An example of unnecessary usage could be when you are sure the records are already unique.

In conclusion, to retrieve unique values from a specific column in an SQL database, the DISTINCT keyword is a powerful, reliable tool. By understanding its correct usage and applications, you can effectively eliminate duplicates from your query results.