Understanding the SQL DISTINCT Statement

The SQL DISTINCT keyword plays a strategic role in database management. It is used to return unique values in the output, eliminating all the duplicate entries. The correct SQL statement to return only distinct or different values from specific column(s) is SELECT DISTINCT column_name FROM table_name.

When working with SQL databases, there could be instances where a column contains duplicate values. And, you might want to fetch all unique values from that column without duplication. This is where the DISTINCT keyword comes into the picture.

Practical Usage of SQL DISTINCT Statement

Consider a 'students' table that contains columns like 'name', 'subject', 'score', etc. If you need to find all the unique 'subject' values, the DISTINCT clause can be used as such:

SELECT DISTINCT subject FROM students;

This statement will return a list of unique subject names present in the 'students' table.

Best Practices with SQL DISTINCT Statement

• Use the DISTINCT keyword sparingly and only when necessary. It is a computationally expensive operation because SQL has to compare every result with every other result to eliminate duplicates. This might slow down the performance of your SQL query, especially with large data sets.

• When using DISTINCT, try to limit the number of columns in the SELECT statement. The more columns you include, the more comparisons SQL has to make, and the slower your query will be.

• Often, it's better to handle the duplicate values at the data source itself rather than using the DISTINCT keyword. Regularly cleaning the data and maintaining its uniqueness at the data entry point will result in optimized query performance.

In summary, the DISTINCT keyword is used in SQL statement to return unique values from a column. It's a valuable tool, but due to its high computational expense, it should be used judiciously and with consideration of the overall database and query performance.