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MySQL DISTINCT clause

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The DISTINCT clause in MySQL is used in the SELECT statement to eliminate duplicate rows from the result set. It allows you to retrieve unique values from a specified column or a combination of columns.

The basic syntax for using the DISTINCT clause is as follows:

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

Here, column1, column2, and so on represent the columns from which you want to retrieve distinct values. The DISTINCT keyword is placed before the column list to indicate that you want only unique values.

Let’s consider an example to illustrate the usage of the DISTINCT clause. Suppose we have a table named employees with columns employee_id, first_name, and department. If we want to retrieve a list of unique departments from the table, we can write the following query:

SELECT DISTINCT department
FROM employees;

In this example, the DISTINCT keyword is used to retrieve unique values from the department column. The result set will contain only one instance of each distinct department.

The DISTINCT clause can be combined with other clauses, such as ORDER BY, WHERE, or JOIN, to perform more complex queries. For instance, you can use it with the ORDER BY clause to sort the unique values in a specific order:

SELECT DISTINCT department
FROM employees
ORDER BY department ASC;

In this case, the result set will contain unique departments sorted in ascending order.

It’s important to note that the DISTINCT keyword operates on all columns specified in the SELECT statement. If you want to retrieve distinct combinations of values from multiple columns, you can include all those columns in the SELECT statement.

Keep in mind that using the DISTINCT clause can have performance implications, especially when dealing with large datasets. It may require additional processing to identify and eliminate duplicate rows. So, use it judiciously when necessary.

In summary, the DISTINCT clause in MySQL is used to retrieve unique values from one or more columns in a table. It eliminates duplicate rows from the result set, allowing you to work with distinct values.