How to Count Distinct Values in a SQL Column?

Distinct Values in SQL: Counting Unique Entries

When working with database tables, it's often necessary to identify and count distinct values within a specific column. This can be achieved using the DISTINCT keyword combined with the COUNT aggregate function.

Obtaining Distinct Values

To select only the distinct values in a column, you have two options:

1. SELECT DISTINCT column_name FROM table_name;
2. SELECT column_name FROM table_name GROUP BY column_name;

Both methods will return a result set containing only the unique values for the specified column.

Counting Distinct Values

To determine the count of distinct values in a column, you can use the following syntax:

SELECT COUNT(DISTINCT column_name) AS some_alias FROM table_name

Here, the DISTINCT keyword is used within the COUNT aggregate function. This operation counts only the distinct values, ignoring duplicates, and returns the result as a new column with an alias of your choosing (e.g., some_alias).

Example

Consider the following table customers:

id	name
1	John
2	Mary
3	John
4	David
5	Mary

To count the distinct values in the name column, we would execute the following query:

SELECT COUNT(DISTINCT name) AS distinct_names FROM customers

The result of this query would be:

distinct_names

3

As you can see, the query returns the count of distinct names in the name column, which is 3.

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