How Can I Count the Number of Distinct Values in an SQL Column?

Finding the Number of Distinct Values in an SQL Column

When working with SQL databases, it's often necessary to determine the number of unique values in a particular column. This count provides valuable insights into the diversity and distribution of data within the table. While querying for distinct values is essential, obtaining the corresponding row count poses an additional challenge.

Getting Distinct Values

As described in the context, the DISTINCT keyword can be used in conjunction with SELECT to retrieve distinct values from a column. There are two common approaches:

• SELECT DISTINCT column_name FROM table_name;
• SELECT column_name FROM table_name GROUP BY column_name;

These queries return the unique values of the specified column, but they do not provide the row count.

Combining DISTINCT with COUNT

To obtain the row count of distinct values, it's necessary to combine the DISTINCT keyword with the COUNT aggregate function. This is achieved as follows:

SELECT COUNT(DISTINCT column_name) AS some_alias FROM table_name;

In this query:

• COUNT(DISTINCT column_name) counts the distinct values of the specified column column_name.
• AS some_alias assigns an alias to the resulting column, allowing it to be easily referenced in further operations or visualizations.

Practical Example

Consider the following table customers:

customer_id	first_name
1	John
2	Mary
3	David
4	John
5	Sarah

To determine the number of distinct values in the first_name column, execute the query:

SELECT COUNT(DISTINCT first_name) AS distinct_names FROM customers;

This query returns the result:

distinct_names

4

This indicates that there are four unique first names in the customers table.

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