MySQL COUNT(DISTINCT) function

COUNT(DISTINCT) function

MySQL COUNT(DISTINCT) function returns a count of number rows with different non-NULL expr values. It is used to count the number of unique or distinct values that match a specified condition within a table.

This function is useful in -

• This helps you understand the diversity and uniqueness of data.
• The COUNT(DISTINCT) identifies and eliminates duplicate values in your data. This is useful for data cleaning and ensuring data accuracy.
• COUNT(DISTINCT) helps in getting this count or categorical data, you might want to know the number of unique categories or options available.
• COUNT(DISTINCT) helps in generating the number of unique occurrences of certain attributes to get accurate and informative reports.
• You can analyze membership in various groups or categories by counting the number of distinct members in each group.
• COUNT(DISTINCT) provides unique value counts for analysis and decision-making

Syntax:

COUNT(DISTINCT expr,[expr...])

Where expr is a given expression.

MySQL Version: 8.0

Example: MySQL COUNT(DISTINCT) function

The following MySQL statement will count the unique 'pub_lang' and average of 'no_page' up to 2 decimal places for each group of 'cate_id'.

Sample table: book_mast

Code:

SELECT cate_id,COUNT(DISTINCT(pub_lang)), ROUND(AVG(no_page),2)
FROM book_mast
GROUP BY cate_id;

Output:

mysql> SELECT cate_id,COUNT(DISTINCT(pub_lang)), ROUND(AVG(no_page),2)
    -> FROM book_mast
    -> GROUP BY cate_id;
+---------+---------------------------+-----------------------+
| cate_id | COUNT(DISTINCT(pub_lang)) | ROUND(AVG(no_page),2) |
+---------+---------------------------+-----------------------+
| CA001   |                         2 |                264.33 | 
| CA002   |                         1 |                433.33 | 
| CA003   |                         2 |                256.67 | 
| CA004   |                         3 |                246.67 | 
| CA005   |                         3 |                245.75 | 
+---------+---------------------------+-----------------------+
5 rows in set (0.00 sec)

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