How Does GROUP BY with Multiple Columns Work in SQL?

Understanding SQL's GROUP BY with Multiple Columns

The SQL GROUP BY clause is a powerful tool for data aggregation. While using GROUP BY with a single column is straightforward, employing it with multiple columns requires a deeper understanding.

When you use GROUP BY x, y, the combination of x and y values defines the grouping criteria. Data is grouped into unique combinations of x and y values, unlike GROUP BY x which groups solely based on x.

Let's illustrate with the "Subject_Selection" table:

Subject	Semester	Attendee
ITB001	1	John
ITB001	1	Bob
ITB001	1	Mickey
ITB001	2	Jenny
ITB001	2	James
MKB114	1	John
MKB114	1	Erica

A GROUP BY Subject query yields:

Subject	Count
ITB001	5
MKB114	2

Showing 5 entries for ITB001 and 2 for MKB114.

However, a GROUP BY Subject, Semester query produces:

Subject	Semester	Count
ITB001	1	3
ITB001	2	2
MKB114	1	2

Here, grouping is based on both Subject and Semester. We see 3 attendees for ITB001 in semester 1, 2 in semester 2, and 2 for MKB114 in semester 1.

In essence, GROUP BY with multiple columns allows simultaneous grouping by multiple criteria, revealing data distribution and relationships between different attributes.

The above is the detailed content of How Does GROUP BY with Multiple Columns Work in SQL?. For more information, please follow other related articles on the PHP Chinese website!