In-depth understanding of the GROUP BY x, y statement in SQL
The GROUP BY x
statement in SQL is used to group records based on the common value of a specified column x. Extending this concept, GROUP BY x, y
represents a more fine-grained grouping, where records are grouped not only based on the value in column x, but also based on the value in column y.
How it works
This grouping effectively separates the data into different sets. Each set contains records that have the same value for both x and y. For example:
-
GROUP BY Subject
Categories data based on unique values in theSubject
column. -
GROUP BY Subject, Semester
further divides these groups based on unique combinations of values in theSubject
andSemester
columns.
Example
Consider the following Subject_Selection
table:
<code>+---------+----------+----------+ | Subject | Semester | Attendee | +---------+----------+----------+ | ITB001 | 1 | John | | ITB001 | 1 | Bob | | ITB001 | 1 | Mickey | | ITB001 | 2 | Jenny | | ITB001 | 2 | James | | MKB114 | 1 | John | | MKB114 | 1 | Erica | +---------+----------+----------+</code>
Apply GROUP BY Subject
to group subjects and calculate attendance:
select Subject, Count(*) from Subject_Selection group by Subject
Output:
<code>+---------+-------+ | Subject | Count | +---------+-------+ | ITB001 | 5 | | MKB114 | 2 | +---------+-------+</code>
expands to GROUP BY Subject, Semester
:
select Subject, Semester, Count(*) from Subject_Selection group by Subject, Semester
Output:
<code>+---------+----------+-------+ | Subject | Semester | Count | +---------+----------+-------+ | ITB001 | 1 | 3 | | ITB001 | 2 | 2 | | MKB114 | 1 | 2 | +---------+----------+-------+</code>
This result shows that three students took ITB001 in the first semester, two students took ITB001 in the second semester, and two students took MKB114 in the first semester.
By grouping multiple columns, you can extract more specific insights and analyze your data in greater detail.
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