How Does SQL's `GROUP BY` Function Work with Multiple Columns?
SQL's GROUP BY Clause: Handling Multiple Columns
The SQL GROUP BY clause aggregates rows in a table based on specified columns. Let's explore how it functions with single and multiple columns.
Single-Column Grouping (GROUP BY x)
GROUP BY x groups rows sharing the same value in column 'x'. All rows with identical 'x' values are treated as a single group.
Multi-Column Grouping (GROUP BY x, y)
Extending this, GROUP BY x, y groups rows based on matching values in both columns 'x' and 'y'. Rows must have identical values for both 'x' and 'y' to be in the same group.
Example: Subject Enrollment
Consider the Subject_Selection table tracking student subject enrollment:
<code>Table: Subject_Selection +---------+----------+---------+ | 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>
Grouping by Subject Only
The query SELECT Subject, COUNT(*) FROM Subject_Selection GROUP BY Subject groups by Subject:
<code>+---------+--------+ | Subject | Count | +---------+--------+ | ITB001 | 5 | | MKB114 | 2 | +---------+--------+</code>
This shows the total enrollment for each subject.
Grouping by Subject and Semester
The query SELECT Subject, Semester, COUNT(*) FROM Subject_Selection GROUP BY Subject, Semester groups by Subject and Semester:
<code>+---------+----------+--------+ | Subject | Semester | Count | +---------+----------+--------+ | ITB001 | 1 | 3 | | ITB001 | 2 | 2 | | MKB114 | 1 | 2 | +---------+----------+--------+</code>
This provides enrollment counts for each subject per semester, offering a more granular view. The key difference is the added level of detail provided by including Semester in the GROUP BY clause.
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