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In SQL how to use GROUP BY on multiple columns?

In SQL how to use GROUP BY on multiple columns?

Richard C.

The ProblemJump To Solution

In SQL, what exactly does GROUP BY do? How does it work when applied to multiple columns instead of just one?

The Solution

Let’s use an example table of people with names and ages. (All commands in this article work on MySQL, PostgreSQL, and MS SQL Server.)

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CREATE TABLE Person ( Id INT PRIMARY KEY, Name VARCHAR(255), Age INT ); INSERT INTO Person (Id, Name, Age) VALUES (1, 'Amir', 25), (2, 'Amir', 25), (3, 'Aya', 2), (4, 'Aya', 30), (5, 'Leila', 8), (6, 'Leila', 35), (7, 'Ndidi', 12), (8, 'Ndidi', 12), (9, 'Ndidi', 25), (10, 'Sofia', 42), (11, 'Mateo', 8), (12, 'Mateo', 67), (13, 'Yara', 35), (14, 'Yara', 35), (15, 'Yara', 4), (16, 'Yara', 35);

First, let’s select Name from the table:

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SELECT Name FROM Person; -- Name -- Amir -- Amir -- Aya -- Aya -- Leila -- Leila -- Ndidi -- Ndidi -- Ndidi -- Sofia -- Mateo -- Mateo -- Yara -- Yara -- Yara -- Yara

Sixteen rows are returned because there are 16 people in the table.

If you want to remove duplicates you can either use DISTINCT or GROUP BY:

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SELECT DISTINCT Name FROM Person; -- Name -- Amir -- Aya -- Leila -- Mateo -- Ndidi -- Sofia -- Yara
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SELECT Name FROM Person GROUP BY Name; -- Name -- Amir -- Aya -- Leila -- Mateo -- Ndidi -- Sofia -- Yara

GROUP BY merges all identical values in a column into a single row, effectively making it return unique values for a column. Unlike DISTINCT, GROUP BY allows you to calculate information related to each group. The functions that do this are called aggregate functions. They are COUNT, SUM, AVG, MIN, and MAX. For example:

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SELECT Name, COUNT(Name) FROM Person GROUP BY Name; -- Name COUNT(Name) -- Amir 2 -- Aya 2 -- Leila 2 -- Ndidi 3 -- Sofia 1 -- Mateo 2 -- Yara 4

If you use GROUP BY on two or more columns, it will merge all pairs of values that are the same in both columns into a single row. In our example, if we group by both Name and Age, then one row will be returned for every person that has the same name and the same age. Here’s the query:

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SELECT Name, Age, COUNT(1) FROM Person GROUP BY Name, Age; -- Name Age COUNT(1) -- Amir 25 2 -- Aya 2 1 -- Aya 30 1 -- Leila 8 1 -- Leila 35 1 -- Mateo 8 1 -- Mateo 67 1 -- Ndidi 12 2 -- Ndidi 25 1 -- Sofia 42 1 -- Yara 4 1 -- Yara 35 3

You can see that there are seven people with the same name, but there are only three people with the same name and age.

Using GROUP BY Name, Age as opposed to GROUP BY Age, Name will make no difference to the results returned (except for the column order).

Do not write GROUP BY Name AND Age instead of GROUP BY Age, Name. Even if your database server accepts this syntax, you will likely get incorrect results.

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