SQL GROUP BY

The GROUP BY statement in SQL is used to arrange identical data into groups based on specified columns. If a particular column has the same values in multiple rows, the GROUP BY clause will group these rows together. Whether we are analyzing sales by region, users by age group or orders by product category, the GROUP BY clause helps transform raw data into structured reports.

Key Points About GROUP BY

• GROUP BY clause is used with the SELECT statement.
• In a SQL query, the GROUP BY clause is placed after the WHERE clause, but before HAVING Clause and ORDER BY clause
• WHERE clause filters rows before grouping, the HAVING clause is used to apply conditions on the grouped data
• Order of Execution in a SQL query is SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY.

Syntax:

SELECT column1, function_name(column2)
FROM table_name
GROUP BY column1, column2

Parameters:

1. function_name: Name of the function used for example, SUM() , AVG().
2. table_name: Name of the table.
3. condition: Condition used.

Examples of GROUP BY in SQL

Let's assume that we have two tables Employee and Student as follows. After adding two tables we will do some specific operations to learn about GROUP BY. Insert some random data into a table and then we will perform some operations in GROUP BY.

Employee Table

CREATE TABLE emp (
emp_no INT PRIMARY KEY,
name VARCHAR(50),
sal DECIMAL(10,2),
age INT
);

INSERT INTO emp (emp_no, name, sal, age) VALUES
(1, 'Aarav', 50000.00, 25),
(2, 'Aditi', 60000.50, 30),
(3, 'Aarav', 75000.75, 35),
(4, 'Anjali', 45000.25, 28),
(5, 'Chetan', 80000.00, 32),
(6, 'Divya', 65000.00, 27),
(7, 'Gaurav', 55000.50, 29),
(8, 'Divya', 72000.75, 31),
(9, 'Gaurav', 48000.25, 26),
(10, 'Divya', 83000.00, 33);
SELECT * from emp;

Output:

Student Table

CREATE TABLE student (
name VARCHAR(50),
year INT,
subject VARCHAR(50)
);

INSERT INTO student (name, year, subject) VALUES
('Alice', 1, 'Mathematics'),
('Bob', 2, 'English'),
('Charlie', 3, 'Science'),
('David', 1, 'Mathematics'),
('Emily', 2, 'English'),
('Frank', 3, 'Science');

Output:

Example 1 : Group By Single Column

Group By single column means, placing all the rows with the same value of only that particular column in one group. Consider the query for Calculating the Total Salary of each Employee by their name in the emp table.

Query:

SELECT name, SUM(sal) FROM emp
GROUP BY name;

Output:

Explanations:

As you can see in the above output, the rows with duplicate NAMEs are grouped under the same NAME and their corresponding SALARY is the sum of the SALARY of duplicate rows. The SUM() function of SQL is used here to calculate the sum. The NAMES that are added are Aarav, Divya and Gaurav.

Example 2 : Group By Multiple Columns

GROUP BY with multiple columns means grouping rows that have the same values in those columns. For example, GROUP BY SUBJECT, YEAR will group all rows that have the same subject and year. Then we can count how many rows are in each group.

Query:

SELECT SUBJECT, YEAR, Count(*)
FROM Student
GROUP BY SUBJECT, YEAR;

Output:

Explantion:

• As we can see in the above output students with same SUBJECT and YEAR are placed in the same group.
• Those whose only SUBJECT is same but YEAR belong to different groups. So here we have grouped the table according to two columns or more than one column.
• The Grouped subject and years are (English,2) , (Mathematics,1) and (Science,3). The above mentioned all groups and years are repeated twice.

HAVING Clause in GROUP BY Clause

The WHERE clause filters rows before grouping, but if you want to filter after grouping, use the HAVING clause. It lets you apply conditions on grouped results like totals or averages.

For example, you can’t use SUM() or COUNT() in WHERE, but you can use them in HAVING to filter groups.

Syntax:

SELECT column1, function_name(column2)
FROM table_name
WHERE condition
GROUP BY column1, column2
HAVING condition
ORDER BY column1, column2;

Key Terms

• function_name: Name of the function used for example, SUM() , AVG().
• table_name: Name of the table.
• condition: Condition used.

Example:

SELECT NAME, SUM(sal) FROM Emp
GROUP BY name
HAVING SUM(sal)>50000;

Output:

Explanation:

In the result, only employees whose total salary (SUM(sal)) exceeds 50,000 are displayed. For example, if Anjali has a total salary less than 50,000, she will be excluded from the output.

Aggregate Functions and GROUP BY Statement in SQL

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