SQL - Group By Clause

---

---

The SQL GROUP BY Clause

The SQL GROUP BY clause is used to group (organize) rows that have the same values in specified columns into summary rows. It is used with aggregate functions like COUNT(), SUM(), AVG(), MAX(), and MIN() to perform calculations on each group of data.

It is useful when you want to summarize data and get totals or averages for each category or group. The GROUP BY clause appears after the WHERE clause (if used), and before the HAVING and ORDER BY clauses in a SQL query.

Syntax

Following is the basic syntax of the SQL GROUP BY clause:

SELECT column_name(s)
FROM table_name
GROUP BY column_name(s);

Where, column_name(s) refers to the name of one or more columns in the table that we want to group the data by and the table_name refers to the name of the table that we want to retrieve data from.

GROUP BY Clause with Aggregate Functions

The GROUP BY clause is commonly used in combination with aggregate functions like SUM(), AVG(), MIN(), MAX(), and COUNT() to perform calculations on grouped data.

This allows you to generate meaningful summaries, such as totals, averages, or counts, for each group of records based on one or more columns.

Example

Assume we have created a table named CUSTOMERS, which contains the personal details of customers including their name, age, address and salary, using the following query:

CREATE TABLE CUSTOMERS (
   ID INT NOT NULL,
   NAME VARCHAR (20) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS CHAR (25),
   SALARY DECIMAL (18, 2),
   PRIMARY KEY (ID)
);

Now insert values into this table using the INSERT statement as follows:

INSERT INTO CUSTOMERS VALUES
(1, 'Ramesh', 32, 'Ahmedabad', 2000.00),
(2, 'Khilan', 25, 'Delhi', 1500.00),
(3, 'Kaushik', 23, 'Kota', 2000.00),
(4, 'Chaitali', 25, 'Mumbai', 6500.00),
(5, 'Hardik', 27, 'Bhopal', 8500.00),
(6, 'Komal', 22, 'Hyderabad', 4500.00),
(7, 'Muffy', 24, 'Indore', 10000.00);

The table created is as shown below:

The following SQL query groups the CUSTOMERS table based on AGE and counts the number of records in each group:

SELECT AGE, COUNT(Name) FROM CUSTOMERS GROUP BY AGE;

Following is the result produced:

Example

In the following query, we are finding the highest salary for each age:

SELECT AGE, MAX(salary) AS MAX_SALARY 
FROM CUSTOMERS GROUP BY AGE;

Following is the output of the above query:

Similarly we can group the records of the CUSTOMERS table based on the AGE column and calculate the maximum salary, average and sum of the SALARY values in each group using the MIN(), AVG() and SUM() functions respectively.

GROUP BY Clause on Single Columns

When we use the GROUP BY clause with a single column, all the rows in the table that have the same value in that particular column will be merged into a single record.

Example

In the following example we are grouping the above created CUSTOMERS table by the ADDRESS column and calculating the average salary of the customer from each city:

SELECT ADDRESS, AVG(SALARY) as AVG_SALARY 
FROM CUSTOMERS GROUP BY ADDRESS;

This would produce the following result:

GROUP BY Clause with Multiple Columns

When we use the GROUP BY clause with multiple columns, all the rows in the table that have the same values in all of the specified columns will be merged into a single group.

Example

In the following query we are grouping the records of the CUSTOMERS table based on the columns ADDRESS and AGE:

SELECT ADDRESS, AGE, SUM(SALARY) AS TOTAL_SALARY 
FROM CUSTOMERS GROUP BY ADDRESS, AGE;

This would produce the following result:

GROUP BY with ORDER BY Clause

We can use the ORDER BY clause with GROUP BY in SQL to sort the grouped data by one or more columns.

Syntax

Following is the syntax for using ORDER BY clause with GROUP BY clause in SQL:

SELECT column1, column2, ..., aggregate_function(columnX) AS alias
FROM table
GROUP BY column1, column2, ...
ORDER BY column1 [ASC | DESC], column2 [ASC | DESC], ...;

Example

In here, we are finding the highest salary for each age, sorted by high to low:

SELECT AGE, MIN(SALARY) AS MIN_SALARY 
FROM CUSTOMERS 
GROUP BY AGE ORDER BY MIN_SALARY DESC;

Following is the result produced:

GROUP BY with HAVING Clause

We can also use the GROUP BY clause with the HAVING clause filter the grouped data in a table based on specific criteria.

Syntax

Following is the syntax for using ORDER BY clause with HAVING clause in SQL:

SELECT column1, column2, aggregate_function(column)
FROM table_name
GROUP BY column1, column2
HAVING condition;

Example

In the following query, we are grouping the customers by their age and calculating the minimum salary for each group. Using the HAVING clause we are filtering the groups where the age is greater than 24:

SELECT ADDRESS, AGE, MIN(SALARY) AS MIN_SUM 
FROM CUSTOMERS 
GROUP BY ADDRESS, AGE HAVING AGE>24;

The result produced is as follows:

GROUP BY with JOINS

We can also use the GROUP BY clause in SQL queries with JOIN operations. This is useful when we need to summarize or aggregate data from multiple related tables.

When using GROUP BY with JOINS, we typically:

• Join two or more tables using an appropriate join type (INNER JOIN, LEFT JOIN, etc.)
• Select the columns we want to group by from either or both tables
• Apply aggregate functions to summarize the data for each group

Syntax

The general syntax for using GROUP BY with JOINs is as follows:

SELECT table1.column1, table2.column2, aggregate_function(columnX) AS alias
FROM table1
JOIN table2 ON table1.common_column = table2.common_column
GROUP BY table1.column1, table2.column2;

Example

Suppose we have another table called ORDERS, which contains information about customer orders, and it is defined as follows:

CREATE TABLE ORDERS (
   ORDER_ID INT NOT NULL,
   CUSTOMER_ID INT,
   AMOUNT DECIMAL(10,2),
   PRIMARY KEY (ORDER_ID)
);

Let us insert some sample data into the ORDERS table:

INSERT INTO ORDERS VALUES
(101, 1, 300.00),
(102, 2, 150.00),
(103, 1, 200.00),
(104, 3, 400.00),
(105, 5, 250.00),
(106, 4, 600.00),
(107, 1, 500.00);

Now let us write a query to find the total order amount per customer name using a JOIN between CUSTOMERS and ORDERS:

SELECT C.NAME, SUM(O.AMOUNT) AS TOTAL_ORDER_AMOUNT
FROM CUSTOMERS C
JOIN ORDERS O ON C.ID = O.CUSTOMER_ID
GROUP BY C.NAME;

Following is the result produced:

In the above example:

• We joined CUSTOMERS and ORDERS using C.ID = O.CUSTOMER_ID
• Then grouped the data by customer NAME
• Calculated the total order AMOUNT per customer using SUM(O.AMOUNT)

This allows us to analyze total sales or purchases made by each customer across the system.