With the assistance of some functions, the GROUP BY statement in SQL is used to organize identical data into groups. i.e. if a specific column has the same values in distinct rows, then these rows will be arranged in a group.
Syntax: the syntax of the GROUP BY clause is given below.
SELECT <Col_1>, <Col_2>, <aggregate_function(Col_3)>....<Col_n> FROM <table_name> WHERE <conditions> GROUP BY <Col_1>, <Col_2>....<Col_N> |
For each the group you can apply an aggregate function like SUM(), COUNT(), etc. The GROUP BY clause appears after FROM or WHERE clause in a SELECT statement.
PostgreSQL GROUP BY clause examples
Suppose we have the following table. You can create an Employee table using the below PostgreSQL statement.
CREATE TABLE public."Employee" ( "Id" bigserial NOT NULL, "FName" text COLLATE pg_catalog."default", "LName" text COLLATE pg_catalog."default", "UserName" text COLLATE pg_catalog."default", "Password" text COLLATE pg_catalog."default", "Contact" text COLLATE pg_catalog."default", "Salary" numeric NOT NULL DEFAULT 0.0, "DOJ" date NOT NULL, CONSTRAINT "Employee_Primarykey" PRIMARY KEY ("Id") ) |
Insert some records in "Employee" table
INSERT INTO public."Employee"( "FName", "LName", "UserName", "Password", "Contact", "Salary", "DOJ") VALUES ('Name1','last1', 'name1@gmail.com', 'name@0123', '9088877787', '20000','2018/07/07'); INSERT INTO public."Employee"( "FName", "LName", "UserName", "Password", "Contact", "Salary", "DOJ") VALUES ('Name2','last2', 'name2@gmail.com', 'name@0123', '9288877787', '30000','2018/07/07'); INSERT INTO public."Employee"( "FName", "LName", "UserName", "Password", "Contact", "Salary", "DOJ") VALUES ('Name3','last3', 'name3@gmail.com', 'name@0123', '9388877787', '40000','2017/09/08'); INSERT INTO public."Employee"( "FName", "LName", "UserName", "Password", "Contact", "Salary", "DOJ") VALUES ('Name4','last4', 'name4@gmail.com', 'name@0123', '9344877787', '70000','2018/06/09'); |
Illustrates of PostgreSQL GROUP BY clause.
1- Using PostgreSQL GROUP BY without an aggregate function example.
Here I am applying GROUP BY clause on the columns DOJ (date of joining) because it has identical data
SELECT "DOJ" FROM public."Employee" GROUP BY "DOJ" |
2- Using PostgreSQL GROUP BY with an aggregate function example.
Suppose, I want to count the employees who have joined the company on the same date so we can query the data from the table using the following statement.
SELECT "DOJ", COUNT(*) FROM public."Employee" GROUP BY "DOJ" |
You can see in the result on dated '2018/07/07', there are two joinings.