In this post, I am going to share a demonstration on how to update the table data using a Subquery in the PostgreSQL.

This is a fundamental help, but I found that most of the beginners always try to find the script for Subquery or Joins.

Below is a small demonstration:

Create two sample tables for Employee:

CREATE TABLE tbl_EmployeeDetails 
(
	EmpID SERIAL PRIMARY KEY
	,EmpName CHARACTER VARYING(50)
	,TotalPerformanceCount INTEGER DEFAULT 0
);

CREATE TABLE tbl_EmployeePerformanceCounter
(
	PerformanceCounterID SERIAL PRIMARY KEY
	,EmpID INTEGER
	,PerformanceCounter INTEGER
	,RecordDateTime TIMESTAMP WITHOUT TIME ZONE
	,CONSTRAINT fk_tbl_EmployeePerformanceCounter_EmpID 
         FOREIGN KEY (EmpID) REFERENCES tbl_EmployeeDetails(EmpID)
);

Insert sample data into both the tables:

INSERT INTO tbl_EmployeeDetails (EmpName)
VALUES ('Anvesh'),('Neevan'),('Martin'),('Roy'),('Jenny');

INSERT INTO tbl_EmployeePerformanceCounter
(EmpID,PerformanceCounter,RecordDateTime)
VALUES
(1,300,'20150101')
,(1,100,'20150201')
,(1,200,'20150301')
,(2,140,'20150101')
,(2,320,'20150201')
,(3,120,'20150101')
,(3,150,'20150201')
,(4,150,'20150101')
,(4,200,'20150201')
,(4,100,'20150301');

Update the count of the employee performance counter using Subquery:

UPDATE tbl_EmployeeDetails
SET TotalPerformanceCount=T.TotalCount   
FROM (SELECT EmpID,SUM(PerformanceCounter) AS TotalCount
      FROM tbl_EmployeePerformanceCounter GROUP BY EmpID) AS T
WHERE tbl_EmployeeDetails.EmpID=T.EmpID;

The Result:

SELECT *FROM tbl_EmployeeDetails ORDER BY 1;