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JOINS (MS SQL Server)

T-SQL statements use joins to retrieve data from two or more tables. You can use joins in the FROM or WHERE clause.

Syntax

SELECT <arguments> FROM <first_table>
[ LEFT | RIGHT | INNER ] JOIN <second_table>
ON <join_condition>
  • <arguments>: use * to select all rows, or use column name(s) to retrieve selected columns.
  • <first_table>: First table name
  • <second_table>: Second table name
  • <join_condition>: condition on which the two tables have a matching pair. This condition has to evaluate to be true.

Pre-requisite

  • MSSQL Server
  • SQL Server Management Studio (SSMS)

Prepare the tables

  1. Create the tables
--Use the 'HASURA' database that you created
USE HASURA;
GO
--Create the first table
CREATE TABLE EMPLOYEE(
id int PRIMARY KEY,
name VARCHAR(10) not null,
--A foreign key reference to 'id' column from the 'department'
dept_id int FOREIGN KEY REFERENCES dbo.DEPARTMENT (id) NULL
);
--Create the second table
CREATE TABLE DEPARTMENT(
id int PRIMARY KEY,
dept_name VARCHAR(10),
location VARCHAR(10)
);
  1. Insert data
--Begin by entering into the 'DEPARTMENT' table first respecting the foreign key reference
INSERT INTO DEPARTMENT VALUES (29, 'R&D', 'USA');
INSERT INTO DEPARTMENT VALUES (17, 'HR', 'England');
INSERT INTO DEPARTMENT VALUES (9, 'Finance', 'Germany');
INSERT INTO DEPARTMENT VALUES (4, 'Product', 'India');
--Insert into the 'EMPLOYEE' table
INSERT INTO EMPLOYEE VALUES (1, 'Zuckerberg', NULL);
INSERT INTO EMPLOYEE VALUES (3, 'Jobs', 17);
INSERT INTO EMPLOYEE VALUES (2, 'Turing', 9);
INSERT INTO EMPLOYEE VALUES (4, 'Musk', 9);
INSERT INTO EMPLOYEE VALUES (5, 'Tesla', 9);
  1. See the result sets of both tables.
SELECT * FROM EMPLOYEE;
SELECT * FROM DEPARTMENT;

LEFT JOIN

--All employees, including the ones that do not have a department assigned, are listed.
SELECT * FROM EMPLOYEE e
LEFT JOIN DEPARTMENT d
ON e.dept_id=d.id;

left-join

RIGHT JOIN

--All departments are listed, but only the employees that have a matching dept_id are shown
SELECT * FROM EMPLOYEE e
RIGHT JOIN DEPARTMENT d
ON e.dept_id=d.id;

right join

FULL JOIN

--All employee and department rows are listed including the ones with no matching records
SELECT * FROM EMPLOYEE e
FULL JOIN DEPARTMENT d
ON e.dept_id=d.id;

full join

INNER JOIN

--Retrieve only the matching rows from both the tables
SELECT * FROM EMPLOYEE e
INNER JOIN DEPARTMENT d
ON e.dept_id=d.id;

inner-join

The keyword INNER is optional. If you omit this and just use JOIN alone, the join defaults to the type INNER JOIN.

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