Joining Tables

In MySQL, JOIN operations are used to combine rows from two or more tables based on a related column between them. Here's an explanation of the different types of joins:

1. INNER JOIN

The INNER JOIN keyword selects records that have matching values in both tables.

SELECT columns
FROM table1
INNER JOIN table2
ON table1.common_column = table2.common_column;

Example
Assume we have two tables, students and enrollments.

SELECT students.id, students.first_name, students.last_name, enrollments.course_id
FROM students
INNER JOIN enrollments ON students.id = enrollments.student_id;

2. LEFT JOIN (or LEFT OUTER JOIN)

The LEFT JOIN keyword returns all records from the left table (table1), and the matched records from the right table (table2). The result is NULL from the right side if there is no match.

SELECT columns
FROM table1
LEFT JOIN table2
ON table1.common_column = table2.common_column;

Example
SELECT students.id, students.first_name, students.last_name, enrollments.course_id
FROM students
LEFT JOIN enrollments ON students.id = enrollments.student_id;

This query returns all students, including those who are not enrolled in any courses. For students not enrolled, course_id will be NULL.

3. RIGHT JOIN (or RIGHT OUTER JOIN)

The RIGHT JOIN keyword returns all records from the right table (table2), and the matched records from the left table (table1). The result is NULL from the left side if there is no match

SELECT columns
FROM table1
RIGHT JOIN table2
ON table1.common_column = table2.common_column;

4. FULL JOIN (or FULL OUTER JOIN)

The FULL JOIN keyword returns all records when there is a match in either left (table1) or right (table2) table records. Note that MySQL does not support FULL OUTER JOIN directly, but it can be simulated using a combination of LEFT JOIN and RIGHT JOIN with UNION.

SELECT columns
FROM table1
LEFT JOIN table2 ON table1.common_column = table2.common_column
UNION
SELECT columns
FROM table1
RIGHT JOIN table2 ON table1.common_column = table2.common_column;

Comments

Post a Comment

Popular posts from this blog

KTU DBMS LAB CSL 333 BTech S5 - Dr Binu V P

 About Me About the course Syllabus and Experiments **************************************** Introduction to RDBMS Working with MySQL ER Diagrams-MySQL Workbench Introduction to SQL Data types in MySQL Built-in Functions MySQL Aggregate Functions Enforcing rules using constraints MySQL -Lab Exercise ****************************************** Joining tables Order by, group by and having clause Nested Queries MySQL -Lab Exercise ******************************************* Insert Data from a CSV file Set Operations Queries using temporary tables TCL commands in MySQL DCL commands in MySQL Views and Assertions Library Database- Lab Exercise ********************************************* PL/SQL and Stored Procedures IF statement in PL/SQL CASE statement in PL/SQL Loops in PL/SQL Procedures in PL/SQL Functions in PL/SQL Triggers in PL/SQL Cursors in PL/SQL Packages in PL/SQL Exception Handling in PL/SQL Backing up and Restoring Databases Query Analysis using Query Plan ***********************
Read more

Procedures in PL/SQL

Procedures in PL/SQL (Procedural Language/Structured Query Language) are a way to encapsulate a set of SQL and PL/SQL statements that perform a specific task. They are stored in the database and can be executed as needed, providing a modular and reusable approach to database programming. Here’s an overview of creating and using procedures in PL/SQL: Creating a Procedure To create a procedure, you use the CREATE PROCEDURE statement followed by the procedure's name, parameters (if any), and the body of the procedure, which contains the executable code. Here’s a basic syntax: CREATE [OR REPLACE] PROCEDURE procedure_name     [ (parameter_name [IN | OUT | IN OUT] datatype [, ...]) ] IS    -- Declaration of variables BEGIN    -- Executable statements EXCEPTION    -- Exception handling END procedure_name; Example Here’s a simple example of a PL/SQL procedure that inserts a new employee record into an employees table: CREATE OR REPLACE PROCEDURE add_employee (     p_emp_id IN employees.emp
Read more

Cursors in PL/SQL

In PL/SQL, cursors are a mechanism for accessing and manipulating query result sets. A cursor allows you to fetch and process rows returned by a query one at a time. There are two main types of cursors in PL/SQL: implicit cursors and explicit cursors. Implicit Cursors PL/SQL automatically creates implicit cursors for SELECT INTO statements, and for INSERT, UPDATE, DELETE, and MERGE statements. Example of Implicit Cursor DECLARE     v_employee_name employees.first_name%TYPE; BEGIN     -- This SELECT INTO statement uses an implicit cursor     SELECT first_name     INTO v_employee_name     FROM employees     WHERE employee_id = 100;     DBMS_OUTPUT.PUT_LINE('Employee Name: ' || v_employee_name); END; In this example, PL/SQL creates an implicit cursor for the SELECT INTO statement and automatically fetches the result into the v_employee_name variable. Explicit Cursors Explicit cursors give you more control over the context area. You can open a cursor, fetch rows from the result s
Read more