MySQL join

MySQL  join :

“JOIN” is an SQL keyword which used to query data from two or more related tables.

MySQL, PostgreSQL, Firebird, SQLite, SQL Server and Oracle are relational database systems. A well-designed database provides a number of tables containing related data. You can see a very simple example :

Here students and course are two table :

id name course
1 pradeep 1
2 amit 1
3 rahul 2
4 Dev 5
5 Raman (NULL)


Course table:

id name
1 Java
2 C#
3 C++
4 C
5 SQL

In essence, MySQL will automatically:Since we’re using InnoDB tables and know that student.course and course.id are related, we can specify a foreign key relationship:

  • re-number the associated entries in the user.course column if the course.id changes
  • reject any attempt to delete a course where users are enrolled.

Note: This is terrible database design!

This database is not efficient. It’s fine for this example, but a student can only be enrolled on zero or one course. A real system would need to overcome this restriction — probably using an intermediate ‘enrollment’ table which mapped any number of students to any number of courses.

JOINs allow us to query this data in a number of ways.

 

Example :


SELECT user.name, course.name

FROM `user`

INNER JOIN `course` on user.course = course.id;

Output :

 

user.name course.name
Pradeep Java
amit Java
rahul C#
Dec SQL

 

Left join :

What if we require a list of all students and their courses even if they’re not enrolled on one? A LEFT JOIN produces a set of records which matches every entry in the left table (user) regardless of any matching entry in the right table (course).


SELECT user.name, course.name

FROM `user`

LEFT JOIN `course` on user.course = course.id;

Right Join :


SELECT user.name, course.name

FROM `user`

RIGHT JOIN `course` on user.course = course.id;

 

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MySQL sorting data from table

MySQL sorting data from table :

MySQL allow us to view data from table in a sorted order. The rows retrieved from the table will be sorted in either ascending order or descending order depending on given condition.
Note : to sort data as ascending use “asc” and descending use “desc”.

Syntax :


select * from <table_name> order by <column_name> <[sorting_order]>;

Example :

Suppose you want to view data from table as salary in descending order. So you will have to write query as follows…


select * from employee order by salary desc;

 

MySQL eliminating duplicate rows with select statement :

MySQL eliminating duplicate rows with select statement :

A table may contain duplicate rows and you need to view only unique row then you can use distinct keyword with
select statement.

Syntax :


select distinct * from <table_name>;

Example :


select distinct * from employee;

MySQL Filtering table data

MySQL Filtering table data :

In database, when you need to view data from table it’s rare required to view all data each time. So sql give us some
method of filtering data that not required.

There are three ways to filter any table :

  • selected columns and all rows
  • selected rows and all columns
  • selected columns and selected rows

selected columns and all rows :

In this way you will write query as selected column name as your need.

Syntax :


select <column_name1>, <column_name2>....<column_name_n> from <table_name>;

Example :


select fname, lname from employee;

selected rows and all columns :

Sql provide where clause to filter row from table.

Syntax :

 


select * from <table_name> where <condition>;

In above query, sql engine compares each records in the table with the condition given in the where clause. Data display
only those records that satisfy the given condition.

Example :

 


select * from employee where salary='20000';

Note : With above condition you can use all standard operators like logical, arithmetic etc.

selected columns and selected rows :

SQL provide facilities to select column as well as row.

Syntax :

 


select <column_name1>, <column_name2>....<column_name_n> from <table_name> where <condition>;

Example :

 


select fname, lname from employee where salary='20000';