Today , lets go into the exciting world of JOIN statements. Before we talk about complex topics such as INNER and OUTER JOINS , default JOIN is an INNER JOIN statement if you are interested , a few words on the concept of JOIN statements. We have , up to this stage , done some basic SQL operations , such as setting up the database and the tables and inserting information into them. We have also looked at basic queries using SELECT statement with WHERE clause. Till now , we are learning what can be done even easier in excel , not much more reason to learn SQL at all. What , then , is the big deal with the databases?
Short and sweet answer to that would be that a database , is a relational database. What this means is that by using common information , we can gather information from two tables at same time. Take a look at the table created , each table is specific to itself. There is no department budget data in employee table and there is no reason why it should be there. If we want to know what is our own department budget , we ask our manager , who has access to it. Same for the SQL database. Instead of adding every information to one table only , we split the tables according to the function or type , pretty much like OOP if you ask me.
Here is a sample JOIN statement ,
select employee.emp_name , department.dept_name , department.dept_budget
from employee , department
where department.dept_budget >500000;
Here , instead of querying from one table , we query from 2 tables , employee and department tables. If you read it carefully 2 or 3 times , you will find it is pretty simple and straightforward. In plain English , it is asking for
“Employee names , department names and department budget for those departments with budget more than 500000.”
There are a few ways of using JOIN statements but for our purpose , this will do. Till next time , have a nice day!
And great works to the original author.