SQL Tutorial >> SQL Join Tables.

Now in SQL Tutorial we want to look at SQL joins. To do joins correctly in SQL requires many of the elements we have introduced so far. Let's assume that we have the following two tables,

Table: Employees
 

FirstName	LastName	Email	DOB	Phone
Austin	Hennery	austin.it@gamail.com	24/11/1978	446 102-2222
Creston	Narvon	narvon_crest@hotmail.com	22/04/1976	443 325-4545
Kate	Bladen	kbladen@usa.net	01/05/1980	326 503-3232
Angela	Julie	julieee@yahoo.co.uk	11/12/1977	326 323-8888
Bobbi	Ferguson	bobi123@kiwibox.com	10/02/1970	506 252-9875
Cute	Alfa	cafa@gmail.com	19/05/1972	506 272-4566

 

Table Employees_Salary

FirstName	Month	Salary
Austin	January	1000$
Creston	January	1000$
Kate	January	1000$
Angela	January	1900$
Bobbi	January	1000$
Cute	January	1000$
Austin	February	1100$
Creston	February	1100$
Kate	February	1100$
Angela	February	1000$
Bobbi	February	1200$
Cute	February	1300$

Now we want to find out salary by employee name. We see that table Employees_Salary includes information about salary of all employees, and table Employees contains general information for each employee. To get the salary information for each employee by his or her name , we have to combine the information from these two tables. Examining the two tables, we find that they are linked via the common field, "FirstName". We will first present the SQL statement and explain the use of each segment later and we again use the alias for each table so that you will learn more about the alias.

Result of SQL Join Tables Example:

First Name of Employee	Salary
Austin	2100$
Creston	2100$
Kate	2100$
Angela	2900$
Bobbi	2200$
Cute	2300$

On above example of SQL Tutorial express that the first two lines tell SQL to select two fields, the first one is the field "FirstName" from table Employees (aliased as First Name of Employee) and the second one is the sum of the field "Salary" from table Employees_Salary (aliased as Salary). Notice how the table aliases are used here: Employees is aliased as emp, and Employees_Salary is aliased as empSal. Without the aliasing, the first line would become

which is much more cumbersome. In essence, table aliases make the entire SQL statement easier to understand, especially when multiple tables are included.

Next, we turn our attention to line three, the SQL WHERE statement. This is where the condition of the SQL join is specified. In this case, we want to make sure that the content in "FirstName" in table Employees matches that in table Employees_Salary, and the way to do it is to set them equal. This WHERE statement is essential in making sure you get the correct output. Without the correct WHERE statement, a SQL Cartesian Join will result. Cartesian SQL joins will result in the query returning every possible combination of the two or whatever the number of tables in the FROM statement tables. In this case, a Cartesian SQL join would result in a total of 4 x 4 = 16 rows being returned.