What are joins in SQL?

[Dëfrim]

Hey guys, I am having my exams after a month and I don’t know what joins in SQL are. I want to the main concept about it so that I could atleast attempt the questions related to joins in SQL. If anyone can help me so that I could understand this topic and can write my exams properly. So, if anyone having any idea then please let me know.

[Dwarner]

The process of forming pairs of rows by matching the contents of related columns is called joining the tables. The resulting table is called a join between the two tables. The Joins can be defined as an operation that includes retrieval of data from more than one table at a time. A join is implemented using the SELECT statement, in which the SELECT list contains the name of the columns to be retrieved from the tables. The FROM clause contains the name of tables from which combined data is to be retrieved. WHERE specifies the rows to be included in the result set with the help of the join operator.

[Ronchi]

The term join applies to any query that combines data from two tables by comparing the values in a pair of columns from the tables. Although joins based on equality between match columns are by far the most common joins, SQL also allows you to join tables based on other comparison operator. This type of join is called as Non-Equi-Join.

[Steyn]

To understand how SQL solves this problem, SQL uses exactly this “imaginary duplicate table” approach to join a table to itself. Instead of actually duplicating the contents of the table, SQL lets you simply refer to it by a different name, called a table alias. Here’s the query, written using the aliases EMPS and MGRS for the Salesreps table.

Select EMPS.Name,MGRS.Name from Salesreps EMPS, Salesres MGRS
Where EMPS.Manager = MGRS.Empl_Num