To create a single table the join clause combines columns of one table to that of another. Join matches up a column with one table to a column in another. A join query does not alter table, but temporarily combines data from each table to be viewed as a single table.Mainly three types of join statements:-

• Inner/Equi Join
• Left Join
• Right Join

Let us we want to find out sales by region. For geting the sales information by region, we have to combine the information from the two tables.Examining the two tables, we find that they are linked via the common field, "store_name". First present the SQL Query and then explain the use:

The first two lines tell SQL to select two fields, the first one is the field "region_name" from table Geography (aliased as REGION), and the second one is the sum of the field "Sales" from table Store_Information (aliased as SALES). Notice how the table aliases are used here: Geography is aliased as A1, and Store_Information is aliased as A2. Without the aliasing, the first line would become

which is much more bulky. 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 3, the WHERE statement. This is where the condition of the join is specified. In this case, we want to make sure that the content in "store_name" in table Geography matches that in table Store_Information, 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 Cartesian Join will result. Cartesian 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 join would result in a total of 4 x 4 = 16 rows being returned.

A Left join returns all rows of the left of the conditional even if there is no right column to match.

A right join will display rows on the right side of the conditional that may or may not have a match.