It is required if the name column names are the same between the two tables. This isn’t always required, however, it IS good practice so that you wont confuse which columns go with what tables. Notice that each of the columns are always preceded by the table name and a period. This is the most common type of “Join” that you will see or use. This particular “Join” is known as an “Inner Join” or “Equijoin”. WHERE customer_info.customer_number = purchases.customer_number

Here is an example of a join statement to accomplish this: SELECT customer_info.firstname, customer_info.lastname, em Using the two new tables, let’s say you would like to select the customer’s name, and items they’ve purchased. This column, which contains the unique customer number will be used to JOIN the two tables. Notice how each of the tables have a common “customer_number” column. Now, whenever a purchase is made from a repeating customer, the 2nd table, “Purchases” only needs to be updated! We’ve just eliminated useless redundant data, that is, we’ve just normalized this database! And the other to keep track of what they purchase:.One for keeping track of your customers.

For example, every time Wolfgang Schultz purchases something, the following rows will be inserted into the table: id Let’s say you have a one-table database that is used to keep track of all of your customers and what they purchase from your store: idĮvery time a new row is inserted into the table, all columns will be be updated, thus resulting in unnecessary “redundant data”. This single table database is also sometimes referred to as a “flat table”. Joins can be explained easier by demonstrating what would happen if you worked with one table only, and didn’t have the ability to use “joins”. Joins allow you to link data from two or more tables together into a single query result–from one single SELECT statement.Ī “Join” can be recognized in a SQL SELECT statement if it has more than one table after the FROM keyword. To put it simply, the “Join” makes relational database systems “relational”. It is time to introduce you to one of the most beneficial features of SQL & relational database systems – the “ Join“. All of the queries up until this point have been useful with the exception of one major limitation – that is, you’ve been selecting from only one table at a time with your SELECT statement.

Create a join in idatabase