Normalization is a way to reduce the amount of time it takes for tables to do their processing. It involves reducing tables to the least number of necessary columns. However, this can create a problem when putting the data back together to present desired output. To solve this problem, JOINs were invented in SQL. In a JOIN two columns from two tables are compared to find out if rows should be meshed together, then the rows are combined. It effectively makes one table an extension of another.

For example, we have a list of customers and a list of transactions. We have a customer number for each customer and that customer number is listed in the transaction log. Since we already have all the info we need about our customers in the customer table, we do not need to also store it in the transaction log. All we need do is list the customer number. So when we need to send an invoice to a customer, we can perform a JOIN on the two tables. This will recall the transaction and with the reference to the customer table get the name, address, etc. Alternatively we could also use a JOIN to find all of the transactions by any specific customer.

For the NEWBIE there is a bewildering list of SQL JOINs. Yet they all have a purpose:

First there is the INNER JOIN. This combines the information from two tables based on a comparison of two columns. It outputs only rows that are found in both tables.

A LEFT JOIN will output all the rows in the left table (or first) table and any associated info from the right table. If there is no info from the right, it will leave those cells blank.

The RIGHT JOIN is similar to the left, only in this case, it is all the rows on the RIGHT that will be listed with unfilled cells left blank.

A FULL JOIN is like a RIGHT and LEFT JOIN together. All rows are listed with the appropriate rows meshed. Blanks will be left in rows with unmeshed fields.

To confuse matters perhaps slightly, there is also the UNION. This works alot like a JOIN only it combines two SELECT statements rather than tables. The result is a list of all the unique info from two selections from like table columns.

Hope that clears things up.