IE, default values remove the necessity to assign a value to a field with the NOT NULL attribute. If you insert a zero into this field mysql will automatically assign a value that is one higher than the highest previous value assigned to this field in this table. This is a simple straightforward method for generating unique identifiers for new widgets as they are entered into the table. We also define several keys. You can only have one primary key per table.
Indexes significantly increase the speed of queries and joins.
Indexes may include more than one field. If you have an index that includes more than one field you do not need to create indexes on the individual fields that make up the composite index. For this reason rather than entering the customers address and name info into this table we assign them a unique identifying number that links us to an entry in a table that contains this information. We'll call this the Customer table. We'll talk about it a bit later.