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The primary consideration for the member_id membership number column is that each of its values should be unique to avoid confusion between members. An AUTO_INCREMENT column is useful here because then we can let MySQL generate unique numbers for us automatically when we add new members. Even though it just contains numbers, the declaration for member_id has several parts:
INT signifies that the column holds integers (numeric values with no fractional part).
UNSIGNED disallows negative numbers.
NOT NULL requires that the column value must be filled in. (This means that no member can be without an ID number.)
AUTO_INCREMENT is a special attribute in MySQL. It indicates that the column holds sequence numbers. The AUTO_INCREMENT mechanism works like this; If the value for the member_id column is missing (or NULL) when you create a new member table record, MySQL automatically generates the next sequence number and assigns it to the column. This makes it easy to assign IDs to new members, because MySQL will do it for us.
The PRIMARY KEY clause indicates that the member_id column is indexed to allow fast lookups and that each value in the column must be unique. The latter property is desirable for member ID values, because it prevents us from using the same ID twice by mistake. (Besides, MySQL requires every AUTO_INCREMENT column to have some kind of unique index, so the table definition is illegal without one.)
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