by Guillermo Madero S..
Hi Wendi,
As you want the field to be optional, I would do either of the following (assuming that the table prefix is mdl and that the collation is utf8_unicode_ci):
1. Allow null values
ALTER TABLE mdl_user ADD city VARCHAR(120) NULL CHARACTER SET utf8 COLLATE utf8_unicode_ci AFTER address;
2. Do not allow null values but assign a default value (e.g. the one used in the location settings)
ALTER TABLE mdl_user ADD city VARCHAR(120) NOT NULL DEFAULT 'Default_City_Value' CHARACTER SET utf8 COLLATE utf8_unicode_ci AFTER address;
If I were doing this, I would go with the second option.