Adding a field: ALTER TABLE Employee ADD gender ENUM('M', 'F')
Remove or drop a field: ALTER TABLE Employee DROP dependents
Note that if you drop a field, there is no undo, no CTRL+Z! The fields and any values that were there are gone.
Change the size of a field: ALTER TABLE Employee MODIFY lastName VARCHAR(20) will change the size of lastName from 15 to 20 and keep all of the existing data.
ALTER TABLE Employee MODIFY lastName VARCHAR(10) will change the size of lastName from 15 to 10. If any names were longer than 10, those names will be truncated.
DROP a table: DROP TABLE tableName
The table and all of the data is gone!
Rename a field: ALTER TABLE People CHANGE gender sex ENUM('M','F')
The format is
ALTER TABLE table-name CHANGE old_field_name new_field_name type
Change the name of the table: ALTER TABLE Employee RENAME People
If you need to see the table definition use SHOW COLUMNS FROM People
Change the Primary Key: You have to drop the existing primary key and add the new one. you can do this in one statement:
ALTER TABLE table_name DROP PRIMARY KEY, ADD PRIMARY KEY(field1,field2)