MySQL - SQL Table Basics - Altering/Adding Columns
You can add new columns to an existing table.
ALTER TABLE phone
ADD inactive_date DATETIME NULL
You can add modify an existing column
ALTER TABLE person
ALTER COLUMN lastname VARCHAR(35) NULL
Considerations for altering a column
- Reducing precision (example, going from CHAR(20) to
CHAR(15)) can cause data truncation and should be avoided unless you
are absolutely sure their will be no impact to the data.
- Changing data types should typically be avoided.
There are exceptions to this. For example, changing a CHAR(20) to a
VARCHAR(20) on columns where the average storage length is 10 can
save disk space.
Alter columns - No Can Do
- You cannot directly alter a a column that is part of the primary key