Oracle - SQL Table Basics - Altering/Adding Columns

Adding column

You can add new columns to an existing table. .

ALTER TABLE PHONE
      MODIFY ( INACTIVE_DATE DATE ) 
/

You can add modify an existing column

ALTER TABLE PERSON
MODIFY ( LASTNAME VARCHAR2(35) )
/

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.

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