Decrease the length of a column : Alter Table « Table « Oracle PL/SQL Tutorial

You can only decrease the length of a column if there are no rows in the table or all the columns contain null values.

create demo table
SQL> create table Employee(
  2    ID                 VARCHAR2(4 BYTE)         NOT NULL,
  3    First_Name         VARCHAR2(10 BYTE),
  4    Last_Name          VARCHAR2(10 BYTE),
  5    Start_Date         DATE,
  6    End_Date           DATE,
  7    Salary             Number(8,2),
  8    City               VARCHAR2(10 BYTE),
  9    Description        VARCHAR2(15 BYTE)
 10  )
 11  /

Table created.

SQL>
SQL>
SQL> -- display data in the table
SQL> select * from Employee
  2  /

no rows selected

SQL>
SQL>
SQL> desc employee;
 Name                Null?    Type
 --------------------

 ID                  NOT NULL VARCHAR2(4)
 FIRST_NAME                   VARCHAR2(10)
 LAST_NAME                    VARCHAR2(10)
 START_DATE                   DATE
 END_DATE                     DATE
 SALARY                       NUMBER(8,2)
 CITY                         VARCHAR2(10)
 DESCRIPTION                  VARCHAR2(15)

SQL>
SQL>
SQL> ALTER TABLE employee
  2  MODIFY first_name VARCHAR2(3);

Table altered.

SQL>
SQL>
SQL> desc employee;
 Name             Null?    Type
 -----------------------------

 ID               NOT NULL VARCHAR2(4)
 FIRST_NAME                VARCHAR2(3)
 LAST_NAME                 VARCHAR2(10)
 START_DATE                DATE
 END_DATE                  DATE
 SALARY                    NUMBER(8,2)
 CITY                      VARCHAR2(10)
 DESCRIPTION               VARCHAR2(15)

SQL>
SQL> select * from Employee
  2  /

no rows selected

SQL>
SQL>
SQL>
SQL> -- clean the table
SQL> drop table Employee
  2  /

Table dropped.

SQL>

6.3.Alter Table
	6.3.1.	Altering a Table
	6.3.2.	Some of the aspects of a column you can modify using ALTER TABLE
	6.3.3.	Adding a Column
	6.3.4.	ADD initially_created DATE DEFAULT SYSDATE NOT NULL;
	6.3.5.	Changing the Size of a Column
	6.3.6.	Decrease the length of a column
	6.3.7.	Changing the Precision of a Numeric Column
	6.3.8.	Decrease the precision of a numeric column
	6.3.9.	Changing the Data Type of a Column
	6.3.10.	Fill data into new added table column
	6.3.11.	Changing the Default Value of a Column
	6.3.12.	Alter table to add new constraint for new added column
	6.3.13.	ALTER TABLE to DISABLE a CONSTRAINT
	6.3.14.	Alter table to add primary key
	6.3.15.	Alter table to add primary key across more than one columns
	6.3.16.	Dropping a Column
	6.3.17.	New added columns are empty
	6.3.18.	Alter table to add unique with tablespace and storage setting
	6.3.19.	Alter table to add primary key with tablespace and storage setting
	6.3.20.	alter table nologging