Modify the field type in the Oracle table

When modifying the Oracle data field type, you can change the data in the table as follows:

Alter table name rename column field name TO field name 1;

Alter table name ADD field name VARCHAR2 (30 );

UPDATE table name SET field name = CAST (field name 1 AS VARCHAR2 (30 ));

Alter table Name drop column field name 1;

The following is the sample code:
Modify the ID_NO Field Type
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Alter table g3_userinfo rename column ID_NO TO ID_NO1;

Alter table g3_userinfo ADD ID_NO VARCHAR2 (20 );

UPDATE g3_userinfo SET ID_NO = CAST (ID_NO1 AS VARCHAR2 (20 ));

Alter table g3_userinfo drop column ID_NO1;

Second:
The alter table name modify (field name type) cannot be used when the column has a value. Therefore, the first type is recommended.