1、 Add, delete and modify table fields:
Syntax for adding fields:alter table tablename add (column datatype [default value][null/not null],….);
Modify the syntax of the field:alter table tablename modify (column datatype [default value][null/not null],….);
Syntax for deleting fields:alter table tablename drop (column);
Note: if multiple columns are added, modified, or deleted, separate them with commas.
Rename the table field: alter table tablename rename column field name to new field name;
2、 Rename table:
alter table tablename rename to new_table_name;
3、 To add, delete, or modify a primary key:
Lookup table constraint:select constraint_name from user_cons_columns a where a.table_name='tablename';
Add table constraint:alter table tablename add constraint pk_name primary key(column);
Delete table constraint:alter table tablename drop constraint constraint_name----（SYS_C002715）;
Modify table constraint: 1) disable table primary key:alter table tablename disable primary key;
2) enable table primary key:alter table tablename enable primary key;
3) rename table primary key:alter table tablename rename constraint pk_id to new_pk_id;
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