sql alter table

Add , delete, and modify fields in ORACLE through SQL statements (alter table)



. tablename modify (column datatype [default value][null/not null],….);



syntax for deleting fields: alter table tablename drop (column); To



add, modify, or delete multiple columns, separate them with commas.



Example of using alter table to add, delete and modify a column.



Create a table structure:

create table test1

(id varchar2(20) not null);



add a field:



alter table test1

add (name varchar2(30) default 'anonymous' not null);



use one SQL statement to add three fields at the same time:



alter table test1

add (name varchar2(30) default 'anonymous' not null,



age integer default 22 not null,



has_money number(9,2)



);



modify a field



alter table test1

modify (name varchar2(16) default 'unknown');



another: the more formal writing is:



-- Add/modify columns

alter table TABLE_NAME rename column FIELD_NAME to NEW_FIELD_NAME;



delete a field



alter table test1

drop column name;



required Note that if there are already values ​​in a column, an error will occur if you change the column width to be smaller than these values.



For example, earlier if we insert a value

insert into test1

values ​​('1', 'We love you');



and then modify the column: alter table test1

modify (name varchar2(8));

will get the following error:

ERROR at the Line 2:

ORA-01441: cannot reduce column length because some values ​​are too large



-------------------------------- -------------

Advanced usage:



rename table

ALTER TABLE table_name RENAME TO new_table_name;



modify the name of the column



Syntax :

ALTER TABLE table_name RENAME COLUMN supplier_name to sname;



example:

alter table s_dept rename column age to age1;



attached: create a table with a primary key >>



create table student (

studentid int primary key not null,

studentname varchar(8),

age int);



1. Create a primary key constraint while creating a table

(1) Unnamed

create table student (

studentid int primary key not null,

studentname varchar(8),

age int);

(2) Named

create table students (

studentid int ,

studentname varchar( 8),

age int,

constraint yy primary key(studentid));



2. Delete the existing primary key constraint in the table

(1) No name

available SELECT * from user_cons_columns;

Find the primary key name in the table and get the primary key in the student table named SYS_C002715

alter table student drop constraint SYS_C002715;

(2) There is a name

alter table students drop constraint yy;



3. Add a primary key constraint to the

table alter table student add constraint pk_student primary key( studentid);