Alter table Command
This command helps us to modify the structure of an existing table.
Structure of an existing table can be modified in two ways
(i) by adding a new column
(ii) by modifying the definition of an existing column.
(i). adding a new column
“Add” clause when used with alter table command helps us to add a new column.
Syntax:
alter table tablename
add (colname1 datatype(size),
Colname2 datatype(size) );
Std(roll int ,name char(10))
Alter table std add(age int);
Alter table std add(age int,per decimal);
Note:
Newly added column will contain NULL of all the existing records.
In order to insert values in the newly added columns we have to use the command “update”.
Update age=10 where roll=101;
(ii). modifying the definition of an existing column
Modify clause when used with alter table command helps us to modify the definition of an existing table.
Syntax:
alter table tablename
modify(oldcolname1 newdatatype(newsize),
oldcolname2 new datatype(newsize));
Example:
Std(roll int,name char(10),age int);
Alter table std
Modify(age decimal);
Alter table std
Modify (name char(20));
