The ALTER table command in SQL is used to modify the structure of an existing table. It can be used to add new columns, delete columns, change column sizes or data types, and add integrity constraints. Some key uses of ALTER table include adding a new column using the ADD command, modifying column properties like data type or default value using the MODIFY command, and changing the structure of a table.

1. ALTER TABLE COMMAND
This is a part of DDL command . The ALTER table command is
used to change definition of existing table usually it can add
columns to a table. It can delete column and change their sizes.
In MY SQL alter table command is used…
1. To Add a New Column
2. To Add an Integrity Constraints
3. To Redefine a column (Data type, size, default value)
1. Adding Column:-
 We can add a new column in existing table with the
help of ALTER table command.
 To ADD a new column use ADD command with the
ALTER table command.
 The new column will be added with NULL values for
all rows currently in the table.
Syntax :-
ALTER TABLE tablename
ADD(newcolumname datatype(size));
Example :-
ALTER TABLE STUDENT
ADD(CITY VARCHAR(20));

2. Q1. To Insert a Record in new column.
UPDATE STUDENT
SET CITY=’ALWAR’;
Insert into student values(‘alwar’);--- wrong query
Modify command:-
We can use the MODIFY command with alter table
command to change structure of table.
We can change following parts of a table:-
 Data Type
 Size
 Default Value
 Order of Column
1) Data Type and Size
Syntax ;-
Alter Table tablename
Modify columnname newdatatype(new size);
Example:-
Alter table student
Modify phoneno varchar(20);

3. Alter table student
Modify city default ‘jaipur’;