Database Reference
In-Depth Information
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Modifying Tables Structure
Modifying the structure of a table is done with the ALTER TABLE command. There are
three basic things that can be modified: columns, column constraints, and table constraints.
ALTER TABLE, statement may varies from one DBMS to another. Making alteration to
existing tables is neither simple nor consistent. A user can add new column or modify ex-
isting column. To change a table using ALTER TABLE, the following information must be
must specified:
The table must exist and the name of the table to be altered after the keywords
ALTER TABLE.
The list of changes to be made.
Adding and Dropping New Column
Syntax:
ALTER TABLE
tablename
ADD
columnname
data type( size);
Or
ALTER TABLE
tablename
DROP COLUMN
columnname
;
Example: Add another column branch to table student
Sol: ALTER TABLE Student
ADD Branch varchar( 15);
Example: Delete a column Phone from table Staff
Sol: ALTER TABLE Staff
DROP COLUMN Phone;
Modifying Existing Column
Syntax:
ALTER TABLE
tablename
MODIFY (
columnname
newdatatype( new size));
Example: Modify table student on column branch
Sol: ALTER TABLE Student
MODIFY( Branch varchar( 20);
Deleting Tables
