1. Instructor: Samia arshad
Alter Table
Instructor: Samia arshad

2. More SQL Data Definition
Changing Tables
Sometimes you want to change the structure of an existing table
One way is to DROP it then rebuild it
This is dangerous, so there is the ALTER TABLE command instead
ALTER TABLE can
Add a new column
Remove an existing column
Add a new constraint
Remove an existing constraint
More SQL Data Definition

3. More SQL Data Definition
ALTERing Columns
Examples:
Suppose we have a table student having id and degree column
ALTER TABLE Student
ADD Degree VARCHAR(20)
DROP COLUMN Degree
To add or remove columns use
ALTER TABLE <table>
ADD <col>
DROP COLUMN <name>
More SQL Data Definition

4. More SQL Data Definition
ALTERing Columns
Examples
Changing attribute type:
ALTER TABLE Student
Modify
Degree CHAR(5)
Renaming attribute:
CHANGE Degree Degree1 varchar(25)
To change existing attribute type and renaming a column
ALTER TABLE <table>
Modify <col>
Alter TABLE <table>
CHANGE <oldname> <newname> type(size)
More SQL Data Definition

5. More SQL Data Definition
Adding Constraints
Examples
Adding constraint:
Unique Constraint
ALTER TABLE STUDENT
ADD CONSTRAINT
ck UNIQUE (DEGREE)
Not Null constraint
alter table STUDENT modify Degree varchar(25) Not Null;
To add or remove columns use
ALTER TABLE <table>
ADD CONSTRAINT
<definition>
ALTER TABLE <table name> modify <column name> type(size) not null;
More SQL Data Definition

6. More SQL Data Definition
Adding Constraints
Primary Key ALTER TABLE STUDENT ADD CONSTRAINT pk Primary key (id)
Foreign key
ALTER TABLE STUDENT
ADD CONSTRAINT
fk Foreign key (column name) references primary table(column name)
More SQL Data Definition

7. More SQL Data Definition
Adding constraints
Check
ALTER TABLE STUDENT
ADD CONSTRAINT
chk check (id>10)
More SQL Data Definition

8. More SQL Data Definition
Dropping Constraint
Dropping Constraint:
ALTER TABLE STUDENT
DROP foreign key constraintname;
Dropping Not Null constraint
ALTER TABLE student CHANGE column name column name int(25) null;
ALTER TABLE <table>
DROP CONSTRAINT
<name>
More SQL Data Definition

9. More SQL Data Definition
Dropping Constraint
To drop primary key, check, unique constraint, the following syntax will be used
Alter table STUDENT drop index constraintname;
More SQL Data Definition

10. More SQL Data Definition
Drop table
Examples
Drop TABLE student
Drop table
Drop TABLE <table name>
More SQL Data Definition