Implementation of SQL DDL commands- CREATE, ALTER, DROP and constraints on relations link Primary Key, Foreign Key and Check

1. Database System Sunita M. Dol
Page 1
HANDOUT#01
Aim:
Implementation of SQL DDL commands- CREATE, ALTER, DROP and constraints on
relations link Primary Key, Foreign Key and Check
Theory:
The SQL(Structured Query Language) Language has several parts:
1. Data-definition language (DDL). The SQL DDL provides commands for defining
relation schemas, deleting relations, and modifying relation schemas.
2. Interactive data-manipulation language (DML). The SQL DML includes a query
language based on both the relational algebra and the tuple relational calculus. It
includes also commands to insert tuples into, delete tuples from, and modify tuples in
the database.
3. View definition. The SQL DDL includes commands for defining views.
4. Transaction control. SQL includes commands for specifying the beginning and
ending of transactions.
5. Embedded SQL and dynamic SQL. Embedded and dynamic SQL define how SQL
statements can be embedded within general-purpose programming languages, such as
C, C++, Java, PL/I, Cobol, Pascal, and Fortran.
6. Integrity. The SQL DDL includes commands for specifying integrity constraints that
the data stored in the database must satisfy. Updates that violate integrity constraints
are disallowed.
7. Authorization. The SQL DDL includes commands for specifying access rights to
relations and views.
Domain Types in SQL:
char(n). Fixed length character string, with user-specified length n.
varchar(n). Variable length character strings, with user-specified maximum length
n.
int. Integer (a finite subset of the integers that is machine-dependent).
smallint. Small integer (a machine-dependent subset of the integer domain type).
numeric(p,d). Fixed point number, with user-specified precision of p digits, with n
digits to the right of decimal point.
real, double precision. Floating point and double-precision floating point numbers,
with machine-dependent precision.
float(n). Floating point number, with user-specified precision of at least n digits.

2. Database System Sunita M. Dol
Page 2
DDL
Data Definition Language (DDL) allows the specification of not only a set of relations but also
information about each relation, including:
The schema for each relation.
The domain of values associated with each attribute.
Integrity constraints
The set of indices to be maintained for each relations.
Security and authorization information for each relation.
The physical storage structure of each relation on disk.
Following are the DDL commands
CREATE
ALTER
DROP
CREATE Command
An SQL relation is defined using the create table command:
create table r (A1 D1, A2 D2, ..., An Dn,
(integrity-constraint1),
...,
(integrity-constraintk))
• r is the name of the relation
• each Ai is an attribute name in the schema of relation r
• Di is the data type of values in the domain of attribute Ai
Constraints on relations
• primary key (Aj1 , Aj2, . . . , Ajm ): The primary-key specification says that
attributes Aj1 , Aj2, . . . , Ajm form the primary key for the relation. The primary
key attributes are required to be nonnull and unique
• foreign key (Ak1 , Ak2, . . . , Akn ) references s: The foreign key specification
says that the values of attributes (Ak1 , Ak2, . . . , Akn ) for any tuple in the relation
must correspond to values of the primary key attributes of some tuple in relation s.
• not null: The not null constraint on an attribute specifies that the null value is not
allowed for that attribute; in other words, the constraint excludes the null value
from the domain of that attribute.
• check(P): The check clause specifies a predicate P that must be satisfied by every
tuple in the relation.
ALTER Command
The Oracle ALTER TABLE statement is used to add, modify, or drop/delete columns in a table.

3. Database System Sunita M. Dol
Page 3
Add column in table
To ADD A COLUMN in a table, the Oracle ALTER TABLE syntax is:
ALTER TABLE table_name
ADD column_name column-definition;
Add multiple columns in table
To ADD MULTIPLE COLUMNS to an existing table, the Oracle ALTER TABLE
syntax is:
ALTER TABLE table_name
ADD (column_1 column-definition,
column_2 column-definition,
...
column_n column_definition);
Modify column in table
To MODIFY A COLUMN in an existing table, the Oracle ALTER TABLE syntax is:
ALTER TABLE table_name
MODIFY column_name column_type;
Modify Multiple columns in table
To MODIFY MULTIPLE COLUMNS in an existing table, the Oracle ALTER TABLE
syntax is:
ALTER TABLE table_name
MODIFY (column_1 column_type,
column_2 column_type,
...
column_n column_type);
Drop column in table
To DROP A COLUMN in an existing table, the Oracle ALTER TABLE syntax is:
ALTER TABLE table_name
DROP COLUMN column_name;
DROP Command
To remove a relation from an SQL database, we use the drop table command. The drop
table command deletes all information about the dropped relation from the database. The
command
drop table r
is a more drastic action than

4. Database System Sunita M. Dol
Page 4
delete from r
The latter retains relation r, but deletes all tuples in r. The former deletes not only all
tuples of r, but also the schema for r. After r is dropped, no tuples can be inserted into r
unless it is re-created with the create table command.
Queries and Output:
CREATE Command and Adding Constraints on Relation
SQL> create table branch
2 (branch_name char(15),
3 branch_city char(15),
4 assets numeric(16,2),
5 primary key(branch_name),
6 check(assets>=0));
Table created.
SQL> desc branch;
Name Null? Type
----------------------------------------- -------- ----------------------------
BRANCH_NAME NOT NULL CHAR(15)
BRANCH_CITY CHAR(15)
ASSETS NUMBER(16,2)
SQL> create table account
2 (account_number char(10),
3 branch_name char(15),
4 balance numeric(12,2),
5 primary key(account_number),
6 foreign key(branch_name)references branch,
7 check(balance>=0));
Table created.
SQL> desc account;
Name Null? Type
----------------------------------------- -------- ----------------------------
ACCOUNT_NUMBER NOT NULL CHAR(10)
BRANCH_NAME CHAR(15)
BALANCE NUMBER(12,2)

5. Database System Sunita M. Dol
Page 5
SQL> create table student(roll_no char(10),first_name char(30),middle_name
char(30),last_name char(3
0),address char(50),city char(20),pincode numeric(6,0),state char(20),mobile_number integer);
Table created.
ALTER Command
SQL> desc student;
Name Null? Type
----------------------------------------- -------- ----------------------------
ROLL_NO CHAR(10)
FIRST_NAME CHAR(30)
MIDDLE_NAME CHAR(30)
LAST_NAME CHAR(30)
ADDRESS CHAR(50)
CITY CHAR(20)
PINCODE NUMBER(6)
STATE CHAR(20)
MOBILE_NUMBER NUMBER(38)
SQL> alter table student drop column mobile_number;
Table altered.
SQL> desc student;
Name Null? Type
----------------------------------------- -------- ----------------------------
ROLL_NO CHAR(10)
FIRST_NAME CHAR(30)
MIDDLE_NAME CHAR(30)
LAST_NAME CHAR(30)
ADDRESS CHAR(50)
CITY CHAR(20)
PINCODE NUMBER(6)
STATE CHAR(20)
DROP Command
SQL> desc student;
Name Null? Type

6. Database System Sunita M. Dol
Page 6
----------------------------------------- -------- ----------------------------
ROLL_NO CHAR(10)
FIRST_NAME CHAR(30)
MIDDLE_NAME CHAR(30)
LAST_NAME CHAR(30)
ADDRESS CHAR(50)
CITY CHAR(20)
PINCODE NUMBER(6)
STATE CHAR(20)
MOBILE_NUMBER VARCHAR2(10)
SQL> drop table student;
Table dropped.
SQL> desc student;
ERROR:
ORA-04043: object student does not exist
Conclusion:
We have studied the various DDL command
a. CREATE Command
b. ALTER Command
c. DROP Command and
d. Constraints like Primary Key, Foreign Key and Check
References:
• Database system concepts by Abraham Silberschatz, Henry F. Korth, S. Sudarshan
(McGraw Hill International Edition) sixth edition.
• Database system concepts by Abraham Silberschatz, Henry F. Korth, S. Sudarshan
(McGraw Hill International Edition) fifth edition.
• http://codex.cs.yale.edu/avi/db-book/db4/slide-dir/
• http://codex.cs.yale.edu/avi/db-book/db5/slide-dir/
• http://codex.cs.yale.edu/avi/db-book/db6/slide-dir/