DBS - STRUCTURE OF SQL QUERIES

1. BY
Ms V.VENNILA MCA., M.Phil..,
Assistant Professor
Bon Secours College for Women, Thanjavur.

2. Data Definition:
 Data definition refers to the set of SQL commands that
can create and manipulate the structures of a database.
 DDL statements are used to create, change and remove
objects including indexes, triggers, tables and views.
 Common DDL statements include
 CREATE
(generates a new table)
 ALTER
(alters table)
 DROP
(removes a table from the database)

3. CREATE:
Syntax:
CREATE TABLE table name(column definitions,table
parameters);
Example:
CREATE TABLE Employee(employee ID integer primary
key, first name char(50),last name(75))
ALTER:
Syntax:
ALTER object type, object name parameters
Example:
ALTER TABLE Employee ADD DOB date

4. Drop:
Syntax:
DROP object type object name
Example:
DROP TABLE Employee
BASIC STRUCTURE OF SQL QUERIES
 SQL is based on set relational operations with
certain modification and enhancements.
 A typical SQL query has the form
 The Select clause
 The From clause
 The Where clause

5. The SELECT clause:
 The select clause list the attributes described in the
result of a query.
 SQL allows duplicates in relations as well as in query
results.
 The select clause can contain arithmetic expressions
involving the operation +,-,* and / operating on
constants o attributes of tuples.
The WHERE clause:
 The where clause specifies conditions that the result
must satisfy.
 Comparison results can be combined using the logical
connectives and, or, not.
 Comparison can be applied to results of arithmetic
expressions.

6. The FROM clause:
 The from clause is a source of a row set to be operated
upon in a data manipulation language(DML) statement.
The from is an SQL reserved word in the SQL standard.
 The from clause is used in conjunction with SQL
statements.
SET OPERATIONS IN SQL
 SQL supports few set operations which can be performed
on the table data.
 These are used to get meaningful results from data
stored in the table under different special conditions.

7. Types of set operations:
 Union
 Union all
 Intersect
 Minus
Union Operation:
Union is used to combine the results of two or more
select statements.
Union All:
This operation is similar to union but it also shows the
duplicate rows.
Intersect:
Intersect operation is used to combine two select
statements.

8. Minus:
The minus operation combines results of two select
statements and return only those in the final result which
belongs to the first set of the result.