SQL Components

Components of SQL :

DDL – Data Definition Language     

Create :– Create a new table.

Alter :– Modifies an existing database.

Truncate:– Remove all record from a table, including all spaces
allocated for the records are removed.

Drop :– Deletes the entire table.

1. CREATE :- Used To Create Table

Syntax:

SQL> CREATE TABLE <TABLE_NAME>(

      2 <COLUMN_1> DATATYPE (SIZE) [CONSTRAINTS],

      3 <COLUMN_2> DATATYPE (SIZE) [CONSTRAINT],

      4 <COLUMN_n> DATATYPE (SIZE) )

      5 ;

Example:

SQL> CREATE TABLE EMPLOYEE(

      2 EMP_NO NUMBER PRIMARY KEY,

      3 EMP_NAME VARCHAR(20) NOT NULL,

      4 LOCATION VARCHAR(20))

      5 ;

Table Created.

2. ALTER :- To Modify Existing Database

Syntax:

SQL> ALTER TABLE <TABLE_NAME>

      2 (ALTER_STATEMENT) <COLUMN_NAME> (SIZE)

      3 ;

Example:

SQL> ALTER TABLE EMPLOYEE

      2 DROP COLUMN LOCATION

      3 ;

Table Altered.

SQL> ALTER TABLE EMPLOYEE

      2 ADD ADDRESS varchar(20)

      3 ;

Table Altered.

3. TRUNCATE :- Remove All Records From Table  and Recreate
Table Structure

Syntax:

SQL> TRUNCATE TABLE <TABLE_NAME>

      2 ;

Example:

SQL> TRUNCATE TABLE EMPLOYEE

      2 ;

Table Truncated.

4. DROP :- To Delete Whole Table 

Syntax:

SQL> DROP TABLE <TABLE_NAME>

      2 ;

Example:

SQL> DROP TABLE EMPLOYEE

      2 ;

Table Dropped.

DQL – Data Query Language        

Select :– Retrieves certain records from one or more tables.

SELECT :- Fetch Data From Database

Syntax:

SQL> SELECT <COLUMN_NAME> 

      2 FROM <TABLE_NAME>

      3 ;

SQL> SELECT * 

      2 FROM <TABLE_NAME>

      3 ;

Example: 

SQL> SELECT EMP_NO 

      2 FROM EMPLOYEE

      3 ;

SQL> SELECT * FROM EMPLOYEE

      2 ;