DDL Full Form - GeeksforGeeks

13 Dec, 2021
DDL stands for Data Definition Language. These commands are used to change the structure of a database and database objects.For example, DDL commands can be used to add, remove, or modify tables with in a database. 
The DDL commands are:  
CREATEALTERDROPTRUNCATERENAME
CREATE
ALTER
DROP
TRUNCATE
RENAME
1. CREATE : This command is used to create table in the relational database . This can be done by specifying the names and datatypes of various columns. 
Syntax:  
CREATE TABLE TABLE_NAME
(
    column_name1 datatype1,
    column_name2 datatype2,
    column_name3 datatype3,
    column_name4 datatype4
);
The column_name in create table command will tell the name of the column and corresponding datatype will specify the datatype of that column.Here in this table the three column_names namely – Student_id is of type int ,Name is of type varchar and Marks is of type int. for example: 
CREATE TABLE 
Employee
(Student_id INT, 
Name VARCHAR(100), 
Marks INT); 
Student_id
Name
Marks
2. ALTER : Alter command is used for altering the table in many forms like: 
Add a columnRename existing columnDrop a columnModify the size of the column or change datatype of the column
Add a column
Rename existing column
Drop a column
Modify the size of the column or change datatype of the column
ADD using ALTER – Syntax to add column : 
ALTER TABLE table_name ADD(
    column_name datatype); 
The above command will add a new column to the table.And the resulting table will have one more column like this: 
ALTER TABLE Student 
ADD
(Address  VARCHAR(200)); 
Here this command will add a new column “Address” in the table Student of datatype varchar(200); 
Student_id
Name
Marks
Address
RENAME using ALTER – Syntax to rename column : 
ALTER TABLE 
table_name 
RENAME 
old_column_name TO new_column_name; 
The above command will rename the existing column to new column. 
ALTER TABLE 
Employee 
RENAME 
Marks TO Age; 
The command above will change the column_name from Marks to Age;  
Student_id
Name
Age
Address
DROP using ALTER – Syntax to Drop a column : 
ALTER TABLE
table_name
DROp
(column_name); 
The above command will delete the existing column. For example: 
ALTER TABLE Employee 
DROP
(Age);  
Here the column_name =”Age”, has been deleted by this command; 
Student_id
Name
Address
MODIFY using ALTER – Syntax to Modify a column : 
ALTER TABLE
Employee MODIFY
(column_name datatype); 
The above command will modify the existing column . For example: 
ALTER TABLE 
student 
MODIFY
(name varchar(300)); 
The above command will modify the column_name “Name” by changing the size of that column. 
Student_id
Name
Address
3. TRUNCATE : This command removes all the records from a table. But this command will not destroy the table’s structure. Syntax :  
TRUNCATE TABLE table_name
This will delete all the records from the table.For example the below command will remove all the records from table student. 
Example: 
TRUNCATE TABLE Student; 
4. DROP : This command completely removes the table from the database along with the destruction of the table structure. 
Syntax –  
DROP TABLE table_name
This will delete all the records as well as the structure of the table. This is the main difference between TRUNCATE AND DROP.-TRUNCATE only removes the records whereas DROP completely destroys the table. 
Example:  
DROP TABLE Student; 
This command will remove the table records as well as destroys the schema too. 
This is all about the DDL commands. 
sooda367
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