Basic Data Definition Language(DDL) Commands

The Data Definition Language( DDL ) commands are used for creating new or modify existing schema of a relation. You can also set constraints such as key constraints, foreign key constraints, etc on a relation.



This command creates a new relation or table. You must specify the fields and data type for each field while using this command.

Figure 1 – Create Table MANAGER


This is most important of DDL commands because it allows us to modify the table anytime.
Here are some examples of ALTER TABLE command

Adding a primary key for the table. A primary key uniquely identifies a tuple in a relation.

Figure 2 – Alter Table MANAGER

Adding a foreign key for the table. A foreign key in a relation refer to primary key of another relation and cannot be null called the Referential Integrity.

Figure 3: Alter Table MANAGER for Foreign Key – Error

In the above example, we receive an error because the attribute ‘DEPTNO’ is not set as primary key in relation to ‘EX_DEPT’.

After adding ‘DEPTNO’ as primary key we are able to set the foreign key for ‘MANAGER’ relation.

Figure 4: Alter Table MANAGER for Foreign Key – Success

Adding a new column in a Table. In the following example, we are adding ‘PCODE‘ in the ‘MANAGER’ table.

Figure 5: Alter Table MANAGER for new column

Removing a column from a relation or table. In the following example, we are removing the ‘PCODE’ column from the ‘MANAGER’ table.

Figure 6: Removing a Column from the Table MANAGER

DESC <Table Name>

The ‘DESC’ command helps view the schema of the relation.

Figure 7: DESC <table_name> display the table schema

DROP TABLE <table name>

This command will drop the table permanently.

Figure 8: DROP < table_name> will delete the Table


Avi Silberschatz, Henry F. Korth, and S. Sudarshan. 27-Jan-2010. Database System Concepts. McGraw-Hill Education.

Ramakrishnan, Johannes Gehrke, and Raghu. 1996. Database Management Systems. McGraw Hill Education; Third edition (1 July 2014).

Wikipedia. n.d. Data definition language. Accessed March 14, 2018.


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