What is DDL data dictionary?

Answer 1

DDL and data dictionary are two different things.

A data dictionary is a collection of unambiguous explanations about data elements. Normally, it is a centralized repository where everyone who needs information about the data being managed (for example, in a computer system) can refer to so they can understand what each data element is and how it is used.

For example, if you have a data element called "ADDRESS," some people might assume that this refers to the street address of a building while other people assume it refers to the IP address of a computer. The data dictionary will explain precisely what the data element called ADDRESS really means. The dictionary might also explain that data elements are made up of other data elements. For example, ADDRESS might be defined as the combination of NUMBER and STREET_NAME. Again, some people might think that NUMBER means the building number on the STREET_NAME, and others might think it means the apartment number, so data dictionaries must be very specific in defining all of the data elements.

Data dictionaries may be written documents expressed in a natural language (like English), or they may also be more formalized, using automated tools to help the organization capture, store and manipulate the information about their data elements. A DDL is a Data Definition Language. These are not natural languages. These are languages that can be understood by computer software (normally, a database management system) and they instruct the computer on what data structures it is to create and manage. A data structure usually involves tables, rows within the table, and then data elements within the rows. For example, you might purchase a database management system. When you first install it, the software has no idea what kind of data you want store and manage, so you have to teach it about what kind of structures you want it to create and what kind of data is going to be stored in those structures. You might, for example, tell if that you want each employee in your company to have a record with their social security number, their name, their home address and their telephone number. Then, you might want to have another record for each social security number that lists the employee's salary and what health insurance benefit they have selected. The way you tell your database management software that this is what you want is through the DDL. Once you have programmed it with the DDL, you can then use a DML (data manipulation language) to load the data into the structures, and use it. Again, a DML is a language that is understandable by the computer software. You may write a DML command that tells you the name of very person who lives on Main Street. You could also tell the database software to increase the annual salary of every employee by 15%.

The DDL tells the computer what data structures you want, and the DML tells the computer what you want to do with the data contained in those structures. The data dictionary tells everyone what all those data elements really mean.