SQL | DDL, DML, TCL and DCL

In this article, we’ll be discussing Data Definition Language, Data Manipulation Language, Transaction Control Language, and Data Control Language.

DDL (Data Definition Language) :

Data Definition Language is used to define the database structure or schema. DDL is also used to specify additional properties of the data. The storage structure and access methods used by the database system by a set of statements in a special type of DDL called a data storage and definition language. These statements define the implementation details of the database schema, which are usually hidden from the users. The data values stored in the database must satisfy certain consistency constraints.
For example, suppose the university requires that the account balance of a department must never be negative. The DDL provides facilities to specify such constraints. The database system checks these constraints every time the database is updated. In general, a constraint can be an arbitrary predicate pertaining to the database. However, arbitrary predicates may be costly to the test. Thus, the database system implements integrity constraints that can be tested with minimal overhead.

  1. Domain Constraints : A domain of possible values must be associated with every attribute (for example, integer types, character types, date/time types). Declaring an attribute to be of a particular domain acts as the constraints on the values that it can take.
  2. Referential Integrity : There are cases where we wish to ensure that a value appears in one relation for a given set of attributes also appear in a certain set of attributes in another relation i.e. Referential Integrity. For example, the department listed for each course must be one that actually exists.
  3. Assertions : An assertion is any condition that the database must always satisfy. Domain constraints and Integrity constraints are special form of assertions.
  4. Authorization : We may want to differentiate among the users as far as the type of access they are permitted on various data values in database. These differentiation are expressed in terms of Authorization. The most common being :
    read authorization – which allows reading but not modification of data ;
    insert authorization – which allow insertion of new data but not modification of existing data
    update authorization – which allows modification, but not deletion.

Some Commands:

CREATE : to create objects in database
ALTER : alters the structure of database
DROP : delete objects from database
RENAME : rename an objects

Following SQL DDL-statement defines the department table :

create table department
(dept_name  char(20),
  building   char(15),
  budget     numeric(12,2));

Execution of the above DDL statement creates the department table with three columns – dept_name, building, and budget; each of which has a specific datatype associated with it.

DML (Data Manipulation Language) :

DML statements are used for managing data with in schema objects.
DML are of two types –

  1. Procedural DMLs : require a user to specify what data are needed and how to get those data.
  2. Declerative DMLs (also referred as Non-procedural DMLs) : require a user to specify what data are needed without specifying how to get those data.

    Declarative DMLs are usually easier to learn and use than procedural DMLs. However, since a user does not have to specify how to get the data, the database system has to figure out an efficient means of accessing data.

Some Commands :

SELECT: retrieve data from the database
INSERT: insert data into a table
UPDATE: update existing data within a table
DELETE: deletes all records from a table, space for the records remain

Example of SQL query that finds the names of all instructors in the History department :

select instructor.name
 from instructor
 where instructor.dept_name = 'History';

The query specifies that those rows from the table instructor where the dept_name is History must be retrieved and the name attributes of these rows must be displayed.

TCL (Transaction Control Language) :

Transaction Control Language commands are used to manage transactions in the database. These are used to manage the changes made by DML-statements. It also allows statements to be grouped together into logical transactions.

Examples of TCL commands –

COMMIT: Commit command is used to permanently save any transaction
            into the database.
ROLLBACK: This command restores the database to last committed state.
            It is also used with savepoint command to jump to a savepoint
            in a transaction.
SAVEPOINT: Savepoint command is used to temporarily save a transaction so
            that you can rollback to that point whenever necessary.

DCL (Data Control Language) :

A Data Control Language is a syntax similar to a computer programming language used to control access to data stored in a database (Authorization). In particular, it is a component of Structured Query Language (SQL).

Examples of DCL commands :

GRANT: allow specified users to perform specified tasks.
REVOKE: cancel previously granted or denied permissions.

The operations for which privileges may be granted to or revoked from a user or role apply to both the Data definition language (DDL) and the Data manipulation language (DML), and may include CONNECT, SELECT, INSERT, UPDATE, DELETE, EXECUTE and USAGE.

In the Oracle database, executing a DCL command issues an implicit commit. Hence, you cannot roll back the command.


References : kakeboksen.td.org.uit.no



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