So, in the example mentioned above, we are trying to specify the data type using the string CHAR. The database server examines the chosen clause, which is the first and one of the last clauses of the select statement. The reason for this is that we need to know all of the alternative columns that could be included in the final result set before we can decide what to include in it. When creating a table, the data types most often used include strings (VARCHAR or CHAR); numbers (NUMBER or INTEGER); and dates (DATE). Other commonly used commands include RENAME and COMMENT.
- In this example, we are going to completely delete a table from the database.
- It is used to retrieve, store, modify, delete, insert and update data in database.
- The commands would be located in a file and would be submitted as a batch to be executed.
- The database server examines the chosen clause, which is the first and one of the last clauses of the select statement.
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.
Relational data Model
DDL statements create, modify and remove database objects, such as tables, indexes and stogroups. DDL is also used in a generic sense to refer to any language that describes data. Data Definition Language (DDL) is used to create and modify the structure of objects in a database using predefined commands and a specific syntax. These database objects include tables, sequences, locations, aliases, schemas and indexes.
The Book table was created previously, but there is no specification for a primary key. To add a primary key, we use the ALTER TABLE command as shown below. Transaction Control Language commands are used to manage transactions in the database.
DDL vs. SQL vs. DML vs. DQL
This lesson has identified data definition language (DDL), a term used to cover SQL statements that update and/or manipulate a database structure. These commands include CREATE, DROP, and ALTER, and are used to add/drop tables, views, indexes, and triggers. DDL statements can be used to add constraints, such as primary or foreign keys to tables. It’s important to have good naming conventions when creating these so that database maintenance is manageable.
The alter command modifies any existing table in a database. The Alter command can add up an additional column, drop the existing columns and even change the data types of various columns involved in a DB table. It is mainly used to modify and establish the structure of the objects present in a database by dealing with the database schema descriptions. The SQL commands that can be used to describe the database structure are collectively known as Data Definition Language. It simply works with database schema descriptions and is used to create and modify the database’s database object structure.
These commands are used to make modifications to database objects, such as indexes, locations and stogroups. DDL also includes several DROP commands to delete objects in a database. DROP commands cannot be undone, so once an object is deleted, it cannot be recovered.
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.
A basis sql (DDL) is a computer language used to create and modify the structure of database objects in a database. These database objects include views, schemas, tables, indexes, etc. DDL is a standardized language with commands to define the storage groups (stogroups), different structures and objects in a database.