DDL statements or commands are used to define and modify the database structure of your tables or schema. When you execute a DDL statement, it takes effect immediately.
Some commands of DDL are:
- CREATE – to create table (objects) in the database
- ALTER – alters the structure of the database
- DROP – delete table from the database
- TRUNCATE – remove all records from a table, including all spaces allocated for the records are removed
- COMMENT – add comments to the data dictionary
- RENAME – rename a table
CREATE :
The create table statement (query) to create a table is given below:
CREATE TABLE <table name> ( <attribute name 1> <data type 1>, ... <attribute name n> <data type n>);
Example:
CREATE TABLE STUDENT ( StudID NUMBER, Name VARCHAR);
The data types that you will use most frequently are character strings, which might be called VARCHAR or CHAR for variable or fixed length strings; numeric types such as NUMBER or NTEGER, which will usually specify a precision; and DATE or related types. Data types are differ according to the databases software whatever you are using to your system.
ALTER :
The alter table statement to make modifications to the table structure such as Key constraints, Column size, etc.
ALTER TABLE <table name> ADD CONSTRAINT <constraint name> PRIMARY KEY(<attribute list>);
Example:
ALTER TABLE STUDENT ADD CONSTRAINT NOT NULL PRIMARY KEY (StudID);
DROP :
The drop table statement (query) to delete a table is given below:
DROP TABLE <table name>;
Example:
DROP TABLE STUDENT;
