rows that have been inserted into your tables.

SQL statements are divided into two major categories:

Data Definition Language (DDL)
Data Manipulation Language (DML)

Data Definition Language (DDL):
DDL statements 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

1. 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:

2. 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);

3. The delete table statement (query) to delete a table is given below:

DROP TABLE <table name>;

Example:

DROP TABLE STUDENT;

Data Manipulation Language

Data Manipulation Language (DML) statements are used for managing data within tables. Some commands of DML are:

• SELECT - retrieve data from the a database
• INSERT - insert data into a table
• UPDATE - updates existing data within a table
• DELETE - deletes all records from a table, the space for the records remain
• MERGE - UPSERT operation (insert or update)
• CALL - call a PL/SQL or Java subprogram
• LOCK TABLE - control concurrency