Chapter 9 - Creating, Deleting, and Altering Tables

Abstract

Tables and all other parts of a database that are not data are collectively known as metadata. Metadata is data about data. Structured query language (SQL) is also known as metadata. The basic table has a name and a set of columns, each with its own data type. One can create a table in SQL using CREATE TABLE. This creates a table named<table name>. The columns of the table are specified in a comma-delimited list of name/data type pairs. Before creating a table, most database management systems (DBMSs) require the creation of a database to hold the new table. When a new row is created, any columns without a specified value are assigned the default value. Unless otherwise specified, the default value of a column is NULL. SQL allows specifying a default value for a column using the DEFAULT clause. DBMS can do much more than just store and access data. It can also enforce rules (called constraints) on what data are allowed in the database. Such constraints are important because they help maintain data integrity. SQL enforces constraints by prohibiting any data in the database that violate any constraint. Any insert, update, or delete that would result in a constraint violation is rejected without changing the database.