Structured Query Language

Most computer software has not yet reached a point where you can literally ask a database, What orders have been placed by customers in New Jersey with ship dates in the third quarter? You must still phrase questions in a restricted syntax that the software can easily parse. You can pose the same question to the demonstration database in the following terms:

SELECT * FROM customer, orders
   WHERE customer.customer_num = orders.customer_num
      AND customer.state = 'NJ'
      AND orders.ship_date
      BETWEEN DATE('7/1/98') AND DATE('9/30/98');

This question is a sample of Structured Query Language (SQL). It is the language that you use to direct all operations on the database. SQL is composed of statements, each of which begins with one or two keywords that specify a function. The HCL® OneDB® implementation of SQL includes a large number of SQL statements, from ALLOCATE DESCRIPTOR to WHENEVER.

You will use most of the statements only when you set up or tune your database. You will use three or four statements regularly to query or update your database. For details on SQL statements, see the HCL OneDB Guide to SQL: Syntax.

One statement, SELECT, is in almost constant use. SELECT is the only statement that you can use to retrieve data from the database. It is also the most complicated statement, and the next two chapters of this book explore its many uses.