Introduction to DB2 for z/OS
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Ways to access data

Introduction to DB2 for z/OS

You can retrieve data by using the SQL statement SELECT to specify a result table that can be derived from one or more tables.

In this information, examples of SQL statements illustrate how to code and use each clause of the SELECT statement to query a table. Examples of more advanced queries explain how to fine-tune your queries by using functions and expressions and how to query multiple tables with more complex statements that include unions, joins, and subqueries. The best way to learn SQL is to develop SQL statements like these examples and then execute them dynamically using a tool such as DB2® QMF™ for Workstation.

The data that is retrieved through SQL is always in the form of a table. Like the tables from which you retrieve the data, a result table has rows and columns. A program fetches this data one or more rows at a time.

Example: Consider this SELECT statement:
SELECT LASTNAME, FIRSTNME
  FROM EMP
  WHERE DEPT = 'D11'
  ORDER BY LASTNAME;

This SELECT statement returns the following result table:

LASTNAME     FIRSTNME
========     ========
BROWN        DAVID
LUTZ         JENNIFER
STERN        IRVING

Many of the examples in this information are based on the sample tables, which represent sample information about a computer company.

  • Ways to select data from columns
    Several different techniques are available for selecting columns from a database for your result tables.
  • How a SELECT statement works
    SQL statements, including SELECT, are made up a series of clauses that are defined by SQL as being executed in a logical order. SELECT statements allow users to definite and organize information that is retrieved from a specified table.
  • SQL functions and expressions
    You can use functions and expressions to control the appearance and values of rows and columns in your result tables. DB2 offers many built-in functions, including aggregate functions and scalar functions.
  • Ways to filter the number of returned rows
    A variety of different comparison operators in the predicate of a WHERE clause let you filter the number of returned rows.
  • Ways to order rows
    You can use the ORDER BY clause to retrieve rows in a specific order.
  • Ways to summarize group values
    You can use the GROUP BY clause to summarize group values.
  • Ways to merge lists of values
    There are several ways to use the UNION keyword for merging lists of values.
  • Ways to specify search conditions
    You can use the HAVING clause in a variety of ways to specify search conditions.
  • Ways to join data from more than one table
    When you want to see information from multiple tables, you can use a SELECT statement. SELECT statements can retrieve and join column values from two or more tables into a single row. The retrieval is based on a specified condition, typically of matching column values.
  • Subqueries
    You can use a subquery to narrow a search condition that is based on information in an interim table.
  • Ways to access DB2 data that is not in a table
    You can access DB2 data even if it is not in a table.
Parent topic: SQL: The language of DB2

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