A join is an operation that combines the rows in tables by comparing the values in specified columns. This section is an overview of SQL Anywhere join syntax.
A relational database stores information about different types of objects in different tables. For example, information particular to employees appears in one table, and information that pertains to departments in another. The Employees table contains information such as employee names and addresses. The Departments table contains information about one department, such as the name of the department and who the department head is.
Most questions can only be answered using a combination of information from different tables. For example, to answer the question "Who manages the Sales department?", you use the Departments table to identify the correct employee, and then look up the employee name in the Employees table.
Joins are a means of answering such questions by forming a new virtual table that includes information from multiple tables. For example, you could create a list of the department heads by combining the information contained in the Employees table and the Departments table. You specify which tables contain the information you need using the FROM clause.
To make the join useful, you must combine the correct columns of each table. To list department heads, each row of the combined table should contain the name of a department and the name of the employee who manages it. You control how columns are matched in the composite table by either specifying a particular type of join operation or using the ON clause.
|Envoyer votre avis sur cette page par email.||Copyright © 2009, iAnywhere Solutions, Inc. - SQL Anywhere 11.0.1|