This chapter describes SilverStream expressions and provides an overview of the operators and built-in functions available in the SilverStream environment. It contains the following sections.
For related information see the following:
For information about using the Expression Builder tool, see the Expression Builder in the online Tools guide.
For reference material about writing expressions, see Expressions, Operators, and Built-in Functions in the online General Reference.
JDBC allows you to pass query strings to an underlying database. The target database must be JDBC compliant. This means that the database must at least have the functionality of ANSI SQL92 Entry Level, which includes all the databases supported by SilverStream.
SilverStream has its own expression engine (parser and evaluator) that is used on top of the JDBC driver (a Java SQL API). This engine is used to help translate SilverStream expression statements back into database statements (mainly SQL). These expressions either do a computation on a column, a validation, or act as a WHERE and ORDER BY clause.
Some parts of the database expression are handled by the SilverStream parser, while other parts are passed through to the database for processing. The Server evaluates any non-database supported expressions in WHERE clauses as each row is retrieved. The processing for non-database supported ORDER BY clauses, on the other hand, is deferred until all the rows have been retrieved.
SilverStream expressions are a powerful way to enhance the functionality of your applications. This section provides an overview of expressions and the different types of expressions you can use in the SilverStream environment.
In its simplest form, an expression is a programming statement that performs an operation on one or more variables, and returns a value. The following Java expression uses the multiplier operator on three declared int variables and assigns the return value to a fourth variable:
int product = a * b * c;
A SilverStream expression is a series of variables, operators, and function calls that can return a single value, a set of values, or, in the case of query expressions, a data set. SilverStream supports many (but not all) of the literals, operators, variables, and functions defined in the Java programming language. The following is a summary of the building blocks of a SilverStream expression.
Variables
Java data type values defined in your application, such as integers, strings, and Boolean values. Examples of variables are table column values, user names, group names, time stamps, and so on.
Operators
Operations or calculations that can be performed on one or more operands.
Built-in functions
A variety of SilverStream functions that return values for specific purposes.
The following example shows what an expression in SilverStream looks like:
employees.employeeID > 100 AND employees.last_name startsWith "W";
This expression represents a WHERE clause in a database query. It uses the greater than > operator with the AND operator and a string function called startsWith to return all rows where the employeeID column is greater than 100, and the employee last name starts with the character W.
SilverStream provides a tool called the Expression Builder, which allows you to create various types of expressions. The following table summarizes the different applications for using expressions.
The Silverstream Expression Builder is a dialog that is tailored to the object currently selected in the Designer or the object's Property Inspector. It lets you build an expression by choosing from lists of variables, operators, and functions available for the selected object.
The Expression Builder supports the following:
The Expression Builder also promotes values for numeric data types. Specifically, the Expression Builder:
The industry standard database access language is called Structured Query Language (SQL). The SilverStream Expression Builder uses SQL as the basis for its own query language. Queries are made up of a query expression and associated clauses. You use query expressions to make selections, which retrieve a subset of the rows in one or more tables, and for projections, which retrieve a subset of the columns in one or more tables. Conventionally, queries take this form:
SELECT list_of_columns
FROM table
WHERE search_criteria
ORDER BY list_of_columns
You can construct WHERE and ORDER BY clauses using the point and click interface of the Expression Builder. The Expression Builder displays a list of table columns and variables that you can choose from to construct your query expression.
You can also construct queries programmatically using the AgiRowSetManager.query() method, which as two variants. See AgiRowSetManager in the API section of the help system.
The WHERE clause specifies the search criteria for the data you want to display. If you omit the WHERE clause, all table rows qualify for selection (this is the default when you create a page, form, or view). The WHERE clause can include column names, constants, logical operators, and relative operators.
Typically, a WHERE clause consists of one or more search conditions (predicates) connected by the logical operators AND, OR, and NOT. For example:
employee.status="Active" and employee.salary < 50000
When SilverStream receives a WHERE or ORDER BY clause, it analyzes the parts of the clause that are linked by an and. When it reaches a part of the clause that contains a function not supported by the underlying database, it sets aside that part of the clause for post-processing. It then takes the parts of the clause that are translatable and applies them to the database. The post-processing parts of the expression are then applied to the result.
NOTE To optimize performance, do not use functions not supported by your native database in WHERE or ORDER BY clauses.
The ORDER BY clause allows you to sort the results of the WHERE clause by any key. You can sort in either ascending (A to Z or 1 to 10) or descending (Z to A or 10 to 1) order. For example:
Operators perform functions on one or more operands. An operand can be a literal value, a variable, or a value returned by an expression. This section summarizes the operator types supported by SilverStream.
For details and examples, see the section in the help system on Operators in Expressions, Operators, and Built-in function in online General Reference.
SilverStream has its own set of built-in functions. Most of the functions are the same as or similar to those used in Java and SQL. This section summarizes the types of functions supported by SilverStream
For details and examples, see Built-in functions in Expressions, Operators, and Built-in functions in the online General Reference.