Expressions

An expression is a combination of one or more values, operators, and built in functions that evaluate to a value.

An expression often assumes the datatype of its components. You can use expressions in many places including:
  • Column expressions in a SELECT clause
  • A condition of the WHERE clause or HAVING clause
Expressions can be simple or compound. A built-in function such as length() or pi() can also be considered an expression.

Simple Expressions

A simple CCL expression specifies a constant, NULL, or a column. A constant can be a number or a text string. The literal NULL denotes a null value. NULL is never part of another expression, but NULL by itself is an expression.

You can specify a column name by itself or with the name of its stream or window. To specify both the column and the stream or window, use the format "stream_name.column_name."

Some valid simple expressions include:
  • stocks.volume
  • 'this is a string'
  • 26

Compound Expressions

A compound CCL expression is a combination of simple or compound expressions. Compound expressions can include operators and functions, as well as the simple CCL expressions (constants, columns, or NULL).

You can use parentheses to change the order of precedence of the expression's components.

Some valid compound expressions include:
  • sqrt (9) + 1
  • ('example' + 'test' + 'string')
  • ( length ('example') *10 ) + pi()

Sequences of Expressions

An expression can contain a sequence of expressions; separated by semicolons and grouped using parentheses, to be evaluated in order. The type and value of the expression is the type and value of the last expression in the sequence. For example,
  • (var1 := v.Price; var2 := v.Quantity; 0.0)
sets the values of the variables var1 and var2, and then returns the value 0.0.

Conditional Expressions

A conditional CCL expression evaluates a set of conditions to determine its result. The outcome of a conditional expression is evaluated based on the conditions set. In CCL, the keyword CASE appears at the beginning of these expressions and follows a WHEN-THEN-ELSE construct.

The basic structure looks like this: 

CASE 
WHEN expression THEN expression
[...]
ELSE expression
END

The first WHEN expression is evaluated to be either zero or non-zero. Zero means the condition is false, and non-zero indicates that it is true. If the WHEN expression is true, the following THEN expression is carried out. Conditional expressions are evaluated based on the order specified. If the first expression is false, then the subsequent WHEN expression is tested. If none of the WHEN expressions are true, the ELSE expression is carried out.

A valid conditional expression in CCL is:
CASE
WHEN mark>100 THEN grade:=invalid
WHEN mark>49 THEN grade:=pass
ELSE grade:=fail
END
Parent topic: CCL Language Components