A simple CCL expression specifies a column, constant, or NULL.
A column name can be specified by itself or with the name of the data stream of which it is a part. To specify both the column and data stream, use this format: data_stream_name.column_name.
A constant can be a number or a text string.
The literal NULL denotes a null value. However, it can only appear in SELECT and in the results of IF/THEN/ELSE and CASE/THEN. It is never in an expression, but is just the literal NULL as the expression.
Some valid simple expressions are:
stocks.volume 'this is a text string' 26