You enter conditional expressions using a syntax similar to Structured Query Language (SQL) with some additional variables.
When you create a conditional expression, use a colon (:) to define a variable. The colon is used to avoid ambiguity when you create expressions that relate the current record to a different, specific record.
You can have subselect in an expression,
exists (select 1 from workorder where wonum=:wonum).
The first wonum is the wonum attribute on the work order object. At run time, the second wonum is replaced with the value of the wonum attribute for the current record.