In a saved search formula, the possible values of mainline are designed to be useful in an HTML context. When mainline is true, the value of {mainline} is the 1-character string * (asterisk). When mainline is false, the value of {mainline} is the 6-character string (non-breaking space, HTML encoded as a character entity reference). These string values can be compared with string literals in an SQL context.
CASE
WHEN {mainline} = '*' THEN expression-when-true
WHEN {mainline} = ' ' THEN expression-when-false
END