IF(valueISNULL,'',#-- value is NULL, but valueStdUnit might not be NULL
CONCAT('(subdata.unit_sig=',unit_sig,' AND subdata.value ',o,' \'',value,'\') OR ')),
IF(unit_sig=stdUnit_sig,'',CONCAT('(subdata.unit_sig=',stdUnit_sig,' AND subdata.value',o,'\'',valueStdUnit,'\') OR ')),'(standard_unit(subdata.unit_sig)=',stdUnit_sig,' AND convert_unit(subdata.unit_sig,subdata.value)',o,valueStdUnit,')'));#-- with unit and non-null valueStdUnit
IF(unit_sig=stdUnit_sig,'',CONCAT('(subdata.unit_sig=',stdUnit_sig,' AND subdata.value',o,'\'',valueStdUnit,'\') OR ')),'(standard_unit(subdata.unit_sig)=',stdUnit_sig,' AND convert_unit(subdata.unit_sig,subdata.value)',o,' ',valueStdUnit,')'));#-- with unit and non-null valueStdUnit