protected function buildColRelVal( $where )
    {
        $rel = $where[ 'rel' ];
        $col = $where[ 'col' ];
        $val = $where[ 'val' ];
        if ( $rel == 'EQ' ) {
            // EQ: equal to another column (i.e. val is an identifier.)
            $val = $this->quote( $val );
            $rel = '=';
        } else {
            $val = $this->formWhereVal( $val );
        }
        // normal case. compose where like col = val
        $col = $this->formWhereCol( $col );

        $where = trim( "{$col} {$rel} {$val}" ) . ' ';
        return $where;
    }