The JDBC Query assertion does not allow for dynamically defining the Order By value in a JDBC query. You can get around this by generating XML result, casting it to text/xml message type variable and using an XSLT stylesheet - ugly but functional. Pass the Order By values as parameters and set them as context variables before calling the Apply XSL Transformation assertion:
<xsl:stylesheet version="1.0" xmlns:L7j="http://ns.l7tech.com/2012/08/jdbc-query-result" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:param name="parameter.sortOrder" select="ascending"/>
<xsl:param name="parameter.sortBy" select="id"/>
<xsl:template match="/L7j:jdbcQueryResult">
<xsl:copy>
<xsl:choose>
<xsl:when test="$parameter.sortOrder = 'descending'">
<xsl:for-each select="L7j:row">
<xsl:sort order="descending" select="L7j:col[@name=$parameter.sortBy]"/>
<xsl:copy-of select="."/>
</xsl:for-each>
</xsl:when>
<xsl:otherwise>
<xsl:for-each select="L7j:row">
<xsl:sort order="ascending" select="L7j:col[@name=$parameter.sortBy]"/>
<xsl:copy-of select="."/>
</xsl:for-each>
</xsl:otherwise>
</xsl:choose>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>