字段为null的查询方式问题

		SELECT
		id.id,
		id.has_yue_coin AS balance,
		id.card_no AS cardNo,
		cc.card_name AS cardName,
		cc.card_price AS cardPrice,
		cv.discount
		FROM
		issue_detail id
		LEFT JOIN card_category cc ON id.card_id = cc.id
		LEFT
		JOIN card_verification cv ON cc.id = cv.card_id
		WHERE
		id.member_id =
		#{memberId}
		<if test="merchantDistrictId != null">
			AND (cc.merchant_district_id = #{merchantDistrictId} or
			cc.merchant_district_id is null)
		</if>
		AND cv.merchant_id = #{merchantId}
		AND id.`is_expire` = 0
		AND
		id.issue_status = 1
		AND cc.`status` = 0
		AND cc.shelf_status = 0
		ORDER BY
		discount

注意问题如下：

1.字段为null的条件匹配：要用 字段 is null。

2.小括号的优先级问题