Question

例如我正在同一个表上进行内连接。我想从表1或表2中选择那些值为空的字段。

以下是查询示例：

SELECT a2.*,a.town
FROM `ads` AS `a`
LEFT JOIN discounts d ON d.ad_id=a.id
INNER JOIN ads a2 ON a2.id=d.discount_ad_id
LEFT JOIN `departments` AS `dp` ON dp.department_id = a.department
LEFT JOIN `map_towns` AS `mt` ON mt.town_id = a.town
WHERE (a.department = 9 OR a.department = 15)
GROUP BY `d`.`discount_id`

a2也有城镇地区，但我可以拥有空值。我需要选择a2.town，如果它不是null，如果它是a.town。这与所有其他字段有关。：）

有可能吗？

谢谢;）

Answer 1

使用ANSI函数COALESCE或MySQL函数IFNULL。根据需要对列进行别名

SELECT
   a2.id,
   a2.col2,
   coalesce(a2.town, a.town) town,
   IFNULL(a2.city, a.city) city,
 ...etc

Answer 2

SELECT  a2.*, COALESCE(a2.town, a.town)
FROM    `ads` AS `a`
JOIN    discounts d
ON      d.ad_id = a.id
JOIN    ads a2
ON      a2.id = d.discount_ad_id
LEFT JOIN
        `departments` AS `dp`
ON      dp.department_id = a.department
LEFT JOIN
        `map_towns` AS `mt`
ON      mt.town_id = a.town
WHERE   a.department IN (9, 15)
GROUP BY
        `d`.`discount_id`

请注意，LEFT JOIN上的discounts在您的查询中是多余的：NULL记录永远不会被返回，因为它们永远不会被以下INNER JOIN到{{ 1}}。

Answer 3

查看COALESCE函数，返回第一个非null的参数。

SELECT COALESCE(a2.town, a.town)

http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#function_coalesce

Mysql查询：INNER JOIN和SELECT *，它不是来自任何表的null

3 个答案: