MySQL if 条件则左连接

Question

我在页面底部有一个有效的选择。如您所见，它有 53 行，在我看来，这太多了。有人告诉我，在 MySQL 中有一个可用的“if”条件，但我无法让它工作。工作 Select unions 4 选择，因为在每个选择中，我需要加入另一个表。是否可以根据内容拼接表格？

SELECT *
FROM (
    SELECT m.`identificator` , ml.id AS link_id, ml.parent, ml.type, 
            ml.destination, ml.disabled, ml.order, mld.`name` AS link_name, 
            mld.`alt` , mld.`title` , cpd.`slug` 
    FROM  `wf_menu` m
    LEFT JOIN  `wf_menu_link` ml ON m.`id` = ml.`menu_id` 
    LEFT JOIN  `wf_menu_link_desc` mld ON ml.`id` = mld.`link_id` 
    LEFT JOIN  `wf_cms_post_desc` cpd ON ml.destination = cpd.post_id
    WHERE mld.`lang_id` =1
    AND mld.`lang_id` = cpd.`lang_id` 
    AND (ml.`type` =  'page'
    OR ml.`type` =  'article')

    UNION

    SELECT m.`identificator` , ml.id AS link_id, ml.parent, ml.type, 
            ml.destination, ml.disabled, ml.order, mld.`name` AS link_name, 
            mld.`alt` , mld.`title` , cpd.`slug` 
    FROM  `wf_menu` m
    LEFT JOIN  `wf_menu_link` ml ON m.`id` = ml.`menu_id` 
    LEFT JOIN  `wf_menu_link_desc` mld ON ml.`id` = mld.`link_id` 
    LEFT JOIN  `wf_cms_category_desc` cpd ON ml.destination = cpd.category_id
    WHERE mld.`lang_id` =1
    AND mld.`lang_id` = cpd.`lang_id` 
    AND ml.`type` =  'cmscat'

    UNION

    SELECT m.`identificator` , ml.id AS link_id, ml.parent, ml.type, 
            ml.destination, ml.disabled, ml.order, mld.`name` AS link_name, 
            mld.`alt` , mld.`title` , spd.`slug` 
    FROM  `wf_menu` m
    LEFT JOIN  `wf_menu_link` ml ON m.`id` = ml.`menu_id` 
    LEFT JOIN  `wf_menu_link_desc` mld ON ml.`id` = mld.`link_id` 
    LEFT JOIN  `wf_shop_category_desc` spd ON ml.destination = spd.category_id
    WHERE mld.`lang_id` =1
    AND mld.`lang_id` = spd.`lang_id` 
    AND ml.`type` =  'shopcat'

    UNION

    SELECT m.`identificator` , ml.id AS link_id, ml.parent, ml.type, 
            ml.destination, ml.disabled, ml.order, mld.`name` AS link_name, 
            mld.`alt` , mld.`title` , 'link' as slug 
    FROM  `wf_menu` m
    LEFT JOIN  `wf_menu_link` ml ON m.`id` = ml.`menu_id` 
    LEFT JOIN  `wf_menu_link_desc` mld ON ml.`id` = mld.`link_id` 
    WHERE mld.`lang_id` =1
    AND ml.`type` =  'link'

) a
ORDER BY `order` DESC

Answer 1

您可以只执行一个查询，其中包含每个左联接，然后使用 IF 语句对生成的“slug”列进行排序。然而，这完全取决于您想要输出什么，以及其他表上是否有多个匹配行。

像这样:-

SELECT m.identificator , ml.id AS link_id, ml.parent, ml.type, 
        ml.destination, ml.disabled, ml.order, mld.name AS link_name, 
        mld.alt , mld.title , cpd.slug 
FROM  wf_menu m
LEFT JOIN  wf_menu_link ml ON m.id = ml.menu_id 
LEFT JOIN  wf_menu_link_desc mld ON ml.id = mld.link_id 
LEFT JOIN  wf_cms_post_desc cpd ON ml.destination = cpd.post_id AND (ml.type = 'page' OR ml.type = 'article')
LEFT JOIN  wf_cms_category_desc cpd ON ml.destination = cpd.category_id AND ml.type =  'cmscat' AND mld.lang_id = cpd.lang_id 
LEFT JOIN  wf_shop_category_desc spd ON ml.destination = spd.category_id AND ml.type =  'shopcat' AND mld.lang_id = spd.lang_id 
WHERE mld.lang_id =1
AND mld.lang_id = cpd.lang_id