MySQL的IF语句中左连接

Question

我希望有人能帮助我。

我有以下查询

SELECT * FROM网格LEFT OUTER JOIN grid_images ON g_img_grid_id = G_ID & &（（g_img_main IS NOT NULL）||（g_img_main IS NULL & & g_img_width> = 475 ）），其中g_blog_id IS NOT NULL GROUP BY G_ID ORDER BY DESC G_ID

它不工作，我怎么想它。我想要发生的是这个。在grid_images的左连接中，我想用g_img_main选择一个图像，如果有一个图像不为null，如果它们为null，那么我希望它选择基于宽度大于474的图像。so g_img_width> = 475.

有没有办法做到这一点？

为了进一步说明我有两个表。一个是“网格”，另一个是“grid_images”。所以每个网格都可以有无限的图像。网格有一个id“G_ID”，并且每个图像连接到“G_ID” ......所以在“grid_images”“g_img_grid_id”告诉我，像属于哪个网格。一些电网将有一个主图像标示出所有的图像“g_img_main”的，有些网格不会。在左边的jin中，我想选择标记为主“g_img_main”的图像（如果有的话），如果不是，我只想选取任何大于特定宽度“g_img_width”的图像。

== Table structure for table grids 

|------ 
|Column|Type|Null|Default 
|------ 
|//**g_id**//|int(11)|No| 
|**g_permalink**|varchar(255)|No| 
|g_type|tinyint(3)|No|3 
|g_function|tinyint(3)|No|1 
|g_format|tinyint(3)|No|1 
|g_category|int(5)|No| 
|g_name|varchar(255)|No| 
|g_company_name|varchar(255)|Yes|NULL 
|g_owner_id|int(11)|No| 
|g_settings|text|No| 
|g_about|text|No| 
|g_website|varchar(2083)|No| 
|g_donate_link|varchar(2083)|Yes|NULL 
|g_donate_paypal|text|Yes|NULL 
|g_image|varchar(255)|Yes|NULL 
|g_view_permission|tinyint(1)|No|1 
|g_post_permission|tinyint(1)|No|1 
|g_allow_voting|tinyint(1)|Yes|NULL 
|g_show_share|tinyint(4)|Yes|NULL 
|g_site_grid|tinyint(1)|Yes|0 
|g_date_created|int(11)|No| 
|g_votes_per_user|int(11)|Yes|NULL 
|g_votes_require_email|tinyint(1)|Yes|NULL 
|g_facebook_eligible|tinyint(1)|Yes|NULL 
|g_facebook_active|tinyint(1)|Yes|NULL 
|g_facebook_app_tab_id|varchar(100)|Yes|NULL 
|g_facebook_page_id|varchar(100)|Yes|NULL 
|g_facebook_page_name|varchar(255)|Yes|NULL 
|g_facebook_page_category|varchar(100)|Yes|NULL 
|g_facebook_page_link|text|No| 
|g_facebook_page_about|text|No| 
|g_votes_rank_display|tinyint(1)|Yes|NULL 
|g_lock_center|tinyint(1)|Yes|NULL 
|g_passthe_item|varchar(255)|Yes|NULL 
|g_facebook_img|varchar(255)|Yes|NULL 
|g_pinterest_img|varchar(255)|Yes|NULL 
|g_montage_img|varchar(255)|Yes|NULL 
|g_views|int(11)|No|0 
|g_blog_id|int(11)|Yes|NULL 
== Dumping data for table grids 

|------ 
|g_id|g_permalink|g_type|g_function|g_format|g_category|g_name|g_company_name|g_owner_id|g_settings|g_about|g_website|g_donate_link|g_donate_paypal|g_image|g_view_permission|g_post_permission|g_allow_voting|g_show_share|g_site_grid|g_date_created|g_votes_per_user|g_votes_require_email|g_facebook_eligible|g_facebook_active|g_facebook_app_tab_id|g_facebook_page_id|g_facebook_page_name|g_facebook_page_category|g_facebook_page_link|g_facebook_page_about|g_votes_rank_display|g_lock_center|g_passthe_item|g_facebook_img|g_pinterest_img|g_montage_img|g_views|g_blog_id 
|------ 
|333|33-plus-ugliest-christmas-sweaters|1|1|4|0|33+ Ugliest Christmas Sweaters|NULL|11|null|<p><span style="line-height: 22.3999996185303px;">Just when you thought you've seen the ugliest Christmas sweaters around these people took ugly to the next level. Please feel free to share your comments, cuz some of these really need commenting...</span><br></p>| | |NULL| |1|2|NULL|1|1|1416865122|NULL|NULL|NULL|NULL|NULL|NULL|NULL|NULL| | |NULL|NULL|NULL|2014/11/26/14170476241649711529-4x3.jpg|2014/11/26/14170476241649711529pin-2x6.jpg|2014/11/26/14170476241649711529.jpg|0|20 


== Table structure for table grid_images 

|------ 
|Column|Type|Null|Default 
|------ 
|//**g_img_id**//|int(11)|No| 
|g_img_mem_id|int(11)|Yes|1 
|g_img_dir|varchar(50)|No| 
|g_img_file|varchar(100)|No| 
|g_img_caption|text|Yes|NULL 
|g_img_link|varchar(2083)|No| 
|g_img_grid_id|int(11)|No|1 
|g_img_date_added|int(11)|No| 
|g_img_uploader_ip|int(11)|No| 
|g_img_lat|decimal(18,12)|Yes|NULL 
|g_img_lng|decimal(18,12)|Yes|NULL 
|g_img_width|int(6)|Yes|NULL 
|g_img_height|int(6)|Yes|NULL 
|g_img_reported|tinyint(1)|Yes|NULL 
|g_img_reported_access_code|varchar(100)|Yes|NULL 
|g_img_last_edit|int(11)|Yes|NULL 
|g_img_flickr_id|varchar(255)|No| 
|g_img_facebook_id|varchar(255)|No| 
|g_img_original_id|int(11)|Yes|NULL 
|g_img_reposted_from|int(11)|Yes|NULL 
|g_img_first_name|varchar(100)|Yes|NULL 
|g_img_email|varchar(255)|Yes|NULL 
|g_img_hearts|int(11)|No|0 
|g_img_views|int(11)|No|0 
|g_img_main|tinyint(1)|Yes|NULL 
== Dumping data for table grid_images 

|------ 
|g_img_id|g_img_mem_id|g_img_dir|g_img_file|g_img_caption|g_img_link|g_img_grid_id|g_img_date_added|g_img_uploader_ip|g_img_lat|g_img_lng|g_img_width|g_img_height|g_img_reported|g_img_reported_access_code|g_img_last_edit|g_img_flickr_id|g_img_facebook_id|g_img_original_id|g_img_reposted_from|g_img_first_name|g_img_email|g_img_hearts|g_img_views|g_img_main 
|------ 
|15918|11|2014/11/22/|13793150131416692879.jpg| | |333|1416692879|849259821|NULL|NULL|453|768|NULL|NULL|NULL| | |NULL|NULL|NULL|NULL|0|0|NULL 
|15919|11|2014/11/22/|4655589171416692816.jpg| | |333|1416692816|849259821|NULL|NULL|613|768|NULL|NULL|NULL| | |NULL|NULL|NULL|NULL|0|0|NULL 
|15920|11|2014/11/22/|463211921416692661.jpg| | |333|1416692661|849259821|NULL|NULL|495|475|NULL|NULL|NULL| | |NULL|NULL|NULL|NULL|0|0|NULL 

Answer 1

你能把条件放在查询的WHERE部分吗？

SELECT 
* 
FROM grids 
LEFT OUTER JOIN grid_images 
ON g_img_grid_id = g_id 
WHERE g_blog_id IS NOT NULL 
AND (g_img_main IS NOT NULL OR (g_img_main IS NULL AND g_img_width >= 475)) 
GROUP BY g_id 
ORDER BY g_id DESC 

Answer 2

这里是下面的查询尝试：获取所有对此有一个主映像，然后工会与记录其中不存在主记录的ID。这应该会给你每个网格的记录...

-- Get the records with a master image. Assumes there is only one master per grid 
SELECT g.g_id, i.g_img_id -- or whatever columns you need 
FROM 
grids g 
INNER JOIN 
grid_images i on i.gm_img_grid_id = g.g_id 
WHERE i.g_img_main IS NOT NULL AND g.g_blog_id IS NOT NULL 

UNION 
-- now get the the other records. 

SELECT g.g_id,min(i.g_img_id) as g_img_id -- columns must match those in the first select 
FROM grids g 
INNER JOIN 
grid_images i on i.gm_img_grid_id = g.g_id 
WHERE i.g_img_main IS NULL 
AND g.g_blog_id IS NOT NULL 
AND i.g_img_width >=475 
GROUP BY g.g_id -- need to group by to get only one record per id