Mysql的CASE WHEN JOIN语句错误

Question

MySQL查询：

SELECT * 
FROM `pet_info` LEFT JOIN 
    `lostpets` 
    ON `pet_info`.`id` = `lostpets`.`petid` LEFT JOIN 
    `pet_images` 
    ON `pet_info`.`id` = `pet_images`.`petid` 
    CASE WHEN `pet_info`.`pet_cat` = 2 
      THEN LEFT JOIN `cat_breeds` 
       ON `cat_breeds`.`id` = `pet_info`.`pet_breed` 
     WHEN `pet_info`.`pet_cat` = 1 
     THEN LEFT JOIN `dog_breeds` 
       ON `dog_breeds`.`id` = `pet_info`.`pet_breed` 
WHERE `pet_info`.`pet_user_id` = 581 

错误：

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'CASE WHEN `pet_info`.`pet_cat` = 2 THEN LEFT JOIN `cat_breeds` ON `cat_breeds`.`' at line 1 

我怎么能解决错误？错误在哪里？请帮帮我。

Answer 1

我想你打算：

SELECT * 
FROM `pet_info` LEFT JOIN 
    `lostpets` 
    ON `pet_info`.`id` = `lostpets`.`petid` LEFT JOIN 
    `pet_images` 
    ON `pet_info`.`id` = `pet_images`.`petid` LEFT JOIN 
    `cat_breeds` 
    ON `cat_breeds`.`id` = `pet_info`.`pet_breed` AND 
     `pet_info`.`pet_cat` = 2 LEFT JOIN 
    `dog_breeds` 
    ON `dog_breeds`.`id` = `pet_info`.`pet_breed` AND 
     `pet_info`.`pet_cat` = 1 
WHERE `pet_info`.`pet_user_id` = 581; 

注：

• 有了这样的查询，你不应该使用SELECT *，你应该明确地选择你想要的列。不同的表格具有相同名称的列。
• 您应该使用列别名。我没有把这些放入查询中，但他们是个好主意。
• 在真正的查询中，您将在SELECT中使用表达式来组合cat_breeds和dog_breeds的列，例如COALESCE(cat_breeds.col1, dog_breeds.col1) as col1。