关于LEFT JOIN语法的CASE条件

问题描述：

在回答这个问题后，在一组左连接中获取CASE将不再烦人！关于LEFT JOIN语法的CASE条件

但是我已经查询以下和平是这是一个正确的apparoache申请一个case语句，只是case语句之前给出一个语法错误（最后括号）说，缺少右括号

from((((((`tbl_workflow_step_item` 
LEFT JOIN `tbl_workflow_step_users` ON ((`tbl_workflow_step_item`.`step_id` = `tbl_workflow_step_users`.`step_id`))) 
LEFT JOIN `tbl_workflow_steps` ON ((`tbl_workflow_step_item`.`step_id` = `tbl_workflow_steps`.`step_id`))) 
LEFT JOIN `tbl_workflow_user_assigned` ON ((`tbl_workflow_step_users`.`user_id` = `tbl_workflow_user_assigned`.`user_id`))) 
LEFT JOIN `tbl_workflow` ON ((`tbl_workflow`.`workflow_id` = `tbl_workflow_steps`.`workflow_id`))) 
LEFT JOIN `tbl_report_type` ON ((`tbl_report_type`.`type_id` = `tbl_workflow_step_item`.`type`))) 
case 
when (`tbl_workflow`.`type` = 2) 
then 
left join `tbl_report_file` on ((`tbl_report_file`.`file_id`=`tbl_workflow_step_item`.`target_item_id`))) 
else 
LEFT JOIN `tbl_blog` ON ((`tbl_blog`.`blog_id` = `tbl_workflow_step_item`.`target_item_id`)))

你不能这样做 - 它没有意义。想想看：根据'case'运算符的结果，返回的行将有不同的模式。 – Phylogenesis

答

不，这不是好。 Case/when语句不能用于JOIN部分。这是你应该做的：

SELECT 
CASE 
    WHEN(`tbl_workflow`.`type` = 2) 
     THEN `tbl_report_file`.YOURFIELDHERE 
    ELSE 
    `tbl_blog`.YOURFIELDHERE 
END AS theAliasYouWant 
FROM((((((`tbl_workflow_step_item` 
LEFT JOIN `tbl_workflow_step_users` ON ((`tbl_workflow_step_item`.`step_id` = `tbl_workflow_step_users`.`step_id`))) 
LEFT JOIN `tbl_workflow_steps` ON ((`tbl_workflow_step_item`.`step_id` = `tbl_workflow_steps`.`step_id`))) 
LEFT JOIN `tbl_workflow_user_assigned` ON ((`tbl_workflow_step_users`.`user_id` = `tbl_workflow_user_assigned`.`user_id`))) 
LEFT JOIN `tbl_workflow` ON ((`tbl_workflow`.`workflow_id` = `tbl_workflow_steps`.`workflow_id`))) 
LEFT JOIN `tbl_report_type` ON ((`tbl_report_type`.`type_id` = `tbl_workflow_step_item`.`type`))) 
LEFT JOIN `tbl_report_file` on ((`tbl_report_file`.`file_id`=`tbl_workflow_step_item`.`target_item_id`))) 
LEFT JOIN `tbl_blog` ON ((`tbl_blog`.`blog_id` = `tbl_workflow_step_item`.`target_item_id`)))

关于LEFT JOIN语法的CASE条件

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