LEFT JOIN語法的CASE條件

Question

在回答這個問題之后，在一組左聯接中使用CASE將不再令人討厭！

但是我有以下查詢疑問，這是否是應用case語句的正確方法，該case語句在case語句（最后一個括號）說缺少右括號之前給出了語法錯誤

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`)))

Answer 1

不，這不是很好。 Case / when語句不能在JOIN部分上使用。 這是您應該做的：

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`)))