LEFT JOIN 语法的 CASE 条件

Question

在回答了这个问题后，在一组左连接中获取一个 CASE 将不再烦人！

但是我有以下疑问，这是应用一个 case 语句的正确方法吗，该语句在 case 语句（最后一个括号）之前给出语法错误，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`)))

Answer 1

不，这不好。 Case/when 语句不能用在 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`)))

LEFT JOIN 语法的 CASE 条件

CASE condition on LEFT JOIN syntax

mysql

join

case

left-join