回答这个问题后，在一组左连接中获取一个 CASE 将不再令人烦恼!
但是我有以下疑问:这是应用 case 语句的正确方法吗？该方法在 case 语句(最后一个括号)之前给出语法错误，表示缺少右括号
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`)))
最佳答案
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`)))
关于mysql - LEFT JOIN 语法上的 CASE 条件，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/39851918/