mysql - mysql 中 SELECT 中的 IF 语句

Question

如果workspace.signed_to = NULL，如何选择列，否则不选择任何内容。

SELECT workspace.*, projects.*,
  CASE 
    WHEN workspace.assigned_to != NULL 
    THEN users.user_id,users.first_name, users.last_name 
   END FROM workspace
INNER JOIN projects ON workspace.workspace_id = projects.project_id 
INNER JOIN users ON workspace.assigned_to = users.user_id
WHERE project_id=:project_id

Answer 1

SELECT workspace.*, projects.*,
  users.user_id,users.first_name, users.last_name
FROM workspace
INNER JOIN projects ON workspace.workspace_id = projects.project_id 
LEFT JOIN users ON workspace.assigned_to = users.user_id
WHERE project_id=:project_id

LEFT JOIN 意味着我们通过分配的用户加入用户，但如果它为空，则用户的 valuea 为空