不是唯一的表/别名

尝试

    SELECt pa.ProjectID, p.Project_Title, a.Account_ID, a.Username, a.Access_Type, c.First_Name, c.Last_Name      FROM Project_Assigned paINNER JOIN Account a        ON pa.AccountID = a.Account_IDINNER JOIN Project p        ON pa.ProjectID = p.Project_IDINNER JOIN Clients c        ON a.Account_ID = c.Account_ID     WHERe a.Access_Type = 'Client';