mysql - 有没有其他方法可以选择此 case 语句中的值而不是子查询

Question

我对 MySQL 有一定程度的了解，但我不确定是否有更正确的方法来完成我在这里所做的事情。

#Select Ticket ID, Ticket Title, Assigned To
SELECT * FROM
    (SELECT
        ti.number, ti.ticket_id, foev.value AS "ticket_name",
        CASE WHEN ti.staff_id = 0 AND ti.team_id = 0
            then
                "** Unassigned **"
            ELSE
                CASE when ti.staff_id = 0
                    THEN
                        (SELECT te.name FROM ost_team te WHERE te.team_id = ti.team_id)
                    ELSE
                        (SELECT CONCAT(st.firstname,LEFT(st.lastname,1)) FROM ost_staff st WHERE st.staff_id = ti.staff_id)
                    END
        END AS "assigned_to"
    FROM ost_ticket ti
        LEFT JOIN ost_form_entry foe ON foe.object_id = ti.ticket_id AND foe.object_type = "T" 
        LEFT JOIN ost_form_entry_values foev ON foev.entry_id = foe.id 
        GROUP BY ti.ticket_id) 
    AS ticket_meta_list 
ORDER BY ticket_id ASC;

是否有更正确的方法来执行这些子查询？我想可能有一些我不太熟悉的连接。

Answer 1

如果您的员工表中没有值为 0 的 staff_id，并且您的团队表中没有值为 0 的 team_id，您可以使用 left join 与coalesce结合使用:

SELECT    ti.number,
          ti.ticket_id,
          foev.value AS ticket_name,
          COALESCE(CONCAT(st.firstname,LEFT(st.lastname,1)),
                   te.name, '** Unassigned **') AS assigned_to
FROM      ost_ticket ti
LEFT JOIN ost_form_entry foe 
       ON foe.object_id = ti.ticket_id 
      AND foe.object_type = 'T' 
LEFT JOIN ost_form_entry_values foev 
       ON foev.entry_id = foe.id 
LEFT JOIN ost_team te 
       ON te.team_id = ti.team_id
LEFT JOIN ost_staff st 
       ON st.staff_id = ti.staff_id
GROUP BY  ti.ticket_id 
ORDER BY  ticket_id