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我想通过比较来获取老师的信息，u.fld_id = ct.fld_teacher_id 如果 ct.fld_teacher_id 为空，我将显示尚未分配。但是 ct.fld_teacher_id 为 null 的行在执行时不会出现。

这是我的查询。

  SELECT ct.fld_id as id, ct.fld_serial, ct.fld_city, ct.fld_venue_name, 
        ct.fld_start_time, ct.fld_end_time, 
        ct.fld_duration, ct.fld_available_seats, 
        ct.fld_course_price, ct.fld_status,
        c.fld_name, u.fld_first_name, u.fld_last_name,
        ct.fld_status, ct.fld_teacher_id, ct.fld_training_name,
        ct.fld_class, ct.fld_level, ct.fld_is_fixed, tt.fld_name as training_type_name
   FROM tbl_course_training as ct, tbl_users as u,
        tbl_courses as c, tbl_training_types as tt
   WHERE c.fld_id = ct.fld_course_id AND u.fld_id = ct.fld_teacher_id AND 
       ct.fld_teacher_id = 'NULL' AND tt.fld_id = ct.fld_training_type_id
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1 回答 1

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在不知道结构的情况下，我无法整理您的查询。但是，我注意到的事情很少。

  • 您不能将 null 值比较为ct.fld_teacher_id = 'NULL'（null 不是字符串）

    ct.fld_teacher_id is null -- Correct way

  • 如果您需要选择即使在ct.fld_teacher_id is null那时

    u.fld_id = isnull(ct.fld_teacher_id, u.fld_id) --or (sql-server) 
u.fld_id = ct.fld_teacher_id or ct.fld_teacher_id is null --(my sql or sql-server)
u.fld_id = COALESCE(ct.fld_teacher_id, u.fld_id)  --(my sql)

  • 还要避免连接表的旧方法，例如From Table1 t1, Table2 t2

    From Table1 t1 JOIN Table2 t2 On t1.Key = t2.Key --Correct way

  • 我想你需要在Left Join这里有几个。请查看此链接以了解 Sql Joins 的工作原理

于 2012-12-19T12:39:39.077 回答