空值和非空值

Question

我想通過比較u.fld_id = ct.fld_teacher_id來獲取老師的信息，如果ct.fld_teacher_id為null，我將顯示尚未分配。 但是ct.fld_teacher_id為null的行在執行時不會顯示。

這是我的查詢。

  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 

Answer 1

我不知道結構便無法整理您的查詢。 但是，我很少注意到。

• 您不能將空值作為ct.fld_teacher_id = 'NULL'進行比較（空值不是字符串 ）

   ct.fld_teacher_id is null -- Correct way 

• 如果即使ct.fld_teacher_id is null時也需要選擇

   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) 

• 還要避免使用舊的聯接表的方法，例如From Table1 t1, Table2 t2

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

• 我認為您需要在此處添加一些Left Join 。 請檢查此鏈接以了解Sql Joins的工作方式