我的 sql 语句在这里遇到了一个奇怪的问题。也许这是愚蠢的事情,我无法抓住它。
SELECT appointment.patient_id,
patient.notes,
patient.first_name,
CONVERT(VARCHAR(27), appointment.start_time, 100),
patient.email_address
FROM appointment,
patient
WHERE Day(appointment.start_time) = 06
AND Month(appointment.start_time) = 06
AND Year(appointment.start_time) = 2013
AND appointment.appointment_type_id != ''
AND appointment.location_id != ''
AND appointment.patient_id = patient.patient_id
AND patient.email_address != '';
上面的查询返回 3 条记录。'notes' 字段的值在所有 3 条记录中均为 NULL。如果我添加一个条件patient.notes != '1 Reminder'
,它不应该返回相同的结果集吗?当我尝试它时,它返回零记录。有人可以解释为什么吗?