如果用户在该字段中提供了数据，则在SQL查询中放入IF语句 - Putting an IF statement in SQL query if user has supplied data in that field

Putting an IF statement in SQL query if user has supplied data in that field

我有一个SQL查询，其中imputs是由用户通过搜索表单生成的。

// Select some data
$sth = $dbh->prepare("SELECT `id`, `type_code`, `connector_type`, `construction_type`,  `author`, `estimate_lead_time`, `last_update`, `confidence_level`, `start_passband`, `stop_passband`, `low_passband`, `high_passband` 
                      FROM `filter_bandpass` 
                      WHERE (`start_passband` = $lowfreq AND `stop_passband` = $highfreq)");

我试图让它允许用户只填写$lowfreq，而不填写$highfreq，结果将显示用户插入数据的$lowfreq =的所有结果，但当他们输入$highfreq的数量时，结果会显示两者。

因此$highfreq包含数据或为空。然后简单地更换

WHERE (`start_passband` = $lowfreq AND `stop_passband` = $highfreq)");

带有

WHERE (`start_passband` = $lowfreq AND (`stop_passband` = $highfreq OR $highfreq = ''))");