DB2、oracle和mysql语句的日期对比及字符串截取

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一、在数据库查询中，我们会经验遇到日期的对比，在常用数据库中，我们使用最多的是oracle,mysql和db2。
DB2：

SELECT timestamp('2014-06-11 08:03:20')-timestamp('2014-06-10 08:01:00'),
specialcheck,SPECIAL_FIELD, CURRENT_DATE, EXAMTYPE, 
patient_name,record_no,register_date,calldate,
 (select swift_number from REGISTER_INFO where 
patient_haveroom.record_no=REGISTER_INFO.record_no) as swift_number,substr( (select 
swift_number from REGISTER_INFO where 
patient_haveroom.record_no=REGISTER_INFO.record_no) ,10,1) as timeStr
 FROM patient_haveroom
 where record_no='10120140605016249'  or record_no='10120140605016247' or 
record_no='10120140605016246'

此处，通过两个日期相减得出的正负数，可以判断日期大小。
二、字符串截取，查询某字段中值截取后特定索引位置的值。
DB2和Oracle:

SELECT timestamp(current timestamp)-timestamp('2014-06-11 08:01:00'),
specialcheck,SPECIAL_FIELD,  EXAMTYPE, 
patient_name,record_no,register_date,calldate,
 (select swift_number from REGISTER_INFO where 

patient_haveroom.record_no=REGISTER_INFO.record_no) as swift_number,
 (select booking_date from REGISTER_INFO where 

patient_haveroom.record_no=REGISTER_INFO.record_no) as booking_date,
substr( (select swift_number from REGISTER_INFO where 

patient_haveroom.record_no=REGISTER_INFO.record_no) ,10,1) as timeStr
 FROM patient_haveroom
 where (patient_name='预约1' or patient_name='预约2' or patient_name='预约3' or 

patient_name='预约4' or patient_name='test2') 
and substr( (select swift_number from REGISTER_INFO where 

patient_haveroom.record_no=REGISTER_INFO.record_no) ,10,1)='0'

mysql：

substring(swift_number,10,1)='2'")

除了函数不一样。其它与mysql和oracle一样的。