SQLPLUS（Oracle） - WHERE子查询，用于确定日期表达式是否大于n天(SQLPLUS (Oracle) - WHERE subquery to determine if date expression greater than n Days)

 我是SQL的新手，在掌握子查询的使用以及将它们放在外部查询的位置时遇到了一些麻烦。 下面的问题是我一直在研究的问题的一部分，似乎无法获得理想的结果。 
 
 我需要提取开始日期和结束日期之间的天数。 然后检查是否大于2并将其应用于外部查询。 此特定尝试返回“Missing Expression”，而其他迭代（2nd Query Bellow）返回一个错误，指出内部查询返回多行。（修改为使用“ALL”关键字也没有产生正确的结果） 
 
 QUERY1 
 
SELECT P.PETNAME,P.PETTYPE
FROM PETTREATMENT PT, EXAMINATION E, PET P, TREATMENT T
WHERE PT.EXAMNO = E.EXAMNO
AND E.PETNO = P.PETNO
AND PT.TREATNO = T.TREATNO 
AND (SELECT TO_DATE(PETTREATMENT.ENDDATE,'DD-MON-YYYY') - TO_DATE(PETTREATMENT.STARTDATE,'DD-MON-YYYY') AS TOTALDAYS FROM PETTREATMENT WHERE TOTALDAYS > 2)
AND T.COST >100
ORDER BY P.PETNAME;
 
 查询2 
 
SELECT P.PETNAME,P.PETTYPE
FROM PETTREATMENT PT, EXAMINATION E, PET P, TREATMENT T
WHERE PT.EXAMNO = E.EXAMNO
AND E.PETNO = P.PETNO
AND PT.TREATNO = T.TREATNO 
AND 2 < (SELECT TO_DATE(PETTREATMENT.ENDDATE,'DD-MON-YYYY') - TO_DATE(PETTREATMENT.STARTDATE,'DD-MON-YYYY') FROM PETTREATMENT)
AND T.COST >100
ORDER BY P.PETNAME;

I am new to SQL and have had some trouble grasping the use of subqueries and where to place them in respect to the outer query. The queries bellow are apart of a problem ive been working on and cant seem to get the desired results. 
 
I need to extract the number of days between a start and end date. Then check if that is greater than 2 and apply that to the outer query. This particular attempt returns "Missing Expression" while other iterations (2nd Query Bellow) have returned an error stating the inner query returns multiple rows.(Modifying to use the "ALL" keyword Did not produce the right results either)
 
QUERY1
 
SELECT P.PETNAME,P.PETTYPE
FROM PETTREATMENT PT, EXAMINATION E, PET P, TREATMENT T
WHERE PT.EXAMNO = E.EXAMNO
AND E.PETNO = P.PETNO
AND PT.TREATNO = T.TREATNO 
AND (SELECT TO_DATE(PETTREATMENT.ENDDATE,'DD-MON-YYYY') - TO_DATE(PETTREATMENT.STARTDATE,'DD-MON-YYYY') AS TOTALDAYS FROM PETTREATMENT WHERE TOTALDAYS > 2)
AND T.COST >100
ORDER BY P.PETNAME;
 
QUERY 2
 
SELECT P.PETNAME,P.PETTYPE
FROM PETTREATMENT PT, EXAMINATION E, PET P, TREATMENT T
WHERE PT.EXAMNO = E.EXAMNO
AND E.PETNO = P.PETNO
AND PT.TREATNO = T.TREATNO 
AND 2 < (SELECT TO_DATE(PETTREATMENT.ENDDATE,'DD-MON-YYYY') - TO_DATE(PETTREATMENT.STARTDATE,'DD-MON-YYYY') FROM PETTREATMENT)
AND T.COST >100
ORDER BY P.PETNAME;

原文：https://stackoverflow.com/questions/36603535