1

I am just curious on how to make sure that the query returns null values too along with other values.

UPDATE: The following query is supposed to output a table with a location id, visit date visit time and bus name for each visits. The query below gives the output but does not include the nulls like in the table below. The result should include nulls also.

There are 3 tables in total: Location1, bus & visit. So I am using this(below) query to extract IDs and join all the tables. The query looks for visits only to Florida.

For instance, take the following query:

SELECT LOC.LOC_ID, VISIT.VISIT_DATE, VISIT.VISIT_TIME, BUS.BUS_NAME 
FROM BUS, LOCATION1, VISIT
WHERE VISIT.BUS_ID = BUS.BUS_ID 
AND VISIT.LOC_ID = LOCATION1.LOC_ID 
AND BUS.BUS_NAME IN (
SELECT BUS_NAME 
FROM BUS 
WHERE BUS_ID IN (
SELECT BUS_ID 
FROM VISIT 
WHERE LOC_ID IN (
SELECT LOC_ID 
FROM LOCATION1 
WHERE LOC_NAME='Florida')));

Let us suppose, there are visits which have not yet been assigned a bus and thus some values are null.

So, from what I believe using IN operator will return set of values but not the null values. What if i use exists instead?

The table can look like this:

 Loc_name   Loc_id         visit_date          visit_time              bus_name
  Florida     1            26-mar-2009           2:00pm                  xyz
  Florida     3            29-jul-2010           3:00pm                  abc
  Florida     8            22-may-2013           2:50pm                  (null)
  .
  .
  .

hope someone can help me with this.

thanks

UPDATE 2:

Found the Answer!

SELECT LOC.LOC_ID, VISIT.VISIT_DATE, VISIT.VISIT_TIME, BUS.BUS_NAME 
FROM LOCATION1
LEFT JOIN VISIT ON LLOCATION1.LOC_ID = VISIT.LOC_ID 
LEFT JOIN BUS ON BUS.BUS_ID = VISIT.BUS_ID
WHERE LOC_NAME = 'Florida';

Thanks again!

4

2 回答 2

1

如果我正确理解了您的问题/查询，我认为可以大大简化（不确定您为什么要使用所有IN语句）。尝试使用 anOUTER JOIN来获得您想要的结果：

SELECT LOC.LOC_ID, VISIT.VISIT_DATE, VISIT.VISIT_TIME, BUS.BUS_NAME 
FROM LOCATION1 LOC
    LEFT JOIN VISIT ON LOC.LOC_ID = VISIT.LOC_ID
    LEFT JOIN BUS ON VISIT.BUS_ID = BUS.BUS_ID 
WHERE LOC.LOC_NAME='Florida'
于 2013-05-22T20:01:07.703 回答
0 
SELECT LOC.LOC_ID, VISIT.VISIT_DATE, VISIT.VISIT_TIME, BUS.BUS_NAME 
FROM BUS, LOCATION1, VISIT
WHERE VISIT.BUS_ID = BUS.BUS_ID(+) 
AND VISIT.LOC_ID = LOCATION1.LOC_ID(+)
AND (BUS.BUS_NAME IN (
SELECT BUS_NAME 
FROM BUS 
WHERE BUS_ID IN (
SELECT BUS_ID 
FROM VISIT 
WHERE LOC_ID IN (
SELECT LOC_ID 
FROM LOCATION1 
WHERE LOC_NAME='Florida'))) OR Bus.Bus_name is null);

您需要进行外部联接而不是完全联接，这就是 (+) 符号的用途。此外，您需要处理 bus.Bus_name 可能为空的位置。

虽然我不确定 (+) 在这种情况下的去向，但我需要了解 visit.bus 和 bus.bus 何时相关以及 visit.loc 和 location 相关的关系。

虽然我认为这更容易阅读

SELECT LOC.LOC_ID, VISIT.VISIT_DATE, VISIT.VISIT_TIME, BUS.BUS_NAME 
FROM Location1 Loc
LEFT JOIN VISIT on Loc1.Loc_ID = Visit.Loc_ID and Loc_Name = 'Florida'
LEFT JOIN BUS on Bus.Bus_ID = Visit.Bus_ID
于 2013-05-22T19:57:49.520 回答