LEFT OUTER JOIN...with differing matching keys - mysql

Question

So...this is a little confusing. I have 2 tables, one is basically a list of Codes and Names of people and topics and then a value, for example:
The second table is just a list of topics, with a value and a "result" which is just a numerical value too:
Now, what I want to do is do a LEFT OUTER JOIN on the first table, matching on topic and value, to get the "Result" field from the second table. This is simple in the majority of cases because they will almost always be an exact match, however there will be some cases there won't be, and in those cases the problem will be that the "Value" in table 1 is lower than all the Values in table 2. In this case, I would like to simply do the JOIN as though the Value in table 1 equalled the lowest value for that topic in table 2.
To highlight - the LEFT OUTER JOIN will return nothing for Row 2 if I match on topic and value, because there's no Geography row in table 2 with the Value 30. In that case, I'd like it to just pick the row where the value is 35, and return the Result field from there in the JOIN instead.
Does that make sense? And, is it possible?
Much appreciated.

Answer 1

You can use Cross Apply here. There may be a better solution performance wise.
declare #people table(
Code int,
Name varchar(30),
Topic varchar(30),
Value int
)
declare #topics table(
[Subject] varchar(30),
Value int,
Result int
)
INSERT INTO #people values (1, 'Doe,John', 'History', 25),
(2, 'Doe,John', 'Geography', 30),
(3, 'Doe,John', 'Mathematics', 45),
(4, 'Doe,John', 'Brad Pitt Studies', 100)
INSERT INTO #topics values ('History', 25, 95),
('History', 30, 84),
('History', 35, 75),
('Geography', 35, 51),
('Geography', 40, 84),
('Geography', 45, 65),
('Mathematics', 45, 32),
('Mathematics', 50, 38),
('Mathematics', 55, 15),
('Brad Pitt Studies', 100, 92),
('Brad Pitt Studies', 90, 90)
SELECT p.Code, p.Name,
case when p.Value < mTopic.minValue THEN mTopic.minValue
else p.Value
END, mTopic.minValue
FROM #people p
CROSS APPLY
(
SELECT [Subject],
MIN(value) as minValue
FROM #topics t
WHERE p.Topic = t.Subject
GROUP BY [Subject]
) mTopic
I am also assuming that:
This is simple in the majority of cases because they will almost always be an exact match, however there will be some cases there won't be, and in those cases the problem will be that the "Value" in table 1 is lower than all the Values in table 2.
is correct. If there is a time when Value is not equal to any topic values AND is not less than the minimum, it will currently return the people.value even though it is not a 'valid' value (assuming topics is a list of valid values, but I can't tell from your description.)
Also technically you only need that case statement in the select statement, not the following mTopic.minValue but I thought the example showed the effect better with it.

Answer 2

Another method of performing this is by using a temporary table to hold the different values.
First insert the exact matches, then insert the non-exact matches that where not found in the initial select and finally grab all the results from the temp table. This solution is more code than the other, so just adding it as an alternative.
Example (SqlFiddle):
Schema first
create table students
( code integer,
name varchar(50),
topic varchar(50),
value integer );
create table subjects
( subject varchar(50),
value varchar(50),
result integer );
insert students
( code, name, topic, value )
values
( 1, 'Doe, John', 'History', 25),
( 2, 'Doe, John', 'Geography', 30),
( 3, 'Doe, Jane', 'Mathematics', 45),
( 4, 'Doe, Jane', 'Brad Pitt Studies', 100);
insert subjects
( subject, value, result )
values
( 'History', 25, 95 ),
( 'History', 30, 84 ),
( 'History', 35, 75 ),
( 'Geography', 35, 51 ),
( 'Geography', 40, 84 ),
( 'Geography', 45, 65 ),
( 'Mathematics', 45, 32 ),
( 'Mathematics', 50, 38 ),
( 'Mathematics', 55, 15 ),
( 'Brad Pitt Studies', 100, 92 ),
( 'Brad Pitt Studies', 90, 90 );
The actual SQL query:
-- Temp table to hold our results
create temporary table tempresult
( code integer,
name varchar(50),
topic varchar(50),
studentvalue integer,
subjectvalue integer,
result integer );
-- Get the exact results
insert tempresult
( code,
name,
topic,
studentvalue,
subjectvalue,
result )
select stu.code,
stu.name,
stu.topic,
stu.value as 'student_value',
sub.value as 'subject_value',
sub.result
from students stu
join
subjects sub on sub.subject = stu.topic
and sub.value = stu.value;
-- Get the non-exact results, excluding the 'students' that we already
-- got in the first insert
insert tempresult
( code,
name,
topic,
studentvalue,
subjectvalue,
result )
select stu.code,
stu.name,
stu.topic,
stu.value as 'student_value',
sub.value as 'subject_value',
sub.result
from students stu
join
subjects sub on sub.subject = stu.topic
-- Business logic here: Take lowest subject value that is just above the student's value
and sub.value = (select min(sub2.value)
from subjects sub2
where sub2.subject = stu.topic
and sub2.value > stu.value)
where not exists (select 1
from tempresult tmp
where tmp.code = stu.code
and tmp.name = stu.name
and tmp.topic = stu.topic)
-- Get our resultset
select code,
name,
topic,
studentvalue,
subjectvalue,
result
from tempresult
order by code,
name,
topic,
studentvalue,
subjectvalue,
result

Answer 3

In this case I would make two joins instead of one. Something like this:
select *
from Table1 T1
LEFT JOIN Table2 T2 on T1.Topic=T2.subject and T1.Value=T2.VALUE
LEFT JOIN Table2 as T3 on T1.Topic=T3.Subject and T1.Value<T2.Value
The do a case to choose the table to take values from. If T2.value is null then use T3.Value ELSE T2.Value. Hope this helps you

Answer 4

A left join is not called for in the requirements. You want to join when T1.Subject = T2.Topic and then either when T1.Value = T2.Value or when T1.Value < T2.Value and T2.Value is the smallest value. Just write it out that way:
select p.*, t.Result
from #People p
join #Topics t
on t.Subject = p.Topic
and( t.Value = p.Value
or( p.Value < t.value
and t.Value =(
select Min( Value )
from #Topics
where Subject = t.Subject )));
Which generates:
Code Name Topic Value Result
---- -------- ----------------- ----- ------
1 Doe,John History 25 95
2 Doe,John Geography 30 51
3 Doe,John Mathematics 45 32
4 Doe,John Brad Pitt Studies 100 92