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"lookup tables" in SQL


I need some advice on adding a certain table into an SQL/Windows client. The table looks something like this:

                          1        2       3       4      5       6
1<x<=2              4.0     2.0    5.0    3.4   6.7    4.7
2<x<=5              3.0     2.3    4.5    3.4   5.1    6.0

The table works in this way. Suppose the variable is y on the top of the table i.e. y = 1,2,3,4,5 or 6. If I have two values y = 3 and x = 1.3, then the table would give me the answer 5.0

My questio comes in two parts, firstly, what is the best way for me to add this type of table to an SQL database. Secondly, what would be the best way to interogate the database to get the answers for a given value of x and y?

Thanks in advance

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3 Solutions
(a) I would recommend having two columns for the x attribute, min_val and max_val respectively. Ensure the values here are unique in that if the max_val is 2, the min_val for the next record is 2.01 or something slightly higher than 2

(b) You could simply pass the x attribute as a parameter and use a double where clause looking for min_val >= parameter and max_val <= the passed x attribute parameter ..

Does this make sense?
You can create a table with columns for Y, minX, maxX and value.
The data would look something like

Y    minX    maxX    Value
1    1       2       4
2    1       2       4
3    1       2       4
1    2       2       4
2    2       2       4
3    2       2       4

You could query this table with a condition similar to:
WHERE Y=myYValue AND myXValue BETWEEN minX AND maxX

You just have to be careful with the minX and maxX values, so they don't overlap and return only one value for every X number.

If either of the axis can be fairly long, I'd suggest the following 3 tables:

XAxisID int identity
XAxisMin numeric
XAxisMax numeric

YAxisID int identity
YAxisMin numeric
YAxisMAx numeric
(or, if they will be distinct integer values, get rid of the min/max and just use YAxisValue int)

AxisIntersectID int identity
XAxisID int
YAxisID int
IntersectValue numeric

Then, if they pass in an X and Y, you can do this:

select IntersectValue from
AxisIntersect ai
inner join XAxis x on ai.XAxisID = x.XAxisID
inner join YXaxis y on ai.YAxisID = y.YAxisID
where @x between x.XAxisMin and x.XAxisMax
and @y = y.YAxisVal
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IssacJonesAuthor Commented:
Hi reb73

Yes, that makes sense to me. I'd have three columns in the table relating to x and y i.e. x_min, x_max and a column for the y values.

Could you give some example SQL code which I could use to interrogate the database?

The 1st 2 solutions provide an easy method if there are a small amount of columns. However, if the # of X-axis values or Y-axis values can grow/vary greatly, I think my solution is a lot more robust.
IssacJonesAuthor Commented:
Hi derrekromm

Your suggestion looks promising and probably more generic. However, I'm only learning SQL so I don't follow everything. In particular, I'm not entirely sure how the intersect of the two tables works. I'm also not too familiar with the "inner join" part of the code.

It looks very interesting though! I'll have to get some reading done I think!

the inner join does what it sounds like, it joins the 2 tables together on a column (or group of columns)

in this case, it connects the Intersect table with the X-axis table and the Y-axis table based on the ID values

your data would look like this

ID Min Max
1 1 2
2 2.01 5

ID Val
1 1
2 2
3 3
4 4
5 5

ID XaxisID YaxisID Val
1 1 1 4
2 1 2 2
3 1 3 5
4 1 4 3.4
5 1 5 6.7
6 1 6 4.7
7 2 1 3

the "ID" fields are simply internal identifiers that are easier for the database system to query/join on
If you go for the simple one table approach, you will probably need 4 columns, the extra one for the result/answer with the query being somewhat like this -

select xyresult  from yourtable
where @x between x_min and x_max
and @y = y_val

You will need x_min, x_max and y_val to be the composite primary key in this case..

But derekkromm has posted a normalized structure which would be more efficient and scalable although there is an extra overhead - populating x, y and answer values in 3 different tables.
IssacJonesAuthor Commented:
Thanks guys - excellent information from you both!

I hope you don't mind if I share the points equally between you both. You have both helped me enourmously.

Many thanks

IssacJonesAuthor Commented:
sorry, "both" should have been "all". put it down to a long day!

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