How would SQL table store Map Legends? - sql

Question

I am not asking which Map legend classification to choose. But assume I have picked one, how would I store it properly in the SQL tables? The reason of asking is, in the end, I need to store more info than I expected... Hope someone could verify.
Consider 3 cases below:
Numeric, Range
Numeric, Single Value
Alphabet, Single Value
To be able to store it properly, SO THAT I am able to do logic classification at real time (do coloring), meaning, I need to store the operators as well (less than [lt], greater than [gt], equal [eq], etc)
Ends up, Say I have 2 tables:
LegendSetup
Column:
1. LegendKey (int)
2. Type (varchar)
3. Min (decimal)
4. Max (decimal)
LegendValueSetup
Column:
1. ValueKey (int) //AutoIncrement PK
2. LegendKey (int) //FK
3. RangeNumeric (decimal) //numeric
4. RangeAlpha (varchar) //alphabet
5. RangeOperator (varchar) //eq, lt, gt
6. RangeShow (varchar) //for display purpose
7. HexColor (varchar)
Is that how it normally works?

Answer 1

As you only need ranges and equalities I suggest you don't use explicit operators, but ranges like this:
CREATE TABLE LegendValueSetup (
ValueAlpha varchar,
ValueNumeric decimal,
LowerNumeric decimal,
UpperNumeric decimal,
... -- other columns
)
For your examples 2 and 3 you would just store the explicit values in the ValueAlpha and ValueNumeric column.
For example 1 you would have to think about if LowerNumeric is an inclusive and UpperNumeric an exclusive bound and vice versa. Then you can store the legend like this:
LowerNumeric | UpperNumeric | Color
NULL | 100 | Black
100 | 200 | Red
200 | 400 | Orange
400 | 600 | Yellow
600 | NULL | Green
If you have now a value and want to get the color, you only have to query
SELECT color FROM LegendValueSetup WHERE LowerNumeric <= $value AND UpperNumeric > $value
You can of course go further and split the table into three different ones, one for each legend type.