Question

This is driving me nuts, I’m sure it’s rather simple but can’t get it right.

Having a table holding ratevalues for different years:

Ratevaluetable

Year   Value A  Value B  Value C
2009     10      15       20
2010     12      18       22
2011     14      21       25

Then I have another persontable:

User    Time      Price
john    2010      value B
Tina    2009      Value C
Bill    2011      Value C

Now what I need is to join the 2 tables so that I get the value in the Price column substituted with the data from the Ratevaluetable.

I can join on time on year, but the I don’t know how to join to the Value fields?

Answer 1

This is not possible with your current database schema. Joins connect tables by matching column values; what you propose to do here is match a column value with a column name, which is impossible.

What would be possible is:

Table ValueTypes:
Id    Description
1     Value A
2     Value B
3     Value C

Table RateValueTable:
Year     ValueTypeId     Value
2009     1               10
2009     2               15
2009     3               20

Table User:
User    Time      ValueTypeId
john    2009      2

Given the above schema, you could join User to RateValueTable in the obvious manner.