SQL query to combine column data from multiple columns - sql

Question

SQL Server 2012, Python 3.
Background Info:
I have two tables; LOADS and FACTORS.
LOADS has the following columns:
FACTORS has the following columns:
Table FACTORS is basically a roadmap how to combine and factor the data in table LOADS.
FACTORS columns mlc and tlc contain the same numbers as LOADS table column subcase.
In this example we will only use value 32771 for LOADS column eid.
Problem:
I need to get a set of results with columns similar to LOADS but with additional columns, like this:
eid, mlc, tlc, fx, fy, fz.
To do this manually:
start with the first row of FACTORS. mlc=1053002, tlc=4053400
select fx, fy, fz from LOADS where subcase=mlc and eid=32771 (this is our mlc loads)
select fx, fy, fz from LOADS where subcase=tlc and eid=32771 (this is our tlc loads)
the final values in columns fx, fy, fz are from these equation:
(mlc.fx * mf + tlc.fx * tf) * cf
(mlc.fy * mf + tlc.fy * tf) * cf
(mlc.fz * mf + tlc.fz * tf) * cf
If tlc=0 then only use mlc. The fx, fy, fz values are from these equations:
(mlc.fx * mf) * cf
(mlc.fy * mf) * cf
(mlc.fz * mf) * cf
I am somewhere between beginner and intermediate using SQL so I have no idea how to do this using only SQL. I have successfully done this using pandas basically doing this the manual way by creating a blank DataFrame, calculating the (3) DOF fx,fy,fz and adding Rows one at a time, building the DataFrame from the ground up. I can share that code if anyone needs to see it, but I really want to do this in SQL if it's possible. The reason is because most general queries using this procedure can take several minutes (for 50,000+ Rows, granted my pandas knowledge might not be too efficient for real world usage) and I really want to get that time down to a few seconds.

Answer 1

Following ZLK's comment, it seems like you want this:
select
LoadsMlc.eid,
Factors.mlc,
Factors.tlc,
(LoadsMlc.fx * Factors.mf + isnull(LoadsTlc.fx * Factors.tf, 0)) * Factors.cf fx,
(LoadsMlc.fy * Factors.mf + isnull(LoadsTlc.fy * Factors.tf, 0)) * Factors.cf fy,
(LoadsMlc.fz * Factors.mf + isnull(LoadsTlc.fz * Factors.tf, 0)) * Factors.cf fz
from
dbo.Factors
join dbo.Loads LoadsMlc on Factors.mlc = LoadsMlc.subcase
left outer join dbo.Loads LoadsTlc on nullif(Factors.tlc, 0) = LoadsTlc.subcase
This assumes there exists exactly one match for subcase against mlc--your question doesn't currently indicate one way or another. You may want to, perhaps, add something like
where
LoadsTlc.eid is null -- We didn't find a tlc (it was 0)
or LoadsMlc.eid = LoadsTlc.eid
If there are potentially multiple matches across the Loads table for each of the Factors. Note that the tlc join is a left outer, as you indicated that we should only use mlc in the case that it is 0.
The second join to Loads uses a nullif to nullify the case when tlc is 0. In that case you'll receive no records for that value. The three columns account for this null case by adding 0.