Using Where clause in Case to compare - sql-server

Question

I am new to T-sql .
I am using the following Query:
SELECT e.Id,e.cAvg,
CASE
WHEN e.cAvg<=0.8 and cAvg>=0 THEN t.Model when t.Cr='0.8' then t.Model
WHEN e.cAvg>0.8 and cAvg<=5.4 THEN t.Model WHEN t.Cr='5.4' then t.Model
WHEN e.cAvg>5.4 and cg<=8 THEN t.Model WHEN t.Cr='8' then t.Model
ELSE 'No Change Required'
END
from A e, B t;
What I am trying to do is:
Select id and cAvg columns in Table A.
Compare cAvg in Table A with Cr in Table B.
Use the comparison in CASE to select the particular row which satisfies the condition.
Use the selected row to give query results.
t.Model is a column of table B. I want to select t.Model value of the selected row in the case statement.
I feel the way is to somehow include a equivalent of the where clause inside When of CASE.
Need Direction!!
The table schema:
Table A:
+----+------+
| id | cAvg |
+----+------+
| 1 | .8 |
| 2 | 5.4 |
| 3 | 6.0 |
+----+------+
Table B:
+-----+-------+
| Cr | Model |
+-----+-------+
| 2 | M1 |
| 5.5 | M2 |
| 8 | M3 |
+-----+-------+
I want to the following:
Compare the values of cAvg with a condition => (cAvg<=8 And cAvg>=5.5 => the model selected must be M3.)
The result I want to get is:
+----+------+-------+
| id | cAvg | Model |
+----+------+-------+
| 1 | .8 | M1 |
| 2 | 5.4 | M2 |
| 3 | 6.0 | M3 |
+----+------+-------+
I tired Join as suggested in the comments, A great thanks , I learnt a lot because of it!!.
My problem is that there are no common columns to join.
Also I need to compare the column in one table with that of another table and then give a result based on the comparison.
I referred to many answers in stack overflow but all the answers are for the premise where there is a common column.
I tried the following:
Inner Join
Cases
I need a direction as to which direction I need to go into.
Thank you!!

Answer 1

1st of all, you're selecting from 2 tables but without any link restrictions, so all rows are compared
If there is a matching key between the tables, so only relevant pairs of rows would be compared, it should be used, in a JOIN statement:
A e JOIN B t ON e.id = t.id
2nd of all, in order to select relevant lines, you should decide what these are..
you can inside a WHERE statement define whatvare the relent cases
WHERE
e.cAvg > 12
You can use the case statement inside WHERE but then the result should be conditioned are returned TRUE
SELECT e.Id,e.cAvg, t.Model
A e JOIN B t ON e.id = t.id
WHERE
CASE WHEN e.cAvg<=6 THEN t.Model when t.Cr=6 then t.Model
WHEN e.cAvg>6 and e.cAvg<=12 THEN t.Model
WHEN t.Cr='12' then t.Model
WHEN e.cAvg>12 and cg<=24 THEN t.Model
WHEN t.Cr='24' then t.Model
ELSE -1 END ! = -1
EDIT
Following you question edit, I think that what you need is a JOIN with a condition
Basically, instead of joining the tables with an equal key, join them with an unequal key.
Since you're looking for cAvg BETWEEN to t. Cr rows, 2 JOINs are needed
SELECT e.Id,e.cAvg, t.Model
FROM
A e JOIN B t ON
e.cAvg >= t.Cr
JOIN B t2 ON
e.cAvg < t2.Cr
WHERE
t.Cr IS NOT NULL
AND t2.Cr IS NOT NULL
The idea is that only where the 2 conditions are met, you would get the results of e
Hope that helps

Answer 2

I found a possible work around for the problem.
Problem Statement:
Compare two tables with no common column.
Use the comparison in CASE to select a particular row.
A WHERE Clause inside CASE is not accepted in T-SQL.
My Work Around :
Add a new column in the second table.
Assign An id from Table B to the column in Table A.
Use the assigned id to select the required row in Table B.
Tables:
Table A:
+----+------+
| id | cAvg |
+----+------+
| 1 | .8 |
| 2 | 5.4 |
| 3 | 6.0 |
+----+------+
Table B
+-----+-----+-------+
| Bid | Cr | Model |
+-----+-----+-------+
| 1 | 2 | M1 |
| 2 | 5.5 | M2 |
| 3 | 8 | M3 |
+-----+-----+-------+
Query to assign id's:
CREATE VIEW [AssignIDView] AS
SELECT DISTINCT e.id,
e.cAvg,
(CASE
WHEN e.cAvg>=0 and e.cAvg<=2 THEN 1
WHEN e.cAvg>2 and e.cAvg<=5.5 THEN 2
WHEN e.cAvg>3 and e.cAvg<=8 THEN 3
ELSE 'Invalid'
END) As BId
FROM A e, B t;
The result of the above Query will be a view as follows:
+----+------+-----+
| id | cAvg | Bid |
+----+------+-----+
| 1 | .8 | 1 |
| 2 | 5.4 | 2 |
| 3 | 6.0 | 3 |
+----+------+-----+
Now use Bid to select rows from table B to assign Model from B:
Query:
CREATE VIEW [ModelAssignView] AS
select e.id,
e.cAvg,
t.Model as [Model]
FROM A e, B t where e.TierID = t.id;
The result of the Query will be as follows:
+----+------+-------+
| id | cAvg | Model |
+----+------+-------+
| 1 | .8 | M1 |
| 2 | 5.4 | M2 |
| 3 | 6.0 | M3 |
+----+------+-------+
The intention of my question was to do the above.
For that I wanted to find an Equivalent of A WHERE Clause inside CASE.
But the above method achieved the solution for me.
Hope it helps:)!