How to build a rules table in sql server - sql-server-2008-r2

Question

There are quite a few business rules which are currently hardcoded within a stored procedure. Wanted to explore the option of setting up a rules table where in we intend to key-in all business rules and based on it execute the stored procedure.
Though the system is little complicated have provided a simple version here.
Create table tblTest
(
TranID int primary key not null,
FName varchar(20) not null,
Age int not null,
Salary money not null,
MaritalStatus char(1) not null
)
Insert into tblTest values (1, 'Alex', 26, '25000.00','Y')
Insert into tblTest values (2, 'Brenda', 25, '14500.00','Y')
Insert into tblTest values (3, 'Peter', 69, '50000.00','N')
Insert into tblTest values (4, 'Paul', 64, '74500.00','Y')
Now to keep the example simple lets assume the business rules to be the following:
1. Age >=25,
2. Age < 65 and
3. Salary > 15K
Create table tblBusRule
(
RuleID int Primary key not null,
ColName varchar(20) not null,
Operator varchar(2) not null,
ColValue varchar(10) not null,
RuleOrder int not null
)
Insert into tblBusRule values (1, 'Age', '>=', '25', 1)
Insert into tblBusRule values (2, 'Age', '<', '65', 2)
Insert into tblBusRule values (3, 'Salary', '>', '15000.00', 3)
The direct query would be something like this which would output the record 1 (Alex) and 4 (Paul) alone.
Select * from tblTest
where
age >=25 and
age < 65 and
salary > '15000.00'
Now how to make this dynamic based on the rules mentioned in tblBusRule?

Answer 1

Using the stuff() with select ... for xml path ('') method of string concatenation and sp_executesql
declare #sql nvarchar(max), #where nvarchar(max);
set #where = stuff((
select ' and '+colname +' '+operator +' ' + colvalue+char(10)
from tblBusRule
order by RuleOrder
for xml path (''), type).value('.','nvarchar(max)')
,1,6,'');
set #sql = 'select * ' +char(10)+'from tblTest'+char(10)+'where '+#where;
select #sql as CodeGenerated;
exec sp_executesql #sql;
rextester demo: http://rextester.com/CGRF91788
returns:
+-------------------------+
| CodeGenerated |
+-------------------------+
| select * |
| from tblTest |
| where Age >= 25 |
| and Age < 65 |
| and Salary > 15000.00 |
+-------------------------+
+--------+-------+-----+------------+---------------+
| TranID | FName | Age | Salary | MaritalStatus |
+--------+-------+-----+------------+---------------+
| 1 | Alex | 26 | 25000,0000 | Y |
| 4 | Paul | 64 | 74500,0000 | Y |
+--------+-------+-----+------------+---------------+
Reference:
- The curse and blessings of dynamic SQL - Erland Sommarskog