Dynamic sql to select a specific value from a column using Joins - tsql

Question

I am attempting to use dynamic sql to select a value based on a field. I have a table of field references I am using for the column names. What I am having troubles with is of course the dynamic sql. My return result is (SELECT ecoa_code FROM CRA_METRO2_BASE WHERE id = 568470) for example. But I really want it to run that select statement. Executing only returns the last row.
DECLARE #BaseCol VARCHAR(250)
SELECT
#BaseCol = '(SELECT ' + FR_base.field_name + ' FROM CRA_METRO2_BASE WHERE id = ' + CONVERT(VARCHAR(15), B.id) + ')'
FROM CRA_INNOVIS_AUDIT_ERROR_FIELDS E
LEFT JOIN CRA_METRO2_BASE B
ON B.id = E.base_id
LEFT JOIN CRA_METRO2_FIELD_REF FR_base
ON FR_base.id = E.base_field_ref
WHERE E.audit_id = #audit_id
EXEC(#BaseCol)

Answer 1

I am not sure I understand your premises correctly and without a mock-up...; so please take this answer with a grain of salt:)
DECLARE #sqlstring VARCHAR(MAX)
SELECT #sqlstring = 'SELECT ' + a.column_name + ' FROM ' + a.[Schema] + '.' + a.table_name
from (
SELECT TOP 1 T.object_id,OBJECT_SCHEMA_NAME(T.[object_id],DB_ID()) AS [Schema],
T.[name] AS [table_name], AC.[name] AS [column_name]
--,TY.[name] AS system_data_type
, AC.[max_length],
AC.[precision], AC.[scale], AC.[is_nullable], AC.[is_ansi_padded]
,AC.column_id
FROM sys.tables AS T
INNER JOIN sys.[all_columns] AC ON T.[object_id] = AC.[object_id]
) a
SELECT #sqlstring
EXEC(#sqlstring)

Answer 2

So I used my above query and now I am using a CTE to build my basic result list. And in my cte I create update statements which then are all put into a temp table.
I extract the update statements and execute them on the temp table. And walla, I have my results!
IF(OBJECT_ID('tempdb..#Temp') IS NOT NULL)
BEGIN
DROP TABLE #Temp
END
CREATE TABLE #Temp
(
usb_data VARCHAR(500),
cra_data VARCHAR(500)
);
WITH ErrorFieldsCTE(id, field, usb_data, cra_data, AUD, SOR, acceptable_variance, is_variance_known, is_reviewed)
AS(
SELECT
+ 'UPDATE #TEMP SET usb_data = (SELECT ' + FR_base.field_name +' FROM CRA_METRO2_BASE WHERE id = '+ CONVERT(VARCHAR(25), B.id) +' ) WHERE id = ' + CONVERT(VARCHAR(15), E.id) + ' ' [usb_data],
+ 'UPDATE #TEMP SET cra_data = (SELECT ' + FR_audit.field_name +' FROM CRA_INNOVIS_INBOUND_AUDIT_INFORMATION WHERE id = '+ CONVERT(VARCHAR(25), A.id) +') WHERE id = ' + CONVERT(VARCHAR(15), E.id) + ' ' [cra_data]
FROM CRA_INNOVIS_AUDIT_ERROR_FIELDS E
LEFT JOIN CRA_METRO2_BASE B
ON B.id = E.base_id
LEFT JOIN CRA_INNOVIS_INBOUND_AUDIT_INFORMATION A
ON A.id = E.audit_id
LEFT JOIN CRA_METRO2_FIELD_REF FR_audit
ON FR_audit.id = E.audit_field_ref
LEFT JOIN CRA_METRO2_FIELD_REF FR_base
ON FR_base.id = E.base_field_ref
WHERE E.audit_id = #audit_id
)
INSERT INTO #Temp
SELECT
id, field, usb_data, cra_data, AUD, SOR, acceptable_variance, is_variance_known, is_reviewed
FROM ErrorFieldsCTE
SELECT -- extract query
#usb_data += usb_data + '',
#cra_data += cra_data + ''
FROM #Temp
EXEC(#usb_data) -- updating temp table, selects usb-data
EXEC(#cra_data) -- updating temp table, selects cra-data
SELECT -- return to web
id, field, usb_data, cra_data, AUD, SOR, acceptable_variance, is_variance_known, is_reviewed
FROM #Temp
IF(OBJECT_ID('tempdb..#Temp') IS NOT NULL)
Begin
Drop Table #Temp
End