Question

I have a few tables:

1. “DCDetails” table which contains the Master data for a few diagnostics centers.
2. “CompanyDetails” table which contains the Master data for Companies
3. “Investigation” table which contains the Investigations(meaning set of medical tests to be conducted)

These are my master tables.

I also have a few mapping tables:
1. “CompanyDCMap” table which contains the MAPPING of Company to Diagnostic centers
2. “InvestigationDCMap” table which contains the MAPPING of Investigation to Diagnostic centers(Or DC for short)

I have to filter a set of DC based on two criteria which are:

1. the DC belongs in the “CompanyDCMap” and
2. Out of the DC filtered in (1), it also belongs in the “InvestigationDCMap” table.

How do I write the query for this so that I get the DC which are in both CompanyDCMap and InvestigationDCMap given I have the primary keys of “CompanyDetails” and “Investigation” tables.

I have almost given up, I am unable to think of a query which filters two sets at the same time.

Kindly help me.

UPDATE
Schema:
CompanyDetails table:

CompanyID(PRIMARY KEY), CompanyName(NVARCHAR(100))
   1                     Company1
   2                     Company2
   3                     Company3

Investigation Table:

InvestigationID(Primary key) , InvestigationName(NVARCHAR(100))
   1                               HIV+ Blood Test
   2                               TMT
   3                               Urine Test

DCDetails Table:

DCID(PRIMARY KEY), DCName(NVARCHAR(100))
   1                 DC1
   2                 DC2
   3                 DC3

CompanyDCMap table

   CompanyDCMapID(Primary key), CompanyID(Foreign key), DCId(Foreign Key)
          1                        1                       1
          2                        1                       2
          3                        2                       2
          4                        2                       3
          5                        3                       1
          6                        3                       3

InvestigationDCMap table

  InvestigationDCMapID(Primary Key), InvestigationID(Foreign Key), DCId(Foreign Key)
          1                                1                           1
          2                                1                           3
          3                                2                           2
          4                                2                           3

Expected Output of a query given CompanyID = 1 and InvestigationID = 2, SELECT DCId and DCName =

     DCId(Int)                   DCName(NVARCHAR(100))
      2                          DC2

Answer 1

SELECT d.DCID, d.DCName
FROM dbo.DCDetails AS d
INNER JOIN dbo.CompanyDCMap AS c
ON d.DCID = c.DCId
INNER JOIN dbo.InvestigationDCMap AS i
ON i.DCId = d.DCID;

To get “distinct” values, you can use:

SELECT DISTINCT d.DCID, d.DCName
FROM dbo.DCDetails AS d
INNER JOIN dbo.CompanyDCMap AS c
ON d.DCID = c.DCId
INNER JOIN dbo.InvestigationDCMap AS i
ON i.DCId = d.DCID;

Or…

SELECT d.DCID, d.DCName
FROM dbo.DCDetails AS d
INNER JOIN dbo.CompanyDCMap AS c
ON d.DCID = c.DCId
INNER JOIN dbo.InvestigationDCMap AS i
ON i.DCId = d.DCID
GROUP BY d.DCID, d.DCName;

Better yet, since no relationships with the other tables are needed:

SELECT d.DCID, d.DCName
FROM dbo.DCDetails AS d
WHERE EXISTS (SELECT 1 FROM dbo.CompanyDCMap WHERE DCId = d.DCID)
AND EXISTS (SELECT 1 FROM dbo.InvestigationDCMap WHERE DCId = d.DCID);

This will be a much more efficient query, but if you need other columns from the other tables, you’ll need to revert to the join version.