SQL Select Data From 3 Tables

Question

I'm developing a C# application for a local business. I want to select data from 3 tables of a SQL Server 2014 database, namely companyinfo , invmain , invtran .

Companyinfo stores the name of business, invmain has sales invoice number and date info while invtran has Invoice Transaction.

I'm using a SELECT statement like this:

SELECT 
    companyinfo.name,    
    invmain.invno, invmain.date, invtrans.itemid, invtrans.unitprice 
FROM 
    companyinfo, invmain, invtrans
WHERE 
    companyinfo.Id = 1 
    AND invmain.invno = 13 
    AND invtrans.invnumber = 13

But this returns null if one of table has no data. Is there any other effective way to do that?

Answer 1

Use the proper ANSI/SQL JOIN syntax like this:

SELECT 
    ci.name,    
    invmain.invno, invmain.date, 
    invtrans.itemid, invtrans.unitprice 
FROM 
    companyinfo ci, 
INNER JOIN
    invmain m ON ci.companyId = m.companyId
INNER JOIN
    invtrans it ON it.invo = m.invno
WHERE 
    companyinfo.Id = 1 

and you need to define JOIN conditions between the tables to establish the "links". I've chosen INNER JOIN which returns only rows that exist in both tables involved, based on the matching condition defined after the ON keyword.

If you want to select rows from companyinfo that don't have any corresponding invoices, you could also use LEFT OUTER JOIN instead of the INNER JOIN to get back data from companyinfo for companies without any invoices