How to open multiple .sql files in only one ssms instance

After spending most of my working day looking for the solution I finally found it. Thanks God, I was almost running out with this issue. Hope it helps somebody else!!!.

Update: linked domain expired, so here’s what it said:

Considering how integrated Microsoft tools usually are the result is
frustrating when you tell Visual Studio to open SQL files using Sql
Server Management Studio (SSMS). I really don’t like using Visual
Studio to edit T-SQL files but in the past, before I discovered this
tip, each SQL file I opened would open in a new instance of SSMS. Try
it:

1. Open a solution which contains SQL files
2. Right-click any SQL file and select “Open With…”
3. Click “Add”
4. Browse to “C:\Program Files\Microsoft SQL Server\100\Tools\Binn\VSShell\Common7\IDE\Ssms.exe” or if you’re
   running x64 Windows “C:\Program Files (x86)\Microsoft SQL
   Server\100\Tools\Binn\VSShell\Common7\IDE\Ssms.exe”, then click “OK”
5. Click “Set as Default” and then “OK”

Now open multiple SQL files. Each time you’ll get a different instance
of SSMS opened. What a pain!

NOTE: This entire article applies to SQL 2005, just replace SSMS with
SQLWB.

How do you resolve this? Repeat steps 1-3 above, but at step #4 enter
the following values:

• Program Name: “explorer.exe”
• Friendly Name: “Windows Explorer”

Repeat step #5 (set as default) above and then click OK. Now, open
additional files. They should all open in the same instance of SSMS.

It would seem that Visual Studio issues a command to SSMS.exe which
includes the path of the file selected in the solution explorer. It is
up to SSMS to check for a new instance, which it doesn’t. But when you
pass the file name to explorer it gets opened up in the same instance.

QUIRK WARNING!

If SSMS is not already open, the first file you attempt to open (not
first time ever, but every time you open an SQL file from Visual
Studio and SSMS isn’t open yet) SSMS will open, but your file will
not. Click the file a 2nd time and it will open the file this time.
Don’t ask me to explain it it just is (and I have no idea why).
Conclusion

The result when you tell Visual Studio that SSMS is the default editor
makes sense, but I don’t get why it would be different when you tell
explorer to open it. Maybe if I were a Windows developer instead of a
web developer I would know the answer. But either way, now you know.
Enjoy.