Hi @Amir Hossein Hozouri
Please use the "Open With" option in Windows to resolve this issue. Open Folder options: right click on the file -> select "properties" -> you should see the option "Open with" where you have a button to change the related application which open the file. Click on the button "change" -> select a different application like SSMS2.0 -> approve and close everything.
By the way, you can open the .sql file as below screenshot. The multiple .sql files are opened in one SSMS.
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