Cmd+O), or drag onto the Dock icon. .sql, .psql and .pgsql all work, and what you get is an ordinary query tab with a file behind it.
Opening
Each file opens in a new tab. Opening one that is already open focuses its existing tab instead of making a second, and files opened before any connection exists wait in a queue until you connect. The title bar carries the filename, and the close button gets the standard macOS unsaved-changes dot.Saving
Cmd+S saves back to the source file, and does a Save As when the tab has no file yet. Cmd+Shift+S always opens Save As.
Cmd+click the title bar for the standard path menu, then pick a folder to open it in Finder.
When a tab has both unsaved file changes and pending data grid edits, Cmd+S saves the grid changes first. Press it again once that finishes to save the file.
Closing a query tab keeps its SQL either way. Reopen it with Cmd+Shift+T or File > Reopen Closed Tab. See Tabs.
When the file changes underneath you
Agit pull or an edit in another editor puts a yellow banner across the top of the tab saying the file was modified on disk, with a Reload button that replaces your tab’s contents with what is now on disk.
Saving over a file that changed since you opened it opens a diff sheet instead of overwriting. Your Changes and On Disk sit side by side, and three buttons resolve it: Keep My Changes, Reload from Disk, or Cancel.
A folder at a time
To watch a whole directory of.sql files rather than opening them one by one, use Linked SQL Folders.