"This only deleted the content of the selected cell."
You got lucky, then!
A single click on a cell selects the cell. Command A or Select All at that stage will Select All items in the same category as the current selection (ie. all Cells), and pressing delete will then delete the contents of all cells in the table.
Clicking twice on a cell selects the cell (first click) AND places the insertion point in that cell.
Pressing command-a or choosing Select All at that point will select all of the text in that cell, and pressing delete will delete what is selected—all the text in that cell.
Regards,
Barry
PS: If you do manage to accidentally delete more than intended…
The OOPS! key is your best friend. Press command-Z to undo that delete.
B.