Has there ever been a case where a scuba diver was attacked by a shark like one of the top three deadly ones the great white bull and tiger? Has there been a case where diver was able to defend himself or herself just with a dive knife?
I'm thinking a small spear would've worked better or an underwater stun gun
Statically this is a low frequency occurrence on divers at depth. The 2019 worldwide total shark bites on humans is 64 confirmed unprovoked cases. Of the total shark/human nibbles roughly 3% of the total annual shark/human interactions (bites) on the planet are SCUBA divers. So 3% of 64 is 1.9 scuba divers had an unpleasant interaction with sharks on the planet. I would worry more about my spouse offing me in my sleep than sharks.
So lets think for just a moment. The yearly average of unprovoked shark "attacks" on humans is 82, of which 3% are SCUBA divers (usually on or near the surface) or 2.46 people on the entire planet! I like those odds. Just don't work holding up dead fish to lure the sharks into the middle of the circle at a shark dive, or spearfish and put the bleeding fish on your stinger at your thigh, If you do either of those two things and get bit by a shark, well, here's your sign.
On one dive trip we saw Tiger, hammerhead, Lemon, White Tip, Silver Tip, Black Tip Oceanic and Reef sharks. Pretty much every dive was 50+ sharks. It was beautiful, and as French Polynesia does not allow feeding they did not bother us. Not to say you don't need to respect them and mind your business. Sharks be Sharks, but we are not on their normal menu and with all the noise our bubbles make that kind of keeps them at a distance.
If I had to worry about a shark it would be the bull shark as they can be a bit nosey. Bow if you live in a certain county and swim at a certain beach in Florida, your odds of interacting with a shark in a bad sort of way get real good. see the above link.
Now quit worrying about sharks attacking you and worry about something with a higher statistical chance of injuring you or killing you and that is you. Los Angeles County CA averages 6 SCUBA deaths per year and we have Mr. White living here. 1000 dives and I have only almost seen one Great White in Catalina (dive buddy saw it but I was busy teaching and missed it.). Poorly maintained equipment (equipment in this case includes heart, arteries, weight etc), OOA and panic are the true killers of SCUBA Divers,