Commentary on the Bible, by Adam Clarke, [1831], at sacred-texts.com
Joab smites the city of Rabbah; and David puts the crown of its king upon his own head, and treats the people of the city with great rigour, Ch1 20:1-3. First battle with the Philistines, Ch1 20:4. Second battle with the Philistines, Ch1 20:5. Third battle with the Philistines, Ch1 20:6, Ch1 20:7. In these battles three giants are slain, Ch1 20:8.
After the year was expired, at the time that kings go out to battle - About the spring of the year; see the note on Sa2 11:1.
After this verse the parallel place in Samuel relates the whole story of David and Bath-sheba, and the murder of Uriah, which the compiler of these books passes over as he designedly does almost every thing prejudicial to the character of David. All he states is, but David tarried at Jerusalem; and, while he thus tarried, and Joab conducted the war against the Ammonites, the awful transactions above referred to took place.
David took the crown of their king - off his head - See Sa2 12:30.
Precious stones in it - The Targum says, "And there was set in it a precious stone, worth a talent of gold; this was that magnetic stone that supported the woven gold in the air." What does he mean?
He brought out the people - See this transaction particularly explained in the notes on the parallel places, Sa2 12:30-31 (note).
Elhanan the son of Jair - See the note on Sa2 21:19. The Targum says, "David, the son of Jesse, a pious man, who rose at midnight to sing praises to God, slew Lachmi, the brother of Goliath, the same day on which he slew Goliath the Gittite, whose spear-staff was like a weaver's beam."
Fingers and toes were four and twenty - See the note on Sa2 21:20.
These were born unto the giant in Gath - "These were born להרפא leharapha, to that Rapha in Gath, or to Arapha." So the Vulgate, Septuagint, and Chaldee.
The compiler of these books passes by also the incest of Amnon with his sister Tamar, and the rebellion of Absalom, and the awful consequences of all these. These should have preceded the fourth verse. These facts could not be unknown to him, for they were notorious to all; but he saw that they were already amply detailed in books which were accredited among the people, and the relations were such as no friend to piety and humanity could delight to repeat. On these grounds the reader will give him credit for the omission. See on Ch1 20:1 (note).