Albert Barnes Commentary 1 Chronicles 11:6-8

"And David said, Whosoever smiteth the Jebusites first shall be chief and captain. And Joab the son of Zeruiah went up first, and was made chief. And David dwelt in the stronghold; therefore they called it the city of David. And he built the city round about, from Millo even round about; and Joab repaired the rest of the city." — 1 Chronicles 11:6-8 (ASV)

The narrative given here supplies a clear omission in 2 Samuel 5:8, where something has evidently dropped out of the text.

Joab's prowess on this occasion, and the part he took in building the city of David (1 Chronicles 11:8), are known to us only from this passage in Chronicles.