* Sun Microsystems Inc., a Mountain View.,...

* Sun Microsystems Inc., a Mountain View., Calif.-based maker of computer workstations, reported 20% improvements in both earnings and revenue for its second fiscal quarter, ended Dec. 27.

Citing strong sales in the United States and Japan, Sun said it earned $47.4 million, or 47 cents per share, compared to $39.1 million, or 39 cents per share, in the same period a year earlier.

Revenue climbed to $909.1 million from $753.5 million during the second quarter a year ago.

Kevin C. Melia, Sun’s chief financial officer, said the weak economy, stiff competition and swift developments in both hardware and software all could cloud the company’s performance in 1992, though executives remain upbeat in the longer term.