SAN DIEGO—San Diego County’s unemployment rate decreased to 4.2 percent in December, down from a revised 4.6 percent in November and below the year-ago estimate of 8 percent, according to figures released on Jan. 21 by the state Employment Development Department. This compares with an unadjusted unemployment rate of 5 percent for California, which decreased from 5.4 percent in November, and 3.7 percent for the nation, which decreased from November’s 3.9 percent, during the same period. Between November and December, nonfarm employment decreased by 1,200 to 1,457,100 according to figures from the Employment Development Department. Agricultural employment contracted by 800 to 8,400. Construction had the largest month-over-month decline with a loss of 2,400 jobs. Seven other industries reported a loss, including educational and health services down 2,100, leisure and hospitality down 1,300, government down 800, financial activities down 600, manufacturing down 500, other services down 400, and mining and logging down …