COALESCE 处理NULL值
现在有需求，计算出每个雇员的收入，收入=工资+福利。
COALESCE (expr1, expr2)
is equivalent to:
CASE WHEN expr1 IS NOT NULL THEN expr1 ELSE expr2 END
Similarly,
COALESCE (expr1, expr2, ..., exprn), for n>=3
is equivalent to:
CASE WHEN expr1 IS NOT NULL THEN expr1 ELSE COALESCE (expr2, ..., exprn) END
