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