For this challenge, you'll change the opacity of an animated element so it gradually fades as it reaches the right side of the screen. In the displayed animation, the round element with the gradient background moves to the right by the 50% mark of th
一.Null的区别 create database scort use scort create table emp ( empno int primary key, ename ), sal int, comm int ) ,); ,,); ,,); ,); ,,); ,,); select * from emp --1.区别null可以参与 is 和 not is 运算,null不能参与 <> 和 != 和 = 运算. select * from emp where comm is nul
oracle在使用函数计算式会遇到这样的情况:例如sum函数 如果计算的sum值为null,则用0替代 方法1(便于理解): select when sum(c.num) is null then 0 else sum(t.num) from class c 方法2(简单粗暴): NVL(Expr1,Expr2)如果Expr1为NULL,返回Expr2的值,否则返回Expr1的值 select NVL(SUM(c.num) ,0) from class c