Oracle入门学习二

null运算：运算的时候如果有空值参与永远返回空，为避免此情况可以借助nvl(param1,param2)函数，param1为空则返回值为param2，否则param1。

select salary+nvl(bonus,0) all_salary from staff;

排重：无论多列还是单列，只需要在列的最前面加“distinct”关键字就可以实现排重，多列排重则是看整个组合是否重复，而非其中一列是否重复。

select distinct salary,name from staff;

where：条件筛选，可以加极限条件例如1=1永远为真，1!=1永远为假。字符串对比是区分大小写的。

-- 判空要用 is null
select salary,name from staff where salary>80000 or bonus is null;

-- 永真条件
select salary,name from staff where 1=1;
-- 永假条件
select salary,name from staff where 1!=1;

-- 字符串比较严格区分大小写,且用单引号括住
select salary,name from staff where name=‘Popo‘;

模糊查询之Like：通配符“_”表示任意一个字符，通配符“%”任意长度的字符串，使用“like”进行模糊查询经常用到这两个通配符。

select salary,name from staff where name like ‘张%‘;
select salary,name from staff where name like ‘张_‘;

select salary,name from staff where name like ‘%o_‘;

模糊查询之between...and...：等价于 “ 值a >= 值b  and 值a <= 值c”，切记包含等于。如果需要不在这个范围内的数据，只需在between前加个not关键字。

select salary,name from staff where salary between 80000 and 90000;
select salary,name from staff where salary>=80000 and salary<=90000;

select salary,name from staff where not salary between 80000 and 90000;

模糊查询之in：表示在某个范围内，但这个范围内的值都可以精确的表示，in(4,5,6)表示字段在4或5或6都可以。不在这个范围则在not in(......)。

select salary,name from staff where salary in (40000,80000);
select salary,name from staff where salary not in (40000,80000);

判null：用“is null”而非=null，而不空使用“is not null”。

-- 判断用is null，非空用 is not null
select salary,name from staff where bonus is null;
select salary,name from staff where bonus is not null;

order by 排序：排序语句永远放末尾，默认是 asc 升序（小->大），desc为降序（大->小），多列排序也只需写一次order by，例如“order by 列1 desc,order by 列2 asc,列3 desc”。

select salary,name from staff order by salary;
select salary,name from staff order by salary asc;
select salary,name from staff order by salary desc;

-- 不写降序升序，默认升序
select salary,name from staff order by salary ,name ;
select salary,name from staff order by salary desc,name desc;

Oracle入门学习二

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