转换函数
to_char to_number to_date
隐式转换 fm 去掉前导0 显式转换 fx 完全精确匹配才会成功，否则报错
NVL(A,B) 若A为NULL，则输出B select last_name,salary,NVL(commission_pct,0),(salary12)+(salary12*NVL(commission_pct,0)) AN_SAL from employees;
NVL2(A,B,C) 若A为NULL，则输出C，若A不为NULL，则输出B select last_name,salary,commission_pct,NVL2(commission_pct,'SAL+COM','SAL') income from employee where department_id IN (50,80);
NULLIF(A,B) 若A=B，则为NULL，若A<>B，则输出A select first_name,LENGTH(first_name) "expr1",last_name,LENGTH(last_name) "expr2",NULLIF(LENGTH(first_name),LENGTH(last_name)) result from employees;
COALESCE(A,B,C,.......Z) 从A开始往后判断是否为NULL，找到不为NULL时则输出值，单最后Z不能设定为NULL
select last_name,salary,commission_pct,COALESCE((salary+(commission_pct*salary)),salary+2000) "new Salary" from employees;
** 条件表达式** CASE select last_name,job_id,salary, case job_id when 'IT_PROG' THEN 1.10salary when 'ST_CLERK' THEN 1.15salary when 'SA_REP' THEN 1.20*salary else salary end "REVISED_SALARY" from employees;
DECODE select last_name,job_id,salary, DECODE(job_id,'IT_PROG',1.10salary, 'ST_CLERK',1.15salary, 'SA_REP' ,1.20*salary, salary) REVISED_SALARY from employees;
