SQL函数

SQL函数可以完成功能如下：

执行数据计算

修改单个数据项

格式化显示的日期和数字

进行数据类型转换

根据行分组操纵输出

字符函数：

–Lower 将字符转换成小写

select lower(first_name)

from employees;

–Upper 将所有字段转换成小写

select upper(first_name) as first_name

from employees;

–Initcap 首字母大写

select initcap(job_id) as first_name

from employees;

–Concat 连接字符串的另一种用法

select concat(first_name||’ ‘,last_name)

from employees;

select concat(concat(first_name,’ ‘),last_name)

from employees;

–Substr 截取字符串

select substr(first_name,1,2)

from employees;

–Replace 字符串替换

select replace(first_name,’a’,’A’)

from employees;

–Length 获取长度

select length(first_name) as name_length

from employees;

–Instr 查找值的位置

select instr(first_name,’e’) as locat

from employees;

–Lpad 从左面进行填充直到字段为指定长度多补充少删除

select lpad(first_name,8,’e’) as locat

from employees;

–Rpad 从右面进行填充直至字段为指定长度多补充少删除

select Rpad(first_name,9,’e’) as locat

from employees;

数字函数

–Round 数字四舍五入 Trunc 数字截断 Mod 取余

–dual表中只有一行一列，且属于sys 但是所有用户否可以看到

select round(7.896544,3)，trunc(7.89456,2)， mod(21,5)

from dual;

日期函数

–获取当前数据库具体时间

select sysdate from dual;

–查询60号部门员工入职了多少时间

select employee_ID,first_name,trunc((sysdate-hire_date)/7)

from employees

where department_id=60;

–查询60号部门员工入职了多少月

select employee_id,trunc(months_between(sysdate,hire_date))

from employees

where department_id=60;

–查询三个月后日期

select add_months(sysdate,3)

from dual;

–查询下周五的日期

select next_day(sysdate,’星期一’)

from dual;

–计算所在月的最后一天

select last_day(sysdate) from dual;

select employee_id,hire_date,

trunc(months_between(sysdate,hire_date)) as workmonth,

add_months(hire_date,3) as threemonth,

next_day(hire_date,’星期一’) as filedate,

last_day(hire_date) as moneydate

from employees

where hire_date between ‘1-1月-07’and ‘30-6月-07’;

转换函数

–对数字进行格式

select employee_id,first_name,

to_char(salary,’$999,999,99’)

from employees;

–to_number把字符串转化成数字

select to_number(‘9527’,’99999’)

from dual;

–to_date函数 加fx为精准匹配，格式要一模一样

select to_date(‘25-12月-17’,’fxDD-Mon-YY’)

from dual;

通用函数

– Nvl 用一个实际的值去替换一个空值

select first_name ,

salary*12+500,

（salary*12+500)*(nvl(commission_pct,0)+1),

commission_pct

from employees;

–Nvl2（first_name，参数1，参数2） 如果第一各参数为空，则返回第二个参数

select first_name ,

salary*12+500,

（salary*12+500)*(nvl(commission_pct,0)+1),

（salary*12+500)*

(nvl(commission_pct,0)+1)+nvl2(commission_pct,salary*0.3,salary*0.4),

nvl2(commission_pct,salary*0.3,salary*0.4),

commission_pct

from employees

where department_id in(80,90);

–Coalesce 如果第一个值为空则返回第二个值，依次类推，只要为空，就返回下一个值

select employee_id , first_name ,

coalesce(to_char(commission_pct),to_char(manager_id),’老板’)

from employees

where department_id in(80,90);