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select to_char(sysdate,‘yyyy-mm-dd hh24:mi:ss‘) currenttime,
to_char(sysdate - interval ‘7‘ year,‘yyyy-mm-dd hh24:mi:ss‘) intervalyear,
to_char(sysdate - interval ‘7‘ month,‘yyyy-mm-dd hh24:mi:ss‘) intervalMonth,
to_char(sysdate - interval ‘7‘ day,‘yyyy-mm-dd hh24:mi:ss‘) intervalday,
to_char(sysdate - interval ‘7‘ hour,‘yyyy-mm-dd hh24:mi:ss‘) intervalHour,
to_char(sysdate - interval ‘7‘ minute,‘yyyy-mm-dd hh24:mi:ss‘) intervalMinute,
to_char(sysdate - interval ‘7‘ second,‘yyyy-mm-dd hh24:mi:ss‘) intervalSecond
from dual;
6）add_months
select add_months(sysdate,12) newtime from dual;
7)extract
select extract(month from sysdate) "This Month",
extract(year from add_months(sysdate,36)) " Years" from dual;
字符函数
--字符函数
select substr(‘abcdefg‘,1,5)substr,--字符串截取
instr(‘abcdefg‘,‘bc‘) instr,--查找子串
‘Hello‘||‘World‘ concat,--连接
trim(‘ wish ‘) trim,--去前后空格
rtrim(‘wish ‘) rtrim,--去后面空格
ltrim(‘ wish‘) ltrim,--去前面空格
trim(leading ‘w‘ from ‘wish‘) deleteprefix,--去前缀
trim(trailing ‘h‘ from ‘wish‘) deletetrailing,--去后缀
trim(‘w‘ from ‘wish‘) trim1,
ascii(‘A‘) A1,
ascii(‘a‘) A2,--ascii(转换为对应的十进制数)
chr(65) C1,
chr(97) C2,--chr(十进制转对应字符)
length(‘abcdefg‘) len,--length
lower(‘WISH‘)lower,
upper(‘wish‘)upper,
initcap(‘wish‘)initcap,--大小写变换
replace(‘wish1‘,‘1‘,‘youhappy‘) replace,--替换
translate(‘wish1‘,‘y‘)translate,--转换，对应一位(前面的位数大于等于后面的位数)
translate(‘wish1‘,‘sh1‘,‘hy‘)translate1,
concat(‘11‘,‘22‘) concat --连接
from dual;
to_number
--to_number(expr)
--to_number(expr,format)
--to_number(expr,format,‘nls-param‘)
select to_number(‘0123‘)number1,--converts a string to number
trunc(to_number(‘0123.123‘),2) number2,
to_number(‘120.11‘,‘999.99‘) number3,
to_number(‘0a‘,‘xx‘) number4,--converts a hex number to decimal
to_number(100000,‘xxxxxx‘) number5
from dual;
聚合函数
student表如下：
count：
--count (distinct|all)
select count(1) as count from student;--效率最高
select count(*) as count from student;
select count(distinct score) from student;
语句1结果：11
avg
--avg (distinct|all)
select avg(score) score from student;
select avg(distinct score) from student;
select classno,avg(score) score from student group by classno;
语句3输出结果：
?
max
--max (distinct|all)
select max(score) from student;
select classno,max(score) score from student group by classno;
min
--min (distinct|all)
select min(score) from student;
select classno,min(score) score from student group by classno;
stddev（standard deviation）标准差
--stddev
select stddev(score) from student;
select classno,stddev(score) score from student group by classno;
sum
--sum
select sum(score) from student;
select classno,sum(score) score from student group by classno;
median--中位数
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