oracle SQL简介（一）

select子句
from 子句
where子句
dual表，一行一列，dummy
函数，树结构查询，连接查询，子查询， 优化

1. 基本函数

1.1. 字符串操作

字符串大小写
select lower(ename),upper(ename),initcap(ename) from emp;

求长度
select length('王五') from dual;
select lengthb('王五') from dual;
--unicode长度
select lengthc('王五') from dual;

取子字符串
select ename,substr(ename,1,1) first,substr(ename,-1) last from emp;
select substr('张三李四王五赵六',1,6) from dual;
select substrb('张三李四王五赵六',1,6) from dual; --按字节取
select substrc('张三李四王五赵六',1,6) from dual; --unicode
select ename,instr(ename, 'A') from emp;

截取和添加
select trim (leading 'a' from 'aadddabcaaaaa') from dual;
select trim (trailing 'a' from 'aadddabcaaaaa') from dual;
select trim (both 'a' from 'aadddabcaaaaa') from dual;
select trim ('a' from 'aadddabcaaaaa') from dual;

select lpad(ename,20, '-'),rpad(ename,20, '-') from emp;

1.2. 操作数字

round,trunc (可对日期操作)
ceil
e为底的幂
select exp(1) value from dual;
取e为底的对数
select ln(2.7182818284590451) value from dual;
取10为底对数
select log(10,10) value from dual;
圆周率
select acos(-1) value from dual;
取符号(-1,0,1)
select sign(-8) value from dual;
求集合最大值
select greatest(1,-2,4,3) value from dual;
求集合最小值
select least(1,-2,4,3) value from dual;

1.3. 操作日期

alter session set nls_date_format='yyyy-mm-dd hh24:mi:ss';
alter session set nls_timestamp_format='yyyy-mm-dd hh24:mi:ss';

select date'1999-02-03' from dual;
select time'5:30:23' from dual;
select timestamp'1999-02-03 5:30:23' from dual;
系统日期时间
sysdate, systimestamp
SELECT to_char(systimestamp,'YYYY-MM-DD HH24:MI:SS.FF3') FROM DUAL;
--日期直接相减
select round(sysdate-hiredate) days,sysdate,hiredate from emp;
--两个日期的月间隔
select trunc(months_between(sysdate,hiredate)),sysdate,hiredate from emp;
--六个月后是哪一天
select add_months(hiredate,6) ,hiredate from emp;
--该日期月底是哪一天
select last_day(hiredate),hiredate from emp;
--当前日期算起，下一个星期五是哪天
select next_day(hiredate,’friday’) hiredate from emp;
select next_day(hiredate,6) hiredate from emp
时间间隔:
select extract(year from sysdate) year,extract(month from sysdate) month, extract(day from sysdate) day from dual;
求某天是星期几
select to_char(sysdate,'day') from dual;
select to_char(sysdate,'d') from dual;

1.4. 类型转换

to_char,to_number,to_date
to_char常用格式:
The following are number examples for the to_char function.
select ename,to_char(sal,'9999.00') from salary from emp;

to_char(1210.73, '9999.9')	would return '1210.7'
to_char(1210.73, '9,999.99')	would return '1,210.73'
to_char(1210.73, '$9,999.00')	would return '$1,210.73'
to_char(21, '000099')	would return '000021'

下面列出了to_char转换日期为一个字符串的有效参数。这些参数可以用来组合。
select to_char(sysdate,'yyyy-mm-dd hh24:mi:ss') from dual;

Parameter	Explanation
YEAR	Year, spelled out
YYYY	4-digit year
YYY YY Y	Last 3, 2, or 1 digit(s) of year.
IYY IY I	Last 3, 2, or 1 digit(s) of ISO year.
IYYY	4-digit year based on the ISO standard
Q	Quarter of year (1, 2, 3, 4; JAN-MAR = 1).
MM	Month (01-12; JAN = 01).
MON	Abbreviated name of month.
MONTH	Name of month, padded with blanks to length of 9 characters.
RM	Roman numeral month (I-XII; JAN = I).
WW	Week of year (1-53) where week 1 starts on the first day of the year and continues to the seventh day of the year.
W	Week of month (1-5) where week 1 starts on the first day of the month and ends on the seventh.
IW	Week of year (1-52 or 1-53) based on the ISO standard.
D	Day of week (1-7).
DAY	Name of day.
DD	Day of month (1-31).
DDD	Day of year (1-366).
DY	Abbreviated name of day.
J	Julian day; the number of days since January 1, 4712 BC.
HH	Hour of day (1-12).
HH12	Hour of day (1-12).
HH24	Hour of day (0-23).
MI	Minute (0-59).
SS	Second (0-59).
SSSSS	Seconds past midnight (0-86399).
FF	Fractional seconds.

The following are date examples for the to_char function.

to_char(sysdate, 'yyyy/mm/dd');	would return '2003/07/09'
to_char(sysdate, 'Month DD, YYYY');	would return 'July 09, 2003'
to_char(sysdate, 'FMMonth DD, YYYY');	would return 'July 9, 2003'
to_char(sysdate, 'MON DDth, YYYY');	would return 'JUL 09TH, 2003'
to_char(sysdate, 'FMMON DDth, YYYY');	would return 'JUL 9TH, 2003'
to_char(sysdate, 'FMMon ddth, YYYY');	would return 'Jul 9th, 2003'

You will notice that in some examples, the format_mask parameter begins with "FM". This means that zeros and blanks are suppressed. This can be seen in the examples below.

to_char(sysdate, 'FMMonth DD, YYYY');	would return 'July 9, 2003'
to_char(sysdate, 'FMMON DDth, YYYY');	would return 'JUL 9TH, 2003'
to_char(sysdate, 'FMMon ddth, YYYY');	would return 'Jul 9th, 2003'

The zeros have been suppressed so that the day component shows as "9" as opposed to "09".

1.5. 操作null

nvl,nvl2,nullif, coalesce

select ename,comm,nvl(comm,-1) from emp;
select nvl(comm,0),nvl2(comm,’A’,’B’) type from emp;
select ename,nullif(ename,'KING') from emp;
COALESCE返回参数列表中的第一个非空值，所有关系型数据库系统都支持的标准函数
select coalesce(comm,sal,100) from emp;

1.6. 分组统计

select sum(comm),count(comm),count(*) from emp;
select avg(comm),avg(nvl(comm,0)) from emp;
求平均工资大于2000的工作
select job,avg(sal) from emp group by job having avg(sal) >2000;
按部门分组
select deptno,sum(sal) from emp group by deptno;
rollup分组
按部门分组并求统计：
select deptno,sum(sal) from emp group by rollup(deptno);
按部门和工种分组并求统计：
select deptno,job,sum(sal) from emp group by rollup(deptno,job);
grouping(列名)，判断列是否参加了分组，0为参加了分组
select deptno,job,grouping(deptno),grouping(job),sum(sal) from emp group by rollup(deptno,job);
cube分组：
select deptno,job,grouping(deptno),grouping(job),sum(sal) from emp group by cube(deptno,job);
为2**n个结果集

1.7. 分支

case
统计各年雇用人数，并转为列
select count(*) total,
sum(case to_char(hiredate,'yyyy') when '1980' then count(hiredate) end ) "1980" ,
sum(case to_char(hiredate,'yyyy') when '1981' then count(hiredate) end ) "1981" ,
sum(case to_char(hiredate,'yyyy') when '1982' then count(hiredate) end ) "1982" ,
sum(case to_char(hiredate,'yyyy') when '1983' then count(hiredate) else 0 end ) "1983" ,
sum(case to_char(hiredate,'yyyy') when '1984' then count(hiredate) end ) "1984" ,
sum(case to_char(hiredate,'yyyy') when '1985' then count(hiredate) end ) "1985" ,
sum(case to_char(hiredate,'yyyy') when '1987' then count(hiredate) end ) "1987"
from emp group by hiredate;

统计各部门各岗位收入总和，并按部门转为列显示
select deptno,job,sum(sal) from emp group by deptno,job order by 1;

select job,
sum(case DEPTNO when 10 then sal else 0 end) "dep 10" ,
sum(case DEPTNO when 20 then sal else 0 end) "dep 20" ,
sum(case DEPTNO when 30 then sal else 0 end) "dep 30" ,
sum(case DEPTNO when 40 then sal else 0 end) "dep 40" ,
sum(sal) total
from emp group by job;

decode
求税率
select ename,sal,decode (trunc(sal/2000,0),
0, 0.00,
1,0.09,
2,0.20,
3,0.30,
0.40) tax_rate
from emp;