/Oracle查询优化改写/
--1、coalesce 返回多个值中，第一个不为空的值
select coalesce('', '', 's') from dual;
--2、order by
-----dbms_random.value 生产随机数,利用随机数对查询结果进行随机排序
select * from emp order by dbms_random.value;
--指定查询结果中的一列进行排序
select * from emp order by 4;
-----order by 中认为null是最大所以null会排在第一或者最后一个
-----可以利用 nulls first 或者 nulls last 对null进行排序处理
select * from emp order by comm nulls first;
select * from emp order by comm nulls last;
----- 多列排序,job 降序排列,如果工作一样，按照工号升序排列
select * from emp order by job desc, empno asc;
------依次按照job,empno降序排序
select * from emp order by job, empno desc;
------将empno = 7934 的排在第一位，其余的按照empno将序排列
------ORDER BY DECODE 按照自定义的顺序排序，如果没有
------定义则按照原始值排序 case when then else end 也是同样的道理
select * from emp order by decode(empno, 7934, 2, 1) asc;
--先按照打的分组排序，然后在分组内按照字段排序
select empno, ename, sal
from emp
order by case
when sal <= 5000 and sal >= 3000 then
0
when sal < 3000 and sal > 1000 then
1
when sal < 1000 then
2
else
3
end asc,
3 asc;
--2
select empno,
ename,
case
when sal <= 5000 and sal >= 3000 then
0
when sal < 3000 and sal > 1000 then
1
when sal < 1000 then
2
else
3
end,
sal
from emp
order by 2 asc, 3 asc;
--3、_ 通配符，代替一个字符。
select T.*, T.ROWID from emp T where t.ename like '_EN%';
---可以通过,将_表示为一个普通的字符,俩种写法
select T.*, T.ROWID from emp T where t.ename like '_EN%' ESCAPE '';
--4、tanslate
-----对应字符一一替换,每一个字符的替换相当于执行一次REPLACE('C','1')
-----会将job字段中的C全部替换为1
select translate(job, 'CRK', '123') from emp t where t.job = 'CLERK';
---第二个字段为空的时候返回null
select translate(job, 'CRK', '') from emp t where t.job = 'CLERK';
---可以利用这个函数，删除字段的部分字符，对应位置字符为空
select translate(job, '1CRK', '1') from emp t where t.job = 'CLERK';
--5、内连接、左连接、右连接、外连接
--SQL-92标准写法,不建议用Oracle特有的= + 来表示连接
--inner join on left join on right join on outer join on
---左右连接的时候，只过滤左边或者右边的数据，用左连接作为例子
select a.empno, a.job, a.comm, b.empno, b.job, b.comm
from emp a
left join emp2 b
on (a.empno = b.empno and b.comm is not null);
select a.empno, a.job, a.comm, b.empno, b.job, b.comm
from emp a, emp2 b
where a.empno = b.empno(+)
and b.comm(+) is not null;
--6、in 多列写法
select *
from emp t
where (t.empno, t.ename) in (select t2.empno, t2.ename from emp t2);
---not in 注意事项：如果在子查询结果中包含null，not in 则返回null
select * from emp t where t.mgr not in (select mgr from emp2);
-- 7、insert into 如果表中有默认字段吗，那么不能显示的插入null,否则表中的字段值
---不会是默认值，依然是null
---8、形成数据结构
---level 代表总共有几层树形结构
select level from dual connect by level;
---9、正则表达式(没必要记住，了解规则和用途就可以了)
---regexp_count 统计匹配的
---regexp_replace 替换匹配的
---regexp_like 用正则表达式模糊查询
select regexp_count('abc,bcd,ddd,4434', ',') from dual;
select regexp_replace('abc,bcd,ddd,4434', ',') from dual;
---X 报表分析精华--Oracle分析函数
--listagg(x,',') within group (order by x)
--将某个字段的多列用逗号(,)连接起来
--同样的方法有wm_concat,但是
select job, listagg(ename, ',') within group(order by ename asc)
from emp
group by job;
---10、instr 字符串位置查找函数
--- 查找分割符的位置，然后截取
--- 从第一个字符开始，检索第二次出现的位置
select instr('zzz,xxx,tt', ',', '1', '2') from dual;
---11、count(*) 当表中没有数据时返回一条数据值为0,当有group by 的时候 没有数据返回
---12、 sum() over (order by x) 按顺序累加
---（如果需要计算累计差,可以将数字转换为负数，然后计算累积和）
select ENAME, SAL, SUM(SAL) OVER(ORDER BY EMPNO) from emp;
---13、分析函数
----按照分组排序获取第一个值或者最后一个值
---- max(ename) keep(dense_rank first order by sal desc) over()
select empno,
ename,
sal,
max(ename) keep(dense_rank first order by sal desc) over(),
max(sal) keep(dense_rank first order by sal desc) over(),
max(ename) keep(dense_rank last order by sal desc) over()
from emp;
---获取分组的最后一个值
select deptno,
max(ename) keep(dense_rank last order by sal desc),
max(sal) keep(dense_rank last order by sal desc),
max(ename) keep(dense_rank last order by sal desc)
from emp group by deptno;
---- lead 获取当前行下一行的数据, lag获取当前行上一行的数据
select ename,
sal,
lead(sal) over(order by sal),
lag(sal) over(order by sal)
from emp
order by sal;
----14、extract 函数返回值为数字，获取时间字段的某一个值
select extract(day from sysdate) from dual;
---- to_char(sysdate,'xxx') d day 1 ww iw ...
---- next_day 1234567 下一个 1 代表周天 2代表周一。。。>
select to_char(sysdate, 'day') from dual;
select next_day(sysdate, 1) from dual;
--月历
select max((case dd
when 2 then
d
end)) d1
from (select to_char(dt, 'iw') weak,
to_char(dt, 'dd') d,
to_number(to_char(dt, 'd')) dd
from (select (trunc(sysdate, 'mm') + level - 1) dt
from dual
connect by level <= 30))
group by weak
order by weak
--rows between 分析函数开窗 (按行)
--range between 按照范围开窗(针对数字和日期列)
select sum(sal) over(order by empno rows between unbounded preceding and 1 preceding)
from emp;
-----15 求余数函数
select mod(34, 4) from dual;
-----16 分页常用伪列 rownum
----- 16.1 先排序，在获取rownum取值
----- 16.2 获取rownum的值后才能按照分页过滤
-----17 SQL动态分割字符串
----- 知道分隔符，但每一个都可能包含多个分割符
---- 针对的是一行数据的结果，
---- level 树形结构查询结构
select regexp_substr(l, '[^,]+', 1, level)
from test6
where id = 1
connect by level <= regexp_count(l, ',');
----18、行转列 pivot 等价与 case when
----带有聚合函数的时候，不要使用俩次或俩次以上的pivot
select *
from (select job, ename, deptno from emp) pivot(count(ename) as c for deptno in(10 as d10,
20 as d20,
30 as d30));
select *
from (select deptno, ename, job from emp) pivot(count(ename) as c for job in('CLERK' as
job_clark,
'SALESMAN' as
job_SALESMAN,
'MANAGER' as
job_MANAGER,
'ANALYST' as
job_ANALYST,
'PRESIDENT' as
job_PRESIDENT));
----19、列转行 unpivot (同样的需求可以用 union all 处理)，要保证转换的列有同样的
----数据类型
-----unpivot include unlls 包含空值
select
from (select
from (select job, ename, deptno from emp) pivot(count(ename) as c for deptno in(10 as d10,
20 as d20,
30 as d30))) unpivot(sal for deptno in(d10_c,
d20_c,
--包含null值 d30_c));
select * from emp unpivot INCLUDE NULLS(salZE for lie in(SAL, COMM));
----20、ceil(rn/5) 返回大于或等于表达式的最小整数
---- ceil 按照5个一组编号,然后在组内排序加序号
---- 然后用序号进行行转列
select *
from (select gp,
ename,
row_number() over(partition by gp order by ename) xh
from (select ceil(rn / 5) gp, ename
from (select rownum rn, ename
from (select ename from emp order by ename)))) pivot(max(ename) as x for xh in(1 as e1,
2 as e2,
----21 ntile(3) over 多数据进行分组,3为分组约定
select ntile(3) over(order by empno),empno,ename from emp where job in ('CLERK', 'MANAGER')
----22 rollup 求统计列的合计值
---- grouping(deptno) 该列被汇总的时候 返回值为1 ，否则返回0
---- 处理分组字段存在空的情况下，与合计行无法区分
select deptno,sum(sal) from emp group by rollup (deptno)
-- emp 按照 deptno, job, empno 分组,同时计算出 deptno, job, empno 的合计、deptno, job的合计
-- deptno 的合计
--grouping 用来区分合计列
select deptno, job, empno, sum(sal), grouping(deptno), grouping(job),grouping(empno)
from emp
group by rollup(deptno, job, empno);
--cube 按照 deptno, job, empno 各种可能组合计算合计，最后加一行总计
--grouping_id deptno,job,empno 三种可能组合合计的分类ID
select deptno, job, empno, sum(sal), grouping_id(deptno,job,empno)
from emp
group by cube(deptno, job, empno);
select deptno,
job,
sum(sal),
grouping_id(deptno, job),
case grouping_id(deptno, job)
when 0 then
'按照部门和工作分组'
when 1 then
'按照部分分组'
when 2 then
'按照工作分组'
when 3 then
'总合计'
end fl
from emp
where emp.deptno is not null
and emp.job is not null
group by cube(deptno, job);
----23 lpad rpad 左右补齐位数
----第二个参数代表字符串
----第二个参数代表期望的长度,不足补齐，超过截取
----第三个参数代表如果字符串长度不够则用这个补齐
select rpad(1,2,',') from dual;
select lpad(1,2,',') from dual;
----24 九九乘法表
with x as(
select level lv from dual connect by level <=9)
, xx as(
select x1.lv lv_a,x2.lv lv_b, to_char(x1.lv) || ' * ' ||
to_char(x2.lv) || ' = ' || to_char(x1.lv * x2.lv) c from x x1,x x2 where x1.lv <= x2.lv)
select lv_b,listagg(c,' ') within group(order by lv_b) from xx group by lv_b; 3 as e3,
4 as e4,
-----25 递归查询
----(PRIOR ename) 获取上一级的信息，可以获取所有列的信息
---- PRIOR 指定按照哪一个字段进行递归
---- connect by PRIOR emp.empno = emp.mgr 找出与本级empno 相等的mgr数据,向下递归
---- connect by emp.empno = PRIOR emp.mgr 找出与本级mgr 相等的empno数据,向上递归
select empno, ename,mgr,(PRIOR ename)
from emp
start with empno = '7902'
connect by PRIOR emp.empno = emp.mgr;
--伪列
--level 层级编码
--connect_by_isleaf 叶子节点标识
select empno, ename,mgr,(PRIOR job),level,connect_by_isleaf
from emp
start with empno = '7902'
connect by PRIOR emp.empno = emp.mgr;
---sys_connect_by_path 可以将层级中的部分字段连接起来(按照层级连接)
--- 3、2、1 连接;2、1连接；1连接
select empno,
ename,
mgr,
(PRIOR job),
level,
connect_by_isleaf,
sys_connect_by_path(ename, ',')
from emp
start with empno = '7902'
connect by emp.empno = PRIOR emp.mgr;
--- order siblings by 树形分支，分别排序,不按照整体结构排序
---无法看清层级结构
select empno, ename,mgr,(PRIOR job),level,connect_by_isleaf
from emp
start with empno = '7566'
connect by PRIOR emp.empno = emp.mgr
order siblings by empno;
--- 树形查询中 where 字段过滤的是查询结果,
--- 所以如果需要树形查询部分数据，必须先过滤，然后作为子查询结构
--- 进行树形查询
select
from (select from emp where deptno = '20')
start with mgr is null
connect by prior empno = mgr;
--如果要过滤一个完整的分支
--需要在connect by prior 后加入过滤语句
--不能在where中加入
select *
from emp
start with mgr is null
connect by prior empno = mgr
and empno !='7566';
----26、取各个分组的 最大最小 第一行 最后一行
select job,
first_value(ename) over(partition by job order by sal desc),
max(ename) keep(dense_rank first order by sal desc) over(partition by job),
last_value(ename) over(partition by job order by sal desc),
max(ename) keep(dense_rank last order by sal desc) over(partition by job)
from emp;
Oracle 在执行SQL语句时，有两种优化方法：即基于规则的RBO和基于代价的CBO。 在SQL执教的时候，到底采用何种优化方法，就由Oracle参数 optimizer_mode 来决定。
SQL> show parameter optimizer_mode
PL/SQL F5 根据不同的选择应用不同的优化方法
来源：https://www.cnblogs.com/zhouxiangting/p/10784583.html