中文字幕亚洲不卡在线亚瑟,国产亚洲精品AAAA片APP,亚洲国产成人精品无码一区二区

Oracle SQL培训�W�记[开发�h员][三]

Anemone — Mon, 01 Oct 2007 04:48:00 GMT

�?索引与分��?-怎么样SQL�q�行的更�?

正确的��用烦�?/strong>
Where条�g落在索引�?br /> 不要在where�?前��用函敎ͼ�否则无法使用索引
Is Null可能无法使用索引
不正��的隐式转换可能不能使用索引
如果能在索引获得数据�Q�就不要回表
如果是复合烦引，注意�W?个字�D�以后，可能使用不到索引
正确的��用hint
如果有别名，一定要有别�?br /> 格式�?*+ index(t index_name) */
无需回表查询的分��写�?/em>
存在以下表T1(A,B,C,D) T1上有索引字段(B,C) .如果只是查B,C两个字段�?
select *
  from (select tt.b, tt.c, rownum as rn
          from (select t.b, t.c from t1 t where c = 2 order by t.c) tt
         where rownum < 3)
where rn > 1
需回表查询的分��写�?/strong>

select /*+ ordered use_nl(t,t1) */
      *
       from (select rid from (
          select rownum rn,rid from (
            select rowid rid from t1
             where c=2
             order by c desc)
          where rownum <= 50)
       where rn >=1) t,
       t1
where t.rid=t1.rowid;

Anemone 2007-10-01 12:48 发表评论

Oracle SQL培训�W�记[开发�h员][二]

Anemone — Mon, 01 Oct 2007 04:25:00 GMT

�?null的那些事

在order 中，��单把null认�ؓ是最�?/li>
与null的运��，�q�回null
SQL> select 1 + null from dual;

    1+NULL
----------

与null的字�W�串合�ƈ�Q�忽略null
SQL> select 'Hi'||null from dual;

'HI'||NULL
----------
Hi

Null的查询�ؓis null

Count(field)�Q�不包括null

如果索引条目全�ؓnull�Q�则索引不记录null

In/not in与null

Exists/not exists与null
SQL> select * from t1;

A          B
---------- ----------
1          1
2
3

SQL> select * from t2;

A          B
---------- ----------
1          1
2

SQL> select * from t1 where b in (select B from t2);

A          B
---------- ----------
1          1

SQL> select * from t1 where b not in (select B from t2);

A          B
---------- ----------

SQL> select * from t1 where exists (select * from t2 where t2.b = t1.b);

A          B
---------- ----------
1          1

SQL> select * from t1 where not exists (select * from t2 where t2.b = t1.b);

A          B
---------- ----------
3
2

exists主要用于片面�?有满��一个条件的卛_��,  所以速度快很�?    in 主要用于具体的集合操�? 有多��满��x��?

Anemone 2007-10-01 12:25 发表评论

Oracle SQL培训�W�记[开发�h员][一]

Anemone — Mon, 01 Oct 2007 04:09:00 GMT

�q�日公司的Oracle牛�h�l�我们开发�h员做了一�ơ有关Oracle的培�?感觉收获颇大,故记录下�?好他日温习之�?

一常用的SQL语句

select name,count(*) from table where .. group by ... 中能查询的字�D�只能�ؓgroup by的字�D?

select * from table where rownum < 5 order by id 中查询出来的�l�果不是按数据中的ID排序�?而只是将select * from table where rownum < 5 的结果集按ID排序,所以如果你要按ID排序,你需要用子查询实�?
select * from ( select * from table order by id ) where rownum < 5

select * from table where name like 'A\_%' escape '\';��?\'后面的字�W�不当关键字来处�?�q�个字符可以自定�?

insert into test(id,name) values(9,'It''s life'); or ||chr(39)|| 如果你想插入'可以使用''或者||chr(39)||方式插入.

如果你想��T1中B更新为T2中的B�?千万要注意限定T1的范�?否则T1的全部列��会更新,如update t1 t set t.B = (select tt.B from t2 tt where tt.A = t.A)��会t1中所有列都更�?如果t2中不存在对应�?则t1中的值则为NULL,所以应该将以上语句攚w��成update t1 t set t.B = (select tt.B from t2 tt where tt.A = t.A) where t.A in (select A from t2)

number(5,2):如果�?insert into test values(123.235)�q�行插入�?��会使用四舍五入的方式插入即��gؓ123.24;如果是insert into test values(12345)则无法插入数�?

�?Oracle 函数

一般函数是数据库设定的字符集来计算,现在一般的oracle都是16�?所以一个汉字长度�ؓ1,而函数后面加b则按字节来计��如:length('中国')=2 lenghtb('中国')=4 .
Substr与substrb 字符串截取函�?负数代表从右开始截�?
SQL> select substr('我是中国�?/span>',2) from dual;

SUBSTR('我是中国�?/span>',2)
----------------------
是中国�h

SQL> select substrb('我是中国�?/span>',2) from dual;

SUBSTRB('我是中国�?/span>',2)
-----------------------
是中国�h

SQL> select substr('我是中国�?/span>',-2) from dual;

SUBSTR('我是中国�?/span>',-2)
-----------------------
国�h

SQL> select substrb('我是中国�?/span>',-2) from dual;

SUBSTRB('我是中国�?/span>',-2)
------------------------
�?/span>

Length与lengthb 长度计算函数

SQL> select length('我是中国�?/span>') from dual;

LENGTH('我是中国�?/span>')
--------------------
                   5

SQL> select lengthb('我是中国�?/span>') from dual;

LENGTHB('我是中国�?/span>')
---------------------
                   10
Instr与Instrb 字符串查扑և��?instr(原字�W�串,查的字符�?起始位置,�W�几个匹�? �q�回字符串位�|?找不到返�? .

SQL> select Instr('abcabcdabcdef','a',1,3) from dual;

INSTR('ABCABCDABCDEF','A',1,3)
------------------------------
                             8

Upper与lower 大小写�{换函�?
SQL> select upper('AaBbCc') from dual;

UPPER('AABBCC')
---------------
AABBCC

SQL> select lower('AaBbCc') from dual;

LOWER('AABBCC')
---------------
aabbcc

Trim/Rtrim/Ltrim 字符串trim函数
SQL> select trim(' A B ') from dual;

TRIM('AB')
----------
A B

SQL> select rtrim('xABx','x') from dual;

RTRIM('XABX','X')
-----------------
xAB

SQL> select ltrim('xABx','x') from dual;

LTRIM('XABX','X')
-----------------
ABx

Trunc 截取函数(不进行四舍五�?

SQL> select trunc(1234.123456,'-2') from dual;

TRUNC(1234.123456,'-2')
-----------------------
                   1200

SQL> select trunc(1234.123456,'2') from dual;

TRUNC(1234.123456,'2')
----------------------
               1234.12

SQL> select trunc(1234.123456,'4') from dual;

TRUNC(1234.123456,'4')
----------------------
             1234.1234

SQL> select trunc(1234.123456,'5') from dual;

TRUNC(1234.123456,'5')
----------------------
            1234.12345

SQL> select trunc(sysdate,'yy') from dual;

TRUNC(SYSDATE,'YY')
-------------------
2007-01-01

SQL> select trunc(sysdate,'mi') from dual;

TRUNC(SYSDATE,'MI')
-------------------
2007-10-01 11:55:00

SQL> select trunc(sysdate,'dd') from dual;

TRUNC(SYSDATE,'DD')
-------------------
2007-10-01

SQL> select trunc(sysdate,'day') from dual;

TRUNC(SYSDATE,'DAY')
--------------------
2007-09-30
Next_day与last_day
SQL> select sysdate from dual;

SYSDATE
-----------
2007-10-01

SQL> select next_day(sysdate,'星期一') from dual;

NEXT_DAY(SYSDATE,'星期一')
--------------------------
2007-10-08 11:57:29

SQL> select next_day(sysdate,1) from dual;

NEXT_DAY(SYSDATE,1)
-------------------
2007-10-07 11:57:42

SQL> select next_day(sysdate,2) from dual;

NEXT_DAY(SYSDATE,2)
-------------------
2007-10-08 11:57:56

SQL> select last_day(sysdate) from dual;

LAST_DAY(SYSDATE)
-----------------
2007-10-31 12:00:
Round 四舍五入函数
SQL> select round(123.456,2) from dual;

ROUND(123.456,2)
----------------
          123.46

SQL> select round(123.456,-2) from dual;

ROUND(123.456,-2)
-----------------
              100

SQL> select round(123.456,-1) from dual;

ROUND(123.456,-1)
-----------------
              120

Ceil与floor 取整函数
SQL> select ceil(1.1) from dual;

CEIL(1.1)
----------
         2

SQL> select floor(9.9) from dual;

FLOOR(9.9)
----------
         9
Decode与nvl Decode相当于一个三元运��函�?nvl 如果��gؓ�I�时默认�?

Anemone 2007-10-01 12:09 发表评论

在SQL语句中��用CASE语句(OR9I中通过)

Anemone — Sat, 22 Sep 2007 12:41:00 GMT

select
'private String'||
(case
when t.type='3' then '[]'
when t.type='4' then '[]'
else
''
end)
||' '||t.name,t.wtd_name_id from jc_wtd_fields t order by t.wtd_name_id,t.name

Anemone 2007-09-22 20:41 发表评论