如果是用主键primary或者唯一索引unique区分了记录的唯一性,避免重复插入记录可以使用：

代码如下:

 
这样当有重复记录就会忽略,执行后返回数字0

还有个应用就是复制表,避免重复记录： 
代码如下:

 
方案二：使用Replace

语法格式：

代码如下:

...算法说明：

REPLACE的运行与INSERT很相像,但是如果旧记录与新记录有相同的值，则在新记录被插入之前，旧记录被删除，即：

尝试把新行插入到表中

当因为对于主键或唯一关键字出现重复关键字错误而造成插入失败时： 
从表中删除含有重复关键字值的冲突行 
再次尝试把新行插入到表中 
旧记录与新记录有相同的值的判断标准就是：
表有一个PRIMARY KEY或UNIQUE索引，否则，使用一个REPLACE语句没有意义。该语句会与INSERT相同，因为没有索引被用于确定是否新行复制了其它的行。 
返回值：
REPLACE语句会返回一个数，来指示受影响的行的数目。该数是被删除和被插入的行数的和
受影响的行数可以容易地确定是否REPLACE只添加了一行，或者是否REPLACE也替换了其它行：检查该数是否为1（添加）或更大（替换）。 
示例:
# eg:(phone字段为唯一索引)

代码如下:

另外,在 SQL Server 中可以这样处理：

代码如下:

方案三：ON DUPLICATE KEY UPDATE

如‍上所写，你也可以在INSERT INTO…..后面加上 ON DUPLICATE KEY UPDATE方法来实现。如果您指定了ON DUPLICATE KEY UPDATE，并且插入行后会导致在一个UNIQUE索引或PRIMARY KEY中出现重复值，则执行旧行UPDATE。 
例如，如果列a被定义为UNIQUE，并且包含值1，则以下两个语句具有相同的效果：

代码如下:

如果行作为新记录被插入，则受影响行的值为1；如果原有的记录被更新，则受影响行的值为2。

注释：如果列b也是唯一列，则INSERT与此UPDATE语句相当： 
代码如下:

如果a=1 OR b=2与多个行向匹配，则只有一个行被更新。通常，您应该尽量避免对带有多个唯一关键字的表使用ON DUPLICATE KEY子句。

您可以在UPDATE子句中使用VALUES(col_name)函数从INSERT…UPDATE语句的INSERT部分引用列值。换句话说，如果没有发生重复关键字冲突，则UPDATE子句中的VALUES(col_name)可以引用被插入的col_name的值。本函数特别适用于多行插入。VALUES()函数只在INSERT…UPDATE语句中有意义，其它时候会返回NULL。

代码如下:

本语句与以下两个语句作用相同：

代码如下:

注释：当您使用ON DUPLICATE KEY UPDATE时，DELAYED选项被忽略。

示例：

这个例子是我在实际项目中用到的：是将一个表的数据导入到另外一个表中，数据的重复性就得考虑(如下)，唯一索引为：email：

代码如下:

再贴一个例子：

代码如下:

其它关键：DELAYED  做为快速插入，并不是很关心失效性，提高插入性能。 
IGNORE  只关注主键对应记录是不存在，无则添加，有则忽略。

php防止重复插入记录实例

附一些删除重复记录的方法

查询及删除重复记录的SQL语句 
1、查找表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断

2、删除表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断，只留有rowid最小的记录

3、查找表中多余的重复记录（多个字段）

4、删除表中多余的重复记录（多个字段），只留有rowid最小的记录

5、查找表中多余的重复记录（多个字段），不包含rowid最小的记录

第二章:表空间管理
1.create tablespaces   创建表空间
sql> create tablespace tablespace_name datafile 'c:\oracle\oradata\file1.dbf' size 100m,
sql> 'c:\oracle\oradata\file2.dbf' size 100m minimum extent 550k [logging/nologging]
sql> default storage (initial 500k next 500k maxextents 500 pctinccease 0)
sql> [online/offline] [permanent/temporary] [extent_management_clause]
2.locally managed tablespace    本地管理表空间
sql> create tablespace user_data datafile 'c:\oracle\oradata\user_data01.dbf'
sql> size 500m extent management local uniform size 10m;
3.temporary tablespace    临时表空间
sql> create temporary tablespace temp tempfile 'c:\oracle\oradata\temp01.dbf'
sql> size 500m extent management local uniform size 10m;
4.change the storage setting    改变存储设置
sql> alter tablespace app_data minimum extent 2m;
sql> alter tablespace app_data default storage(initial 2m next 2m maxextents 999);
5.taking tablespace offline or online    同时表空间离线或在线
sql> alter tablespace app_data offline;
sql> alter tablespace app_data online;
6.read_only tablespace    只读表空间
sql> alter tablespace app_data read only|write;
7.droping tablespace    删除表空间
sql> drop tablespace app_data including contents;
8.enableing automatic extension of data files   enableing自动延长数据档案
sql> alter tablespace app_data add datafile 'c:\oracle\oradata\app_data01.dbf' size 200m
sql> autoextend on next 10m maxsize 500m;
9.change the size fo data files manually  尺寸变化的数据档案手动
sql> alter database datafile 'c:\oracle\oradata\app_data.dbf' resize 200m;
10.Moving data files: alter tablespace  移动数据档案:改变表空间
sql> alter tablespace app_data rename datafile 'c:\oracle\oradata\app_data.dbf'
sql> to 'c:\oracle\app_data.dbf';
11.moving data files:alter database   移动数据档案:改变数据库
sql> alter database rename file 'c:\oracle\oradata\app_data.dbf'
sql> to 'c:\oracle\app_data.dbf';

第三章:表
1.create a table   新建表
sql> create table table_name (column datatype,column datatype]....)
sql> tablespace tablespace_name [pctfree integer] [pctused integer]
sql> [initrans integer] [maxtrans integer]
sql> storage(initial 200k next 200k pctincrease 0 maxextents 50)
sql> [logging|nologging] [cache|nocache]
2.copy an existing table   复制一个现有的表
sql> create table table_name [logging|nologging] as subquery
3.create temporary table   创建临时表
sql> create global temporary table xay_temp as select * from xay;
on commit preserve rows/on commit delete rows
4.pctfree = (average row size - initial row size) *100 /average row size
pctused = 100-pctfree- (average row size*100/available data space)
5.change storage and block utilization parameter  改变存储和利用块参数
sql> alter table table_name pctfree=30 pctused=50 storage(next 500k
sql> minextents 2 maxextents 100);
6.manually allocating extents
sql> alter table table_name allocate extent(size 500k datafile 'c:/oracle/data.dbf');
7.move tablespace   移动表空间
sql> alter table employee move tablespace users;
8.deallocate of unused space  deallocate的闲置空间
sql> alter table table_name deallocate unused [keep integer]
9.truncate a table   截断表
sql> truncate table table_name;
10.drop a table  删除表
sql> drop table table_name [cascade constraints];
11.drop a column  删除列
sql> alter table table_name drop column comments cascade constraints checkpoint 1000;
alter table table_name drop columns continue;
12.mark a column as unused
sql> alter table table_name set unused column comments cascade constraints;
alter table table_name drop unused columns checkpoint 1000;
alter table orders drop columns continue checkpoint 1000
data_dictionary : dba_unused_col_tabs

　　第四章:索引
1.creating function-based indexes  建立基于功能的索引
sql> create index summit.item_quantity on summit.item(quantity-quantity_shipped);
2.create a B-tree index    创造一个B树索引
sql> create [unique] index index_name on table_name(column,.. asc/desc) tablespace
sql> tablespace_name [pctfree integer] [initrans integer] [maxtrans integer]
sql> [logging | nologging] [nosort] storage(initial 200k next 200k pctincrease 0
sql> maxextents 50);
3.pctfree(index)=(maximum number of rows-initial number of rows)*100/maximum number of rows
4.creating reverse key indexes   创造逆转关键指标
sql> create unique index xay_id on xay(a) reverse pctfree 30 storage(initial 200k
sql> next 200k pctincrease 0 maxextents 50) tablespace indx;
5.create bitmap index   创建位图索引
sql> create bitmap index xay_id on xay(a) pctfree 30 storage( initial 200k next 200k
sql> pctincrease 0 maxextents 50) tablespace indx;
6.change storage parameter of index   改变存储参数指标
sql> alter index xay_id storage (next 400k maxextents 100);
7.allocating index space    分配索引空间
sql> alter index xay_id allocate extent(size 200k datafile 'c:/oracle/index.dbf');
8.alter index xay_id deallocate unused;    改变指数xay_id deallocate剩

第五章:约束
1.define constraints as immediate or deferred     界定制约即期或延期
sql> alter session set constraint[s] = immediate/deferred/default;
set constraint[s] constraint_name/all immediate/deferred;
2. sql> drop table table_name cascade constraints
sql> drop tablespace tablespace_name including contents cascade constraints
3. define constraints while create a table    界定限制,同时创建一个表
sql> create table xay(id number(7) constraint xay_id primary key deferrable
sql> using index storage(initial 100k next 100k) tablespace indx);
primary key/unique/references table(column)/check
4.enable constraints    使制约
sql> alter table xay enable novalidate constraint xay_id;
5.enable constraints     使制约
sql> alter table xay enable validate constraint xay_id;

第六章:LOAD数据
1.loading data using direct_load insert
sql> insert /*+append */ into emp nologging
sql> select * from emp_old;
2.parallel direct-load insert
sql> alter session enable parallel dml;
sql> insert /*+parallel(emp,2) */ into emp nologging
sql> select * from emp_old;
3.using sql*loader
sql> sqlldr scott/tiger \
sql> control = ulcase6.ctl \
sql> log = ulcase6.log direct=true

第七章:数据整理
1.using expoty
$exp scott/tiger tables(dept,emp) file=c:\emp.dmp log=exp.log compress=n direct=y
2.using import
$imp scott/tiger tables(dept,emp) file=emp.dmp log=imp.log ignore=y
3.transporting a tablespace
sql>alter tablespace sales_ts read only;
$exp sys/.. file=xay.dmp transport_tablespace=y tablespace=sales_ts
triggers=n constraints=n
$copy datafile
$imp sys/.. file=xay.dmp transport_tablespace=y datafiles=(/disk1/sles01.dbf,/disk2
/sles02.dbf)
sql> alter tablespace sales_ts read write;
4.checking transport set
sql> DBMS_tts.transport_set_check(ts_list =>'sales_ts' ..,incl_constraints=>true);
在表transport_set_violations 中查看
sql> dbms_tts.isselfcontained 为true 是， 表示自包含
第八章: 密码安全与资源管理
1.controlling account lock and password
sql> alter user juncky identified by oracle account unlock;
2.user_provided password function
sql> function_name(userid in varchar2(30),password in varchar2(30),
old_password in varchar2(30)) return boolean
3.create a profile : password setting
sql> create profile grace_5 limit failed_login_attempts 3
sql> password_lock_time unlimited password_life_time 30
sql>password_reuse_time 30 password_verify_function verify_function
sql> password_grace_time 5;
4.altering a profile
sql> alter profile default failed_login_attempts 3
sql> password_life_time 60 password_grace_time 10;
5.drop a profile
sql> drop profile grace_5 [cascade];
6.create a profile : resource limit
sql> create profile developer_prof limit sessions_per_user 2
sql> cpu_per_session 10000 idle_time 60 connect_time 480;
7. view => resource_cost : alter resource cost
dba_Users,dba_profiles
8. enable resource limits
sql> alter system set resource_limit=true;
第九章:用户管理
1.create a user: database authentication
sql> create user juncky identified by oracle default tablespace users
sql> temporary tablespace temp quota 10m on data password expire
sql> [account lock|unlock] [profile profilename|default];
2.change user quota on tablespace
sql> alter user juncky quota 0 on users;
3.drop a user
sql> drop user juncky [cascade];
4. monitor user
view: dba_users , dba_ts_quotas
第十章:特权管理
1.system privileges: view => system_privilege_map ,dba_sys_privs,session_privs
2.grant system privilege
sql> grant create session,create table to managers;
sql> grant create session to scott with admin option;
with admin option can grant or revoke privilege from any user or role;
3.sysdba and sysoper privileges:
sysoper: startup,shutdown,alter database open|mount,alter database backup controlfile,
alter tablespace begin/end backup,recover database
alter database archivelog,restricted session
sysdba: sysoper privileges with admin option,create database,recover database until
4.password file members: view:=> v$pwfile_users
5.O7_dictionary_accessibility =true restriction access to view or tables in other schema
6.revoke system privilege
sql> revoke create table from karen;
sql> revoke create session from scott;
7.grant object privilege
sql> grant execute on dbms_pipe to public;
sql> grant update(first_name,salary) on employee to karen with grant option;
8.display object privilege : view => dba_tab_privs, dba_col_privs
9.revoke object privilege
sql> revoke execute on dbms_pipe from scott [cascade constraints];
10.audit record view :=> sys.aud$
11. protecting the audit trail
sql> audit delete on sys.aud$ by access;
12.statement auditing
sql> audit user;
13.privilege auditing
sql> audit select any table by summit by access;
14.schema object auditing
sql> audit lock on summit.employee by access whenever successful;
15.view audit option : view=> all_def_audit_opts,dba_stmt_audit_opts,dba_priv_audit_opts,dba_obj_audit_opts
16.view audit result: view=> dba_audit_trail,dba_audit_exists,dba_audit_object,dba_audit_session,dba_audit_statement
第十一章: 规则管理器
1.create roles
sql> create role sales_clerk;
sql> create role hr_clerk identified by bonus;
sql> create role hr_manager identified externally;
2.modify role
sql> alter role sales_clerk identified by commission;
sql> alter role hr_clerk identified externally;
sql> alter role hr_manager not identified;
3.assigning roles
sql> grant sales_clerk to scott;
sql> grant hr_clerk to hr_manager;
sql> grant hr_manager to scott with admin option;
4.establish default role
sql> alter user scott default role hr_clerk,sales_clerk;
sql> alter user scott default role all;
sql> alter user scott default role all except hr_clerk;
sql> alter user scott default role none;
5.enable and disable roles
sql> set role hr_clerk;
sql> set role sales_clerk identified by commission;
sql> set role all except sales_clerk;
sql> set role none;
6.remove role from user
sql> revoke sales_clerk from scott;
sql> revoke hr_manager from public;
7.remove role
sql> drop role hr_manager;
8.display role information
view: =>dba_roles,dba_role_privs,role_role_privs,dba_sys_privs,role_sys_privs,role_tab_privs,session_roles
第十二章: 备份与恢复
1. v$sga,v$instance,v$process,v$bgprocess,v$database,v$datafile,v$sgastat
2. Rman need set dbwr_IO_slaves or backup_tape_IO_slaves and large_pool_size
3. Monitoring Parallel Rollback
> v$fast_start_servers , v$fast_start_transactions
4.perform a closed database backup (noarchivelog)
> shutdown immediate
> cp files /backup/
> startup
5.restore to a different location
> connect system/manager as sysdba
> startup mount
> alter database rename file '/disk1/../user.dbf' to '/disk2/../user.dbf';
> alter database open;
6.recover syntax
--recover a mounted database
>recover database;
>recover datafile '/disk1/data/df2.dbf';
>alter database recover database;
--recover an opened database
>recover tablespace user_data;
>recover datafile 2;
>alter database recover datafile 2;
7.how to apply redo log files automatically
>set autorecovery on
>recover automatic datafile 4;
8.complete recovery:
--method 1(mounted databae)
>copy c:\backup\user.dbf c:\oradata\user.dbf
>startup mount
>recover datafile 'c:\oradata\user.dbf;
>alter database open;
--method 2(opened database,initially opened,not system or rollback datafile)
>copy c:\backup\user.dbf c:\oradata\user.dbf (alter tablespace offline)
>recover datafile 'c:\oradata\user.dbf' or
>recover tablespace user_data;
>alter database datafile 'c:\oradata\user.dbf' online or
>alter tablespace user_data online;
--method 3(opened database,initially closed not system or rollback datafile)
>startup mount
>alter database datafile 'c:\oradata\user.dbf' offline;
>alter database open
>copy c:\backup\user.dbf d:\oradata\user.dbf
>alter database rename file 'c:\oradata\user.dbf' to 'd:\oradata\user.dbf'
>recover datafile 'e:\oradata\user.dbf' or recover tablespace user_data;
>alter tablespace user_data online;
--method 4(loss of data file with no backup and have all archive log)
>alter tablespace user_data offline immediate;
>alter database create datafile 'd:\oradata\user.dbf' as 'c:\oradata\user.dbf''
>recover tablespace user_data;
>alter tablespace user_data online
5.perform an open database backup
> alter tablespace user_data begin backup;
> copy files /backup/
> alter database datafile '/c:/../data.dbf' end backup;
> alter system switch logfile;
6.backup a control file
> alter database backup controlfile to 'control1.bkp';
> alter database backup controlfile to trace;
7.recovery (noarchivelog mode)
> shutdown abort
> cp files
> startup
8.recovery of file in backup mode
>alter database datafile 2 end backup;
9.clearing redo log file
>alter database clear unarchived logfile group 1;
>alter database clear unarchived logfile group 1 unrecoverable datafile;
10.redo log recovery
>alter database add logfile group 3 'c:\oradata\redo03.log' size 1000k;
>alter database drop logfile group 1;
>alter database open;
or >cp c:\oradata\redo02.log' c:\oradata\redo01.log
>alter database clear logfile 'c:\oradata\log01.log';

如在ms   sqlserver   用此语句：  
  select   top   2   *   from   test01   就会只显示前2条记录，  
   
 

MYSQL

select   *   from   your_table   where   ....   limit   2;  
   
  使用limit就可以了.

Oracle的语句: 
select * from (select rownum r ,* from test) tt 
where tt.r > 50 and tt.r <= 100;  (最好用PLSQL游标来解决)

DB2中： 
select * from payment fetch first 5 row only --查前5条记录 

MySQL

 
select * from Cat limit ?,?  
select * from Cat limit 20000,100  
从Cat表中第20000条开始取出100条记录，即记录号为20000到20099的记录  
  
select * from Cat limit ?  
select * from Cat limit 100  
从Cat表中取出记录号为1到100的记录  
  
select * from Cat order by rand() limit ?  
select * from Cat order by rand() limit 100  
从Cat表中随机取出100条记录  

Oracle

 
select * from (select *, rownum rownum_ from Cat where rownum <= ?) where rownum_ > ?  
select * from (select *, rownum rownum_ from Cat where rownum <= 20099) where rownum_ > 19999  
从Cat表中第20000条开始取出100条记录，即记录号为20000到20099的记录  
  
select * from Cat where rownum <= ?  
select * from Cat where rownum <= 100  
从Cat表中取出记录号为1到100的记录  
  
select * from (select * from Cat order by dbms_random.value) where rownum < ?  
select * from (select * from Cat order by dbms_random.value) where rownum < 100  
从Cat表中随机取出100条记录  

HSQLDB

select * from Cat limit ? ?  
select * from Cat limit 20000 100  
从Cat表中第20000条开始取出100条记录，即记录号为20000到20099的记录  
  
select * from Cat top ?  
select * from Cat top 100  
从Cat表中取出记录号为1到100的记录