（3 ）Postgresql 中的 || 用 法与其他数据库不同：
select a|| b from table1;
当a 或b 其中一个为null 时， 该查询返回null ，

（4 ）Postgresql 中没有concat 函数，且由于|| 用法的问题，无法使用|| 替换，解决 方法为在public schema 中创建函数concat

create or replace function concat(text,text)
returns text as
$body$select coalesce($1,'') || coalesce($2,'')$body$
language 'sql' volatile;
alter function concat(text,text) owner to postgres;

-- 无需特殊授权即可在其他schema 中使用

（4 ）Postgresql 中没有dual 虚拟表，为保 证程序兼容性，可创建伪视图（view ）替代：

CREATE OR REPLACE VIEW dual AS
SELECT NULL::"unknown"
WHERE 1 = 1;

ALTER TABLE dual OWNER TO postgres;
GRANT ALL ON TABLE dual TO postgres;
GRANT SELECT ON TABLE dual TO public;

必须授权public 以select 权 限

（5 ）关联查询用法区别

ORACLE:

简单外连接：
SELECT COUNT(DISTINCT(A.COL1)) AS RCOUNT FROM
SCHEMA.PREFIX_TABLE1 A,SCHEMA.PREFIX_TABLE2 B
WHERE 1 = 1
AND A.COL2 = B.COL2(+)
AND A.COL3 > 0
AND A.COL4 = '1'

超级变态外连接：
SELECT COUNT(DISTINCT(A.COL1)) AS RCOUNT FROM
SCHEMA.PREFIX_TABLE1 A,SCHEMA.PREFIX_TABLE2 B,SCHEMA.PREFIX_TABLE3 C,SCHEMA.PREFIX_TABLE4 D
WHERE 1 = 1
AND A.COL2 = B.COL2
AND A.COL3 = C.COL3(+)
AND A.COL4 = D.COL4(+)
AND A.COL5 > 0
AND A.COL6 = '1'

POSTGREsql:

简单外连接：
select count(distinct(a.col1)) as rcount from
schema.prefix_table1 a left outer join schema.prefix_table2 b on (a.col2 = b.col2)
where 1 = 1
and a.col3 > 0
and a.col4 = '1'

超级变态外连接：
select count(distinct(a.col1)) as rcount from
schema.prefix_table1 a inner join schema.prefix_table2 b on (a.col2 = b.col2)
left outer join schema.prefix_table3 c on (a.col3 = c.col3)
left outer join schema.prefix_table4 d on (a.col4 = d.col4)
where 1 = 1
and a.col5 > 0
and a.col6 = '1'

（6 ）Postgresql 中子查询较为规范，子查询结果集必须拥有alias

ORACLE:
SELECT * FROM (
SELECT * FROM (
SELECT * FROM SCHEMA.PREFIX_TABLE ORDER BY COL1
) WHERE X=1 ORDER BY COL2
) WHERE Y=2 ORDER BY COL3

POSTGREsql:

SELECT * FROM (
SELECT * FROM (
SELECT * FROM SCHEMA.PREFIX_TABLE ORDER BY COL1 ALIAS1
) WHERE X=1 ORDER BY COL2 ALIAS2
) WHERE Y=2 ORDER BY COL3

（7 ） Postgresql 中没有rownum ，无法 使用where rownum < = X 的方法进行分页，取而代之的是limit X ，offset Y 方法, 而ORACLE 中不允许使用LIMIT X 的 方法
ORACLE:

SELECT * FROM ( SELECT * FROM (SELECT * FROM SCHEMA.PREFIX_TABLE1 ORDER BY COL1 DESC,COL2 ASC) where ROWNUM <= 50 ORDER BY COL3 ASC,COL4 DESC)
WHERE ROWNUM <= 20 ORDER BY COL5 DESC,COL6 ASC;

POSTGRES:

select * from ( select * from (SELECT * FROM SCHEMA.PREFIX_TABLE1 ORDER BY COL1 DESC,COL2 ASC) selb order by col3 asc,col4 desc limit 50 ) sela
order by col5 desc,col6 asc limit 20;

注意！！limit 必须用于order by 之后

例：取1 到50 条数据

select * from VOIP_FEE_RATE temp offset 0 limit 50

（8 ）序列使用的区别

ORACLE:
SELECT SCHEMA.PREFIX_TABLE1_SEQUENCE.NEXTVAL AS nCode FROM DUAL

POSTGRES:
SELECT NEXTVAL('SCHEMA.PREFIX_TABLE1_SEQUENCE') AS nCode FROM DUAL
-- 注意，此方法前提是dual 视图已建立，如没有，可省略FROM DUAL

（9 ）字段取别名必须用as
Oracle -- SELECT A.COL1 A_COL1 ，A.COL2 A_COL2 FROM A_TABLE A
Postgresql---- SELECT A.COL1 AS A_COL1 ，A.COL2 AS A_COL2 FROM A_TABLE A

（10 ）NVL 用法
Oracle --SELECT NVL(SUM(VALUE11),0) FS_VALUE1,NVL(SUM(VALUE21),0) FS_VALUE2 FROM FIELD_SUM
Postgresql--SELECT COALESCE(SUM(VALUE11),0) AS FS_VALUE1,COALESCE(SUM(VALUE21),0) AS FS_VALUE2
FROM FIELD_SUM

（11 ）TO_NUMBER 用法
Oracle -- SELECT COL1 FROM A_TABLE ORDER BY TO_NUMBER(COL1)
Postgresql- select TO_NUMBER(COL1,'99G999D9S') from A_TABLE [ 注：'999999' ---- 6 位数为COL1 字段的长度]

（12 ）DECODE 用法
Oracle -- SELECT DECODE(ENDFLAG,'1','A','B') ENDFLAGFROM TEST
Postgresql- SELECT (CASEENDFLAG WHEN '1' THEN 'A'ELSE 'B' END) AS ENDFLAG FROM TEST

（13 ）统计 相关 用法
Oracle -- SELECT ROUND(AVG(SUM(BASICCNT1))) BASICCNT FROM ACCESS_INFO_SUM1_V
WHERE YEARCODE BETWEEN '200305' AND '200505' GROUP BY SCCODE

Postgresql-- SELECT ROUND(AVG(AIV.BASICCNT)) AS BASICCNT FROM ( SELECT SUM(BASICCNT1) AS BASICCNT
FROM ACCESS_INFO_SUM1_V WHERE YEARCODE BETWEEN '200305' AND '200505' GROUP BY sccode ) AIV

（14 ）时间计算 用法
Oracle -- SELECT CEIL(SYSDATE - TO_DATE('20051027 14:56:10','YYYYMMDD HH24:MI:SS')) AS DAYS
FROM DUAL
Postgresql-- SELECT EXTRACT(DAY FROM (TO_TIMESTAMP(CURRENT_TIMESTAMP,'YYYY-MM-DD-HH24-MI-SS') - TO_TIMESTAMP('2005-10-27 14:56:10','YYYY-MM-DD-HH24-MI-SS') ))+1 AS DAYS FROM DUAL
Oracle -- add_months(date,int)
Postgresql-- 创建函数来解决
CREATE FUNCTION add_months(date,int)
RETURNS date AS
'SELECT ($1 + ( $2::text || ''months'')::interval)::date;'
LANGUAGE 'sql' 、

（15）关于事务

在Postgresql中，默认情况下，一条sql语句是一个独立的事务。在一条sql语句开始执行时，Postgresql自动为其创建一个事务。如果语句执行成功，Postgresql自动提交该语句的事务，否则回滚事务。