PostgreSQL和Oracle的sql差异

1.rownum

（1）Oracle分页查询使用rownum，PostgreSQL使用limit offset

ORACLE	POSTGRESQL
select * from (select rownum r,e.* from emp e where rownum <=5) t where r>0;	select * from emp limit 5 offset 0;

（2）Oracle中rownum=1，PostgreSQL中使用limit 1

ORACLE	POSTGRESQL
select * from emp where rownum = 1;	select * from emp limit 1;

（3）Oracle中序号列rownum，PostgreSQL使用窗口函数

ORACLE	POSTGRESQL
select rownum,t.* from emp t;	select row_number() over(), t.* from emp t;

2.系统日期

ORACLE	POSTGRESQL
SYSDATE	current_timestamp, current_date

3.delete语句

Oracle delete语句可以没有from，pg必须要有from

ORACLE	POSTGRESQL
delete from emp where empno = xxx; delete emp where empno = xxx	delete from emp where empno = xxx

4.类型自动转换

Oracle支持类型自动转换，例如数字自动换换为字符串等；PG中需要显示转换，或者添加CAST

5.子查询别名

PostgreSQL在from关键字后的子查询必须要有别名，Oralce可以没有。

6. group by having

PG having语句必须在group by之后，oracle可以在group by之前

7.递归查询

Oracle中使用start with ... connect by..., PG中使用with recusive

ORACLE	POSTGRESQL
select * from larearrelation where rearedgens = 1 and rearflag = 1 and rearlevel = '01' connect by prior agentcode = rearagentcode start with rearagentcode = '10032226';	with recursive rs as ( select * from larearrelation where rearagentcode = '10032226' union all select a. from larearrelation a, rs where a.rearagentcode = rs.agentcode ) select * from rs where rearedgens = 1 and rearflag = '1' and rearlevel = '01'

8.update语句别名

postgresql中update语句时，set的字段不能有别名

ORACLE	POSTGRESQL
update emp t set t.name = 'xxx' where t.empno = 2	update emp set name = 'xxx' where empno = 2

9. 日期相减

oracle日期相减自动转换为数字，结果为相差的天数。 pg日期相减为interval类型，得到相差天数需要进行类型转换

10.递归查询中的level

oracle的递归查询中level表示查询深度（或者递归层次），在PG中没有此含义的关键字，需要自行在with recursive实现

ORACLE	POSTGRESQL
select max(level) from larearrelation where rearedgens = 1 and rearflag = 1 and rearlevel = '01' connect by prior agentcode = rearagentcode start with rearagentcode = '10032226';	with recursive rs as ( 　　select larearrelation., 1 depth from larearrelation where rearagentcode = '10032226' 　　union all 　　select a./*, rs.depth + 1 depth from larearrelation a, rs where a.rearagentcode = rs.agentcode ) select max(rs.depth) from rs where rearedgens = 1 and rearflag = '1' and rearlevel = '01'

11.序列的调用

序列: PostgreSQL 序列（SEQUENCE） - Merlin_Tang - 博客园

ORACLE	POSTGRESQL
select seqname.nextval from dual;	select nextval('seqname')

12.外连接

Oralce外连接支持使用 (+)， PostgreSQL需使用left jion或者right join标准sql语法

13.distinct去重复

oracle支持unique关键字去重复，pg中只能使用distinct

14.字符串分割

ORACLE	POSTGRESQL
listagg	string_agg

15.集合相减

ORACLE	POSTGRESQL
Minus	except

16.null与 ''

null和''在oracle中是一致的，最终都会存储为null，在PG中会进行区分

17.不等于

Oracle中**! =** ，**< >**操作符中间允许有空格，PG中不可以

18.别名

PG中无效的别名，可以尝试加as关键字，例如name

19.正则表达式

ORACLE	POSTGRESQL
SELECT REGEXP_SUBSTR('17,20,23',',+',1,1,'i') FROM DUAL;	select (regexp_matches('17,20,23', ',+')) $1$

20.字段大小写

oracle字段名大写，PG字段名小写

21.NVL --- COALESCE

ORACLE	POSTGRESQL
SELECT NVL(SUM(VALUE1),0) S_VALUE1 FROM FIELD_SUM ;	SELECTCOALESCE(SUM(VALUE1),0) AS S_VALUE1 FROM FIELD_SUM ;

22.TO_NUMBER

ORACLE	POSTGRESQL
SELECT COL1 FROM A_TABLE ORDER BYTO_NUMBER(COL1);	SELECT COL1 FROM A_TABLE ORDER BY TO_NUMBER(COL1, 999999) $注：'999999' ---- 6位数为COL1字段的长度$

23.DECODE --- CASE WHEN

ORACLE	POSTGRESQL
SELECTDECODE(ENDFLAG, '1', 'A', 'B') ENDFLAG FROM TEST;	SELECT (CASE ENDFLAG WHEN '1' THEN 'A' ELSE 'B' END) AS ENDFLAG FROM TEST;

24.ADD_MONTHS

ORACLE	POSTGRESQL
add_months(date, int)	--创建函数来解决 CREATE FUNCTION add_months(date, int) RETURNS date AS 'SELECT (1+(1+(2::text \|\| ''months'')::interval)::date;' LANGUAGE 'sql'

25.LAST_DAY

ORACLE	POSTGRESQL
LAST_DAY(DATE)	--创建函数来解决 CREATE FUNCTION LAST_DAY(DATE) RETURNS DATE AS 'SELECT date(substr(text($1 + interval(''1 month'')),1,7)\|\|''-01'')-1' LANGUAGE 'sql';

26.LAST_DAY

ORACLE	POSTGRESQL
LAST_DAY(DATE)	--创建函数来解决 CREATE FUNCTION LAST_DAY(DATE) RETURNS DATE AS 'SELECT date(substr(text($1 + interval(''1 month'')),1,7)\|\|''-01'')-1' LANGUAGE 'sql';