oracle中强制类型转换,Oracle vs PostgreSQL，研发注意事项（7）- 类型转换
本节以数值型相互转换以及数值型和字符型的转换为例大体介绍了Oracle和PostgreSQL类型转换上的部分异同，可据此思路推广到其他类型。
一、数值类型转换
下面以数值类型为例子说明,包括运算结果的转换和强制类型转换.
运算结果
以除运算为例说明.
PostgreSQL的除运算
testdb=# select 1/4;
?column?
----------
0
(1 row)
Oracle的除运算
TEST-orcl@server4>select 1/4 from dual;
1/4
----------
.25
两个整型值1和4参与除法运算,结果PostgreSQL为整型的0,Oracle为浮点型的0.25,两者的行为不一致.
为何PostgreSQL执行整型运算返回的结果是整型?当然,这是PG的机制(整型/整型=整型)使然,在PG中,运算的结果类型可查询pg_operator获得:
testdb=# \x
Expanded display is on.
testdb=# select * from pg_operator where oprname = '/' and oprleft=21 and oprright = 21;
-[ RECORD 1 ]+--------
oprname | / -->运算符
oprnamespace | 11
oprowner | 10
oprkind | b
oprcanmerge | f
oprcanhash | f
oprleft | 21 -->int2(占用2个字节的整型,通过select * from pg_type where oid=21查询可得)
oprright | 21 -->同上
oprresult | 21 -->整型/整型,结果也是整型
oprcom | 0
oprnegate | 0
oprcode | int2div
oprrest | -
oprjoin | -
在PostgreSQL中,要想获得0.25的结果,需要进行转换:
testdb=# select 1/4::float;
?column?
----------
0.25
(1 row)
二、强制类型转换
以字符型->整型为例说明.
PostgreSQL
testdb=# drop table if exists t_cast ;
DROP TABLE
testdb=# create table t_cast (c_int int,c_s varchar(20));
CREATE TABLE
testdb=# insert into t_cast values(1,'1');
INSERT 0 1
testdb=# insert into t_cast values(2,'2');
INSERT 0 1
testdb=# select * from t_cast where c_int = 1;
c_int | c_s
-------+-----
1 | 1
(1 row)
testdb=# select * from t_cast where c_s = 1;
ERROR: operator does not exist: character varying = integer -->可变长字符型转换为整型
LINE 1: select * from t_cast where c_s = 1;
^
HINT: No operator matches the given name and argument types. You might need to add explicit type casts.
Oracle
TEST-orcl@server4>drop table t_cast;
Table dropped.
TEST-orcl@server4>create table t_cast (c_int int,c_s varchar2(20)) tablespace users;
Table created.
TEST-orcl@server4>insert into t_cast values(1,'1');
1 row created.
TEST-orcl@server4>insert into t_cast values(2,'2');
1 row created.
TEST-orcl@server4>select * from t_cast where c_int = 1;
C_INT C_S
---------- --------------------
1 1
TEST-orcl@server4>select * from t_cast where c_s = 1;
C_INT C_S
---------- --------------------
1 1
PG,整型不能转换为字符型,而Oracle可以.
PG可以通过显式类型转换或者自定义类型转换的机制实现字符型->整型的转换:
-- 显式转换
testdb=# select * from t_cast where c_s = 1::varchar;
c_int | c_s
-------+-----
1 | 1
(1 row)
-- 自定义类型转换
testdb=# create cast(varchar as integer) with inout as implicit;
CREATE CAST
testdb=# select * from t_cast where c_s = 1;
c_int | c_s
-------+-----
1 | 1
(1 row)
通过数据字典表pg_cast可查询PG支持的类型转换.
testdb=# select oid,a.* from pg_cast a where castsource=1043 and casttarget = 23;
oid | castsource | casttarget | castfunc | castcontext | castmethod
-------+------------+------------+----------+-------------+------------
16774 | 1043 | 23 | 0 | i | i --> 这是新加的记录
三、参考资料
CREATE CAST
PostgreSQL 自定义自动类型转换(CAST)