postgresql与lightdb中的null行为及兼容性

null首先跟char、varchar、bpchar（blank padded character）、text有关系。在oracle中，则与char、varchar2、clob有关。

其次，涉及的范围比较广，''、null、=、!=、is null、is not null、替换、计算长度、类型强转如cast(null as date)，以及它们之间的组合。

zjh@postgres=# select null = '';
 ?column? 
----------
 f
(1 row)

zjh@postgres=# select null = null;
 ?column? 
----------
 t
(1 row)

zjh@postgres=# select null is null;
 ?column? 
----------
 t
(1 row)

zjh@postgres=# select '' is null;
?column?
----------
f
(1 row)

zjh@postgres=# select '' = '';
 ?column? 
----------
 t
(1 row)

zjh@postgres=# select '' = ' ';
 ?column? 
----------
 f
(1 row)

zjh@postgres=# select '' = ' '::bpchar;
 ?column? 
----------
 t
(1 row)

zjh@postgres=# select '' = ' '::char;
 ?column? 
----------
 t
(1 row)

zjh@postgres=# select '' = ' '::varchar;
 ?column? 
----------
 f
(1

zjh@postgres=# select '' = ' '::text;
 ?column? 
----------
 f
(1 row)

zjh@postgres=# select '' = trim(' ')::text;
 ?column? 
----------
 t
(1 row)

zjh@postgres=# select '' = trim(' '::text);
 ?column? 
----------
 t
(1

zjh@postgres=# select 1 from t where '' = null;
 ?column? 
----------
(0 rows)

zjh@postgres=# select sum(id) from t where '' = null;
 sum 
-----
    
(1 row)

zjh@postgres=# select max(id) from t where '' = null;
 max 
-----
    
(1

zjh@postgres=# select 'abc' = 'abc '::text;
 ?column? 
----------
 f
(1 row)

zjh@postgres=# select 'abc' = 'abc '::bpchar;
 ?column? 
----------
 t
(1 row)

zjh@postgres=# select 'abc' = ' abc'::bpchar;
 ?column? 
----------
 f
(1 row)

zjh@postgres=# select 'abc' = ' abc'::varchar;
 ?column? 
----------
 f
(1 row)

zjh@postgres=# select 'abc' = ' abc'::text;
 ?column? 
----------
 f
(1

zjh@postgres=# select trim('abc ') = 'abc '::bpchar;
 ?column? 
----------
 t
(1

zjh@postgres=# select replace('12345','4','');
 replace 
---------
 1235
(1 row)

zjh@postgres=# select replace('12345','4',null);
 replace 
---------
 
(1 row)

zjh@postgres=# 
zjh@postgres=# select replace('12345',null,null);
 replace 
---------
 
(1 row)

zjh@postgres=# select replace('12345','','');
 replace 
---------
 12345
(1

zjh@postgres=# select length(null);
 length 
--------
       
(1 row)

zjh@postgres=# select length('');
 length 
--------
      0
(1

zjh@postgres=# select 1 from t where cast('' AS DATE) is null;
ERROR:  invalid input syntax for type date: ""
LINE 1: select 1 from t where cast('' AS DATE) is null;
                                   ^
zjh@postgres=# select 1 from t where cast(null AS DATE) is null;
 ?column? 
----------
        1
(1 row)

zjh@postgres=# select 1 from t where cast('' AS bpchar(10)) is null;
 ?column? 
----------
(0 rows)

zjh@postgres=# select 1,cast('' AS bpchar(10)) from t;
 ?column? |   bpchar   
----------+------------
        1 |           
(1 row)

zjh@postgres=# select 1,length(cast('' AS bpchar(10))) from t;
 ?column? | length 
----------+--------
        1 |      0
(1 row)

zjh@postgres=# select 1,length(cast(null AS bpchar(10))) from t;
 ?column? | length 
----------+--------
        1 |       
(1

-- 虽然pg中null和''都被诊断为unkown，但是内部处理貌似仍然不同
zjh@postgres=# select pg_typeof('');
 pg_typeof 
-----------
 unknown
(1 row)

zjh@postgres=# select pg_typeof(null);
 pg_typeof 
-----------
 unknown
(1

======================下面是不等于=====================

​​https://www.postgresql.org/docs/current/functions-conditional.html#FUNCTIONS-COALESCE-NVL-IFNULL​​

​​https://linuxhint.com/bpchar-data-type-postgres/​​

​​https://www.postgresql.org/docs/current/datatype-character.html​​

​​https://www.geeksforgeeks.org/postgresql-difference-between-char-varchar-and-text/​​

oracle中null的行为

SQL> select 1 from dual where '' = '';

no rows selected

SQL> select 1 from dual where null = null;

no rows selected

SQL> select 1 from dual where null is null;

     1
----------
     1

SQL> select 1 from dual where '' is null;

     1
----------
     1

SQL> select sum(id) from t_null where '' = null;

   SUM(ID)
----------


SQL> select * from t_null where '' = null;

no rows selected

SQL> select max(id) from t_null where '' = null;

   MAX(ID)
----------


SQL> select count(id) from t_null where '' = null;

 COUNT(ID)
----------
     0

SQL> select replace('12345','4','') from dual;

REPL
----
1235

SQL> select replace('12345','4',null) from dual;

REPL
----
1235

SQL> select replace('12345',null,null) from dual;

REPLA
-----
12345

SQL> select replace('12345','','') from dual;

REPLA
-----
12345

SQL> select length('') from dual;

LENGTH('')
----------


SQL> select length(null) from dual;

LENGTH(NULL)
------------


SQL>

SQL> select 1 from t where cast('' AS DATE) is null;

     1
----------
     1

SQL> select 1 from t where cast(null AS DATE) is null;

     1
----------
     1

SQL> select 1 from t where cast('' AS char(10)) is null;

     1
----------
     1
SQL> select 1,cast('' AS char(10)) from t;

     1 CAST(''ASC
---------- ----------
     1

SQL> select 1,length(cast('' AS char(10))) from t;

     1 LENGTH(CAST(''ASCHAR(10)))
---------- --------------------------
     1

SQL> select 1,length(cast(null AS char(10))) from t;

     1 LENGTH(CAST(NULLASCHAR(10)))
---------- ----------------------------
     1

==============================下面不等于=================== 

​​https://community.oracle.com/tech/developers/discussion/1053012/difference-between-null-and​​

​​https://cloud.tencent.com/developer/article/1052571​​

总结

　　在oracle里面，除了存储层面（包括PL/SQL）的char类型外，可以认为''几乎就是当做null来处理的，除了!=/=/in/not in操作外（因为本质上in、not in也是调用=/!=操作符实现的）。

　　但是在原生pg中，''和null的实现是不一样的，虽然都是unknown，但是内部仍然不是相同的。

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