char 是定长型，varchar 是不定长型。简单例子

'aa '='aa' 吗？这个问题很有意思。可以相等也可以不等，char类型就相等，varchar类型就不等。

SQL> create table test( a char(3));

Table created

SQL> insert into test values ('aa ');

1 row inserted

SQL> insert into test values ('aa');

1 row inserted

SQL> commit;

Commit complete

SQL> select distinct a from test;

A
---
aa

SQL> select count(distinct a) from test;

COUNT(DISTINCTA)
----------------
1

SQL> select count(a) from test;

COUNT(A)
----------
2

显然char类型会认为2个是一样的。再看varchar

SQL> create table test1(a varchar2(3));

Table created

SQL> insert into test1 values('aa');

1 row inserted

SQL> insert into test1 values('aa ');

1 row inserted

SQL> commit;

Commit complete

SQL> select count(distinct a) from test1;

COUNT(DISTINCTA)
----------------
2

SQL> select count( a) from test1;

COUNT(A)
----------
2

很明显，2个值是不一样的。distinct区分出来了。呵呵。

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char，varchar字段型数据的比较