首先，创建两个表。

CREATE DATABASE IF NOT EXISTS test;
USE test;
DROP TABLE IF EXISTS student_info;
CREATE TABLE IF NOT EXISTS student_info(
id string COMMENT 'student id',
name string COMMENT 'student name'
)
PARTITIONED BY (l_date string)
ROW FORMAT DELIMITED
FIELDS TERMINATED BY '\t';
ALTER TABLE test.student_info SET SERDEPROPERTIES('serialization.null.format' = '');

DROP TABLE IF EXISTS student_score;
CREATE TABLE IF NOT EXISTS student_score(
id string COMMENT 'student id',
class string COMMENT 'class',
score int COMMENT 'class score'
)
PARTITIONED BY (l_date string)
ROW FORMAT DELIMITED
FIELDS TERMINATED BY '\t';
ALTER TABLE test.student_score SET SERDEPROPERTIES('serialization.null.format' = '');

学生信息表中有4条记录，

1   jobs
2   cook
3   gates
4   musk

学生成绩表中有3项记录，

1   math    98
2   math    96
3   math    94

我要找一个没有分数，id是4的学生。

select * from test.student_info a
left join test.student_score b
on a.id=b.id
where (b.id='' or b.id is null)
and a.id='4';

我什么都没有。但是，我添加了“trim（）”。

select * from test.student_info a
left join test.student_score b
on a.id=b.id
where (b.id='' or b.id is null)
and trim(a.id)='4';

我能得到我想要的。

a.id    a.name  b.id    b.class b.score
4   musk    NULL    NULL    NULL

所以，我认为有一个错误。