Research on the problem of SQL query set returning to [null,null]

In the Java project, if you query some fields of a table in the database through Mybatis, the return value is list. Sometimes the [null, null] phenomenon occurs.

The specific reason is that the query SQL only fetches some fields, and these fields are null.

But for other fields of the database table, the query has a value, but it just so happens that the field you query is Null.

Execute: SELECT * FROM user2 WHERE name='rtr'

The result is:

A rtr 2019-02-19

B rtr 2019-02-19

Execute: SELECT department_id FROM user2 WHERE name='rtr'

The result is:

Null

Null

Execute: SELECT COUNT (*) FROM

(SELECT department_id FROM user2 WHERE name='rtr') AS t

The result is: 2.

Solution: several fields that need to be queried must be added with a judgment that is not empty.

SELECT department_id FROM user2 WHERE NAME = 'rtr' AND department_id IS NOT NULL

Results: null

SELECT

COUNT ()

FROM

(SELECT FROM user2 WHERE NAME = 'rtr' AND department_id IS NOT NULL) AS t

Result: 0