The mysql5.7 base changes a NOT NULL column to NULL

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Operating System:ubuntukylin 16.04 LTS 64bit
Mysql:ver 14.14 Distrib 5.7.17, for Linux (x86_64) using Editline Wrapper

Step1:

Log in and show all databases and enter a database

[email protected]:~$ mysql -u admin -penter  password: welcome to the mysql monitor.  commands end with  ; or \g.Your MySQL connection id is 29Server version:  5.7.17-0ubuntu0.16.04.1  (Ubuntu) copyright  (c)  2000, 2016, oracle and/or its  affiliates. all rights reserved. oracle is a registered trademark of oracle corporation and/or  Itsaffiliates. other names may be trademarks of their respectiveowners . type  ' help; '  or  ' \h '  for help. Type  ' \c '  to clear the current input  statement. 

mysql> show databases;+--------------------+| database            |+--------------------+| information_ schema | |  data               | |  exception          | |  logininformation   | |  usinginformation   | |  mysql              | |  performance_schema | |  phpmyadmin         | |  sys                |+--- -----------------+9 rows in set  (0.00 sec) 

mysql> use usinginformation; Reading table information for completion of table and column namesyou can turn off this feature to get a quicker startup W Ith-adatabase changed

Step2:

Show all tables under the current database and view the structure of a table

mysql> show tables;+----------------------------+|  tables_in_usinginformation |+----------------------------+| using_engineer              | |  Using_STM                   |+----------------------------+2 rows in set  (0.00 sec) 

mysql> describe using_engineer;+------------------+----------+------+-----+---------+-------+|  Field            | Type      | null | key | default | extra |+------------------+ ----------+------+-----+---------+-------+| id_engineer      | char (5)   | NO   | PRI | NULL    |        | |  using_engineer   | bit (1)    | NO   |      | null    |       | |  time_build_stm   | char ( | NO   |   )   | null    |       | |  Time_Abandon_STM | char ( | NO   |     | NULL   )  |       |+------------------+----------+------+-----+---------+------ -+4 rows in set  (0.00 SEC)

Step3:

Change a non-empty column to a column that can be empty and show the results

Mysql> ALTER TABLE Using_engineer modify time_abandon_stm char (+) null; Query OK, 0 rows affected (0.67 sec) records:0 duplicates:0 warnings:0

mysql> describe using_engineer;+------------------+----------+------+-----+---------+-------+|  Field            | Type      | null | key | default | extra |+------------------+ ----------+------+-----+---------+-------+| id_engineer      | char (5)   | NO   | PRI | NULL    |        | |  using_engineer   | bit (1)    | NO   |      | null    |       | |  time_build_stm   | char ( | NO   |   )   | null    |       | |  Time_Abandon_STM | char ( | yes  |     | null    |)        |+------------------+----------+------+-----+---------+-------+4  rows in set  (0.00 SEC)

Note:

My question:

No null--null no thing, then what's the reverse? Null-> no null would it be OK?

What should I do with null items in a column?

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mysql5.7 base changes a NOT NULL column to null