What problem does this PR solve?

fix bug for #11831。should display collate of the column， but did not。

create table ttt4(a varchar(123) default null collate utf8mb4_unicode_ci)engine=innodb default charset=utf8mb4 collate=utf8mb4_unicode_ci;
show create table ttt4;
mysql> show create table ttt4\G
*************************** 1. row ***************************
Table: ttt4
Create Table: CREATE TABLE ttt4 (
a varchar(123) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
1 row in set (0.00 sec)

What is changed and how it works?

mysql> show create table ttt4\G
*************************** 1. row ***************************
Table: ttt4
Create Table: CREATE TABLE ttt4 (
a varchar(123) COLLATE utf8mb4_unicode_ci DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
1 row in set (0.00 sec)

Check List

Tests

• Unit test