Update "Type system differences" for MySQL Compatibility

[dveeden]

What is changed, added or deleted? (Required)

The "Type system differences" section was updated as it TiDB supports some of the types that were listed as unsupported.

TiDB:

mysql> SELECT tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v4.0.8
Edition: Community
Git Commit Hash: 66ac9fc31f1733e5eb8d11891ec1b38f9c422817
Git Branch: heads/refs/tags/v4.0.8
UTC Build Time: 2020-10-30 08:21:16
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.00 sec)

mysql> CREATE TABLE t1 (id SERIAL, y SQL_TSI_YEAR, f FIXED(4,2));
Query OK, 0 rows affected (0.11 sec)

mysql> SHOW CREATE TABLE t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `y` year(4) DEFAULT NULL,
  `f` decimal(4,2) DEFAULT NULL,
  UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin
1 row in set (0.01 sec)

MySQL:

mysql> SELECT VERSION();
+-----------+
| VERSION() |
+-----------+
| 8.0.20    |
+-----------+
1 row in set (0.00 sec)

mysql> CREATE TABLE t1 (id SERIAL, y SQL_TSI_YEAR, f FIXED(4,2));
Query OK, 0 rows affected (0.06 sec)

mysql> SHOW CREATE TABLE t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `id` bigint unsigned NOT NULL AUTO_INCREMENT,
  `y` year DEFAULT NULL,
  `f` decimal(4,2) DEFAULT NULL,
  UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Have version specific changes
• Might cause conflicts

[CLAassistant]

All committers have signed the CLA.

[ghost]

LGTM

[TomShawn]

LGTM
@dveeden Thanks!

[ti-srebot]

cherry pick to release-4.0 in PR #4118

[Joyinqin]

/label translation/done

[Joyinqin]

/unlabel translation/doing