cannot select session scope ScopeNone variable

[SunRunAway]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> select @@global.explicit_defaults_for_timestamp;
+------------------------------------------+
| @@global.explicit_defaults_for_timestamp |
+------------------------------------------+
| 1                                        |
+------------------------------------------+
1 row in set (0.00 sec)

mysql> select @@session.explicit_defaults_for_timestamp;
ERROR 1238 (HY000): Variable 'explicit_defaults_for_timestamp' is a GLOBAL variable

2. What did you expect to see? (Required)

mysql> select @@global.explicit_defaults_for_timestamp;
+------------------------------------------+
| @@global.explicit_defaults_for_timestamp |
+------------------------------------------+
|                                        1 |
+------------------------------------------+
1 row in set (0.00 sec)

mysql> select @@session.explicit_defaults_for_timestamp;
+-------------------------------------------+
| @@session.explicit_defaults_for_timestamp |
+-------------------------------------------+
|                                         1 |
+-------------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

[Patrick0308]

Let me fix it.

[sre-bot]

Integrity check：
component RCA symptom affect_version fix_version fields are empty
@Patrick0308
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