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### What problem does this PR solve? <!--add issue link with summary if exists-->
https://github.com/pingcap/parser/issues/147https://github.com/pingcap/parser/pull/148
MySQL compatibility: `show create database if not exists` syntax
See: https://dev.mysql.com/doc/refman/5.7/en/show-create-database.html
MySQL
```
MariaDB [(none)]> show create database b2;
+----------+----------------------------------------------------------------+
| Database | Create Database |
+----------+----------------------------------------------------------------+
| b2 | CREATE DATABASE `b2` /*!40100 DEFAULT CHARACTER SET utf8mb4 */ |
+----------+----------------------------------------------------------------+
1 row in set (0.002 sec)
MariaDB [(none)]> show create database if not exists b2;
+----------+-----------------------------------------------------------------------------------------+
| Database | Create Database |
+----------+-----------------------------------------------------------------------------------------+
| b2 | CREATE DATABASE /*!32312 IF NOT EXISTS*/ `b2` /*!40100 DEFAULT CHARACTER SET utf8mb4 */ |
+----------+-----------------------------------------------------------------------------------------+
1 row in set (0.000 sec)
```
TiDB
```
MySQL [b1]> show create batabase b2;
ERROR 1105 (HY000): line 1 column 20 near " b2" (total length 23)
MySQL [b1]> show create database b2;
+----------+-----------------------------------------------------------------+
| Database | Create Database |
+----------+-----------------------------------------------------------------+
| b2 | CREATE DATABASE `b2` /* !40100 DEFAULT CHARACTER SET utf8mb4 */ |
+----------+-----------------------------------------------------------------+
1 row in set (0.000 sec)
MySQL [b1]> show create database if not exists b2;
ERROR 1105 (HY000): line 1 column 23 near " not exists b2" (total length 37)
```
### What is changed and how it works?
Insert `/*!32312 IF NOT EXISTS*/` before shema name when statement contains `if not exists`
### Check List <!--REMOVE the items that are not applicable-->
Tests <!-- At least one of them must be included. -->
- Unit test
Code changes
Side effects
Related changes
- Need to cherry-pick to the release branch
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