Check whether the `op` condition is null rejected, if yes, derive
`col is not null` for both sides of inner join, and for inner side
of outer join.
Also, remove duplicate conditions before pushing them down. Check
if the condition is mutable before removing duplicates.
For AntiSemiJoin, AntiLeftOuterSemiJoin and LeftOuterSemiJoin, do
not generate new `is not null` conditions.
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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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