mroongaでも以下SQLコマンドが利用可能なようです。もし差し支えなければ、ドキュメントの追加をご検討頂けると幸いです。 http://mroonga.org/ja/docs/reference.html

mroonga-CREATE-TABLE-IF-NOT-EXISTS.md

CREATE TABLE IF NOT EXISTS table_name (...)形式での動作テストは以下の通りです。

環境

• CentOS-6.4
• MySQL-5.6.11
• mysql-mroonga-3.03

InnoDBでの動作

mysql> CREATE TABLE IF NOT EXISTS sample_table (
    ->   id INT PRIMARY KEY AUTO_INCREMENT,
    ->   contents TEXT
    -> ) ENGINE = InnoDB DEFAULT CHARSET utf8;
Query OK, 0 rows affected (0.02 sec)

mysql> CREATE TABLE IF NOT EXISTS sample_table (
    ->   id INT PRIMARY KEY AUTO_INCREMENT,
    ->   contents TEXT
    -> ) ENGINE = InnoDB DEFAULT CHARSET utf8;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> show warnings;
+-------+------+-------------------------------------+
| Level | Code | Message                             |
+-------+------+-------------------------------------+
| Note  | 1050 | Table 'sample_table' already exists |
+-------+------+-------------------------------------+
1 row in set (0.00 sec)

mysql> DROP TABLE IF exists sample_table;
Query OK, 0 rows affected (0.00 sec)

mroongaでの動作

mysql> CREATE TABLE  IF NOT EXISTS sample_table (
    ->   id INT PRIMARY KEY AUTO_INCREMENT,
    ->   contents TEXT
    -> ) ENGINE = mroonga DEFAULT CHARSET utf8;
Query OK, 0 rows affected (0.01 sec)

mysql> CREATE TABLE  IF NOT EXISTS sample_table (
    ->   id INT PRIMARY KEY AUTO_INCREMENT,
    ->   contents TEXT
    -> ) ENGINE = mroonga DEFAULT CHARSET utf8;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> show warnings;
+-------+------+-------------------------------------+
| Level | Code | Message                             |
+-------+------+-------------------------------------+
| Note  | 1050 | Table 'sample_table' already exists |
+-------+------+-------------------------------------+
1 row in set (0.00 sec)

mysql> DROP TABLE IF exists sample_table;
Query OK, 0 rows affected (0.01 sec)