If [not] exists in MySQL

Recently MySQL Developing distributed database based on database , Need to support a if [not] exists grammar . To study the SQL Grammar analysis part , Sum up ：

1、 stay MySQL in , Support when creating tables create table if not exists db.table_name ....

create table if not exists test1
(
    c1 int primary key,
    c2 varchar(50)
)engine = innodb;

If the table to be created exists , Then return directly , You are not recreating the table .

If the watch doesn't exist , Then create the table .

2、drop One table , Need to support if exists grammar ：

drop table if exists test1;

If you want to drop The specified table does not exist , directly drop fall

If you want to drop The table of does not exist , Will display a warnings, as follows ：

+-------+------+-----------------------------+
| Level | Code | Message                     |
+-------+------+-----------------------------+
| Note  | 1051 | Unknown table 'mysql.test1' |
+-------+------+-----------------------------+
1 row in set (0.00 sec)

Come to a general process , Include create/drop if [not] exists Prompt for statement ：

Cluster[mysql]> create table test1
    -> (
    -> c1 int primary key,
    -> c2 varchar(50)
    -> )engine = innodb;
Query OK, 0 rows affected (0.06 sec)

Cluster[mysql]> create table test1
    -> (
    -> c1 int primary key,
    -> c2 varchar(50)
    -> )engine = innodb;
ERROR 1050 (42S01): Table 'test1' already exists
Cluster[mysql]> 
Cluster[mysql]> 
Cluster[mysql]> 
Cluster[mysql]> 
Cluster[mysql]> create table if not exists test1
    -> (
    -> c1 int primary key,
    -> c2 varchar(50)
    -> )engine = innodb;
Query OK, 0 rows affected, 1 warning (0.02 sec)

Cluster[mysql]> show warnings;
+-------+------+------------------------------+
| Level | Code | Message                      |
+-------+------+------------------------------+
| Note  | 1050 | Table 'test1' already exists |
+-------+------+------------------------------+
1 row in set (0.00 sec)

Cluster[mysql]> drop table test1;
Query OK, 0 rows affected (0.05 sec)

Cluster[mysql]> drop table test1;
ERROR 1051 (42S02): Unknown table 'mysql.test1'
GreatDB Cluster[mysql]> 
GreatDB Cluster[mysql]> 
GreatDB Cluster[mysql]> drop table if exists test1;
Query OK, 0 rows affected, 1 warning (0.02 sec)

Cluster[mysql]> show warnings;
+-------+------+-----------------------------+
| Level | Code | Message                     |
+-------+------+-----------------------------+
| Note  | 1051 | Unknown table 'mysql.test1' |
+-------+------+-----------------------------+
1 row in set (0.00 sec)