select ... information_schema.columns ...; exception when querying with alias

[wenyh1]

• dble version：3.20.07.99
• preconditions ：
  no
• configs：

cluster.cnf

bootstrap.cnf

db.xml

user.xml

sharding.xml

• steps：
  step1. select column_name as column_name from information_schema.columns where table_schema = 'schema_xx' and table_name = 'table_xx';
• expect result：
  1. success
• real result：
  1. ERROR 1003 (HY000): occur Exception, so see dble.log to check reason
     See in the log that the issued sql is overappended with 'as column_name', excption message:

2020-09-24 05:48:04.074  INFO [complexQueryExecutor3] (com.actiontech.dble.sqlengine.SQLJob.errorResponse(SQLJob.java:129)) - 
error response errNo:1064, You have an error in your SQL syntax; check the manual that corresponds to your MySQL 
server version for the right syntax to use near 'as column_name FROM INFORMATION_SCHEMA.COLUMNS WHERE 
table_schema = 'schema_xx' at line 1 from of sql :SELECT column_name AS `column_name` as column_name FROM 
INFORMATION_SCHEMA.COLUMNS WHERE table_schema = 'schema_xx'AND table_name = 'table_xx'

• supplements：
  1.

[quexiuping]

verified on 5.7.21-dble-9.9.9.9-81073d64a76fb91036bbd28a8a932338133ba09c-20201016023638