TDengine - invalid operation

Question

I was trying to execute sql:

select last(*) from mytable where index=index-1

Describe mytable:

             Field              |         Type         |   Length    |   Note   |
=================================================================================
 ts                             | TIMESTAMP            |           8 |          |
 gauge                          | DOUBLE               |           8 |          |
 index                          | NCHAR                |          11 | TAG      |

The result occur error:

Invalid operation

Answer 1

index is nchar type so you should change the SQL to

select last(*) from mytable where index='some_value';