**Use case**

PostgreSQL is using `ALTER TABLE t ALTER COLUMN c TYPE type` instead of `ALTER TABLE t MODIFY COLUMN c type`.
There is a risk that some people already get used to PostgreSQL syntax.

Let's implement it in ClickHouse.

```
tutorial=# ALTER TABLE hits_100m_obfuscated MODIFY COLUMN UserAgentMinor TEXT
tutorial-# ;
ERROR:  syntax error at or near "MODIFY"
LINE 1: ALTER TABLE hits_100m_obfuscated MODIFY COLUMN UserAgentMino...
                                         ^
tutorial=# ALTER TABLE hits_100m_obfuscated ALTER COLUMN UserAgentMinor TYPE TEXT
;
ALTER TABLE
tutorial=#
```

**Proposed implementation**

Implement it in parser level.
else if (s_to_table.ignore(pos))
                {
                    if (!parseDatabaseAndTableName(pos, expected, command->to_database, command->to_table))
                        return false;
                    command->move_destination_type = DataDestinationType::TABLE;
                }
                else
                    return false;
                if (command->move_destination_type != DataDestinationType::TABLE)
                {
                    ASTPtr ast_space_name;
                    if (!parser_string_literal.parse(pos, ast_space_name, expected))
                        return false;
                    command->move_destination_name = ast_space_name->as<ASTLiteral &>().value.get<const String &>();
                }
            }
            else if (s_add_constraint.ignore(pos, expected))
            {
                if (s_if_not_exists.ignore(pos, expected))
                    command->if_not_exists = true;
                if (!parser_constraint_decl.parse(pos, command->constraint_decl, expected))
                    return false;
                command->type = ASTAlterCommand::ADD_CONSTRAINT;
            }
            else if (s_drop_constraint.ignore(pos, expected))
            {
                if (s_if_exists.ignore(pos, expected))
                    command->if_exists = true;
                if (!parser_name.parse(pos, command->constraint, expected))
                    return false;