if else syntax in ansible playbook

I am new to Ansible and trying to understand what is wrong with my syntax.

My goal is that only one of the roles will be selected. I do not want to use 'when'.

Here is what I wrote (I am using Ansible v2.9.5):

- name: Install external DB for Cloudera Manager Server
  hosts: db_server
  roles:
 - {{% if (databases_type == "postgresql") %} role: postgresql {% else %} {% endif %}
   {% if (databases_type == "mysql") %} role: mariadb {% else %} {% endif %} 
   {% if (databases_type == "oracle") %} role: oracledb}

When I run the playbook I get a syntax error but it is not clear enough.

Thanks in advance.

A simple dictionary might be a cleaner option. For example,

shell> cat playbook.yml
- name: Install external DB for Cloudera Manager Server
  hosts: db_server

  vars:

    my_roles:
      postgresql: postgresql
      mysql: mariadb
      oracle: oracledb

  tasks:

    - include_role:
        name: "{{ my_roles[databases_type] }}"

Example

Let's create the roles

shell> cat roles/postgresql/tasks/main.yml
- debug:
    var: role_name

shell> cat roles/mariadb/tasks/main.yml
- debug:
    var: role_name

shell> cat roles/oracledb/tasks/main.yml
- debug:
    var: role_name

Next, let's create an inventory with three servers, group_vars with default databases_type and host_vars with the variables for two hosts test_01 and test_02. The third host test_03 will use the variables from group_vars.

shell> cat hosts
[db_server]
test_01
test_02
test_03

shell> cat group_vars/db_server 
databases_type: mysql

shell> cat host_vars/test_01 
databases_type: postgresql

shell> cat host_vars/test_02
databases_type: oracle

Then, the playbook gives (abridged)

shell> ansible-playbook -i hosts playbook.yml 

PLAY [Install external DB for Cloudera Manager Server] *************

TASK [include_role : {{ my_roles[databases_type] }}] ***************

TASK [postgresql : debug] ******************************************
ok: [test_01] => 
  role_name: postgresql

TASK [oracledb : debug] ********************************************
ok: [test_02] => 
  role_name: oracledb

TASK [mariadb : debug] *********************************************
ok: [test_03] => 
  role_name: mariadb