Add blog post about SQL servers

docs/blog/index.rst

@@ -13,3 +13,4 @@ Blog
    private-engines
    command-line-engines
    search-indexer-engines
+   sql-engines

docs/blog/sql-engines.rst

@@ -0,0 +1,117 @@
+=================
+Query SQL servers 
+=================
+
+Now you can query SQL servers using searx. The following ones are supported:
+
+* `PostgreSQL`_
+* `MySQL`_
+* `SQLite`_
+
+All of the engines above are added to ``settings.yml`` just commented out, as you have to
+set the required attributes for the engines, e.g. ``database``. By default, the engines use
+``key-value`` template for displaying results. If you are not satisfied with the original result layout,
+you can use your owm template by placing the template under
+``searx/templates/{theme_name}/result_templates/{template_name}`` and setting
+``result_template`` attribute to ``{template_name}``.
+
+As mentioned in previous blog posts, if you do not wish to expose these engines on a
+public instance, you can still add them and limit the access by setting ``tokens``
+as described in the `blog post about private engines`_.
+
+Configure the engines
+=====================
+
+The configuration of the new database engines are similar. You must put a valid
+SELECT SQL query in ``query_str``. At the moment you can only bind at most
+one parameter in your query. By setting the attribute ``limit`` you can
+define how many results you want from the SQL server. Basically, it
+is the same as the LIMIT keyword in SQL.
+
+Please, do not include LIMIT or OFFSET in your SQL query as the engines
+rely on these keywords during paging. If you want to configure the number of returned results
+use the option ``limit``.
+
+PostgreSQL
+----------
+
+PostgreSQL is a powerful and robust open source database.
+
+Before configuring the PostgreSQL engine, you must install the dependency ``psychopg2``.
+
+You can find an example configuration below:
+
+.. code:: yaml
+
+  - name : postgresql
+    engine : postgresql
+    database : my_database
+    username : searx
+    password : password
+    query_str : 'SELECT * from my_table WHERE my_column = %(query)s'
+    shortcut : psql
+
+
+MySQL
+-----
+
+MySQL is said to be the most popular open source database. 
+
+Before enabling MySQL engine, you must install the package ``mysql-connector-python``.
+
+The authentication plugin is configurable by setting ``auth_plugin`` in the attributes.
+By default it is set to ``caching_sha2_password``.
+
+This is an example configuration for quering a MySQL server:
+
+.. code:: yaml
+
+  - name : mysql
+    engine : mysql_server
+    database : my_database
+    username : searx
+    password : password
+    limit : 5
+    query_str : 'SELECT * from my_table WHERE my_column=%(query)s'
+    shortcut : mysql
+
+
+SQLite
+------
+
+SQLite is a small, fast and reliable SQL database engine. It does not require
+any extra dependency.
+
+You can read from your database ``my_database`` using this example configuration:
+
+.. code:: yaml
+
+  - name : sqlite
+    engine : sqlite
+    shortcut: sq
+    database : my_database
+    query_str : 'SELECT * FROM my_table WHERE my_column=:query'
+
+
+Next steps
+==========
+
+The next step is to add support for more data stores, e.g. Redis and MongoDB.
+
+Acknowledgement
+===============
+
+This development was sponsored by `Search and Discovery Fund`_ of `NLnet Foundation`_ .
+
+.. _PostgreSQL: https://www.postgresql.org/
+.. _MySQL: https://www.mysql.com/
+.. _SQLite: https://www.sqlite.org/index.html
+.. _blog post about private engines: private-engines.html#private-engines
+.. _Search and Discovery Fund: https://nlnet.nl/discovery
+.. _NLnet Foundation: https://nlnet.nl/
+
+
+| Happy hacking.
+| kvch // 2021.05.23 23:16
+
+