Wsrtjtyk

I have been asked to refactor a query that contains a snippet :

SELECT @xi:=@xi+1 AS xc

FROM

   (SELECT 1

    UNION SELECT 2

    UNION SELECT 3) xc1,



   (SELECT 1

    UNION SELECT 2

    UNION SELECT 3)xc2,



   (SELECT 1

    UNION SELECT 2

    UNION SELECT 3) xc3,



   (SELECT @xi:=-1) xc0

I believe the rest of the query is written in MySQL.

I am completely new to this style of writing queries, and have not come across it in any Postgresql, MySQL or BigQuery Standard SQL documentation.

Unfortunately there are other elements of the query that are not working so I cannot produce an output or describe what I'm trying to achieve (because I have no idea what this does) but thought while I work on those other parts someone might be able to provide some insight to the syntax to this particular piece of logic or maybe provide a topic that I can find out more about this snippet so that I can develop my knowledge.

It's an user defined variable and will display :

    0 (-1+1)

    1 (0+1)

    2 (1+1)

    3 (2+1)

...

    26 (25+1)