"这个 MySQL 查询是什么意思？"

Question

I'm trying to understand some queries

(select (@a) 
 from (select(@a:=0x00)
     ,(select (@a) 
       from (information_schema.schemata)
       where (@a) in (@a:=concat(@a,schema_name,'<br>'))
       )
 ) a
)

Answer 1

Not much to explain, it is a horrendous abuse of session variables. It looks like it is trying to get a concatenated list of database names, in the same manner GROUP_CONCAT would with a delimiter of <br> ; but it looks very unreliable.

Answer 2

Taking out many of the redundant parens that are not important, so we can strip those. Then some slight formatting for readability.

select
      @a as ThisIsTheFinalAnswer 
   from 
      ( select @a := 0x00,
              ( select @a
                   from information_schema.schemata
                   where @a in @a: = concat(@a, schema_name,'<br>')
              )
      )