why we use @:=0 and such other characters in sql dios?

why we use @:=0 and such other characters
what is the use of these in SQL Queries and DIOS?

to store some value to local variable. That value is then used later... In DIOS you are building output in iterations and in each iteration you are adding new table name (column name) to some local variable. When DIOS finish you publish that local variable which holds complete output (all tables, all columns.... depends what you want). Sometimes we use local variable also as a intermediate storage of some data, or as a counter (for example when we are numbering tables in output), ....