Описание тега mysql-variables
For use with special handling of queries for such things as ranking, grouping, and segmenting. The use of cross-joins that initialize variables, Producing otherwise difficult or impossible queries without them. One should not use this tag merely if you are using variables, in simply fashion.
Pertaining to special use-cases where MySQL User-Defined Variables play an essential or unique role in solving queries or other related MySQL functionality.
These User-Defined variables start with an @ sign, are not DECLARED, and are session-specific. The variables are destroyed upon the close of the connection.
The variables often play a crucial role in special functionality such as ranking, grouping, and special ordering of results. Some of these use-cases include crafting tricky queries to achieve certain, challenging, resultsets.