Just like in a Programming Language you need to Declare the variable first, this creates the storage location for that variable
DECLARE variable_name data_type [DEFAULT literal_value];
SELECT column_1[, column_2]...
INTO var1[, var2]...
SET variable_name =
{literal_value|expression};
Loading Variable with a Literal Value or an Expression
Loading Variable with Data from a Table
The code used to load the variable depends on what you want to load into it
Optional