SQL statements never appear outside of another language, called the host language (in this case EGL). Because SQL is a distinct language from EGL, it does not automatically have access to the variables declared in your EGL program. As in other languages that host SQL, you can use host variables to give EGL this access.
A host variable has the same name as a variable in the host language, with an additional initial colon character (:). The initial colon indicates that the variable is defined in the host language (EGL) and not SQL.
SELECT customer_balance INTO :myCustomer.customerBalance FROM Customer WHERE customer_number = :myCustomer.customerNumber
The names customer_balance and customer_number refer to columns in the database table Customer; :myCustomer.customerBalance and :myCustomer.customerNumber are host variables (variables or fields that SQL looks for in your EGL program).