Nice! It has a succinct overview of the Relational Model before discussing ISBL.

There was a earlier thread today which is an interview with Chris Date where he mentions that QBE and ISBL are more "Relational" than SQL - https://news.ycombinator.com/item?id=39189015

Skip to page 11 in the PDF for examples.

    MEMBERS(NAME, ADDRESS, BALANCE)
    ORDERS(ORDER_NO, NAME, ITEM, QUANTITY)
    SUPPLIERS(SNAME, SADDRESS, ITEM, PRICE)

    1. Print the names of members in the red:
    LIST MEMBERS : BALANCE < 0 % NAME

    i.e. select members with negative balance and
    project out their names