7.2. Precedence Charts

You will plan a query using operators and inputs expressed in a precedence chart. A precedence chart depicts a partial ordering of the work (“do this sometime before doing that”), but not a complete ordering (“do this first and that next”). The former is all you need to plan a query, and if you are thinking the latter too soon, then you are wasting your time.

You have seen a simple chart already that looks like this:

Achievement and Filter operator example

A precedence chart in this book follows these conventions:

When starting out to learn how the operators work, we will take care when naming the base of each relation and underlining it, and we will make note of what the identifying columns are in the input and output relation(s).

The shapes and arrows appear in patterns that will make it easy for you to discern the relationship between the bases of the input relations and the bases of the result relations.

A precedence chart has several notable characteristics.

Notice that I call each precedence chart a “Fragment.” Fragment implies that what is shown could be only a portion of a larger precedence chart, or be incomplete.

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