staging lec7 for db/networking + part of lab stuff

cst363/lec/lec7.md

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+# lec7
+
+## Lab Activity
+
+This lecture has two correspondnig lab activities in `lab/` using `1994-census-summary.sql` with instrucctions on `aggregation-lab.pdf` and `nested-subqueries-lab.pdf`.
+
+## Null Operations
+
+take the following table as a trivial example of working data
+
+```
+a | b
+-----
+1 | 2
+3 | N
+```
+
+Where `a` and `b` are attributes and N signifiies a NULL value.
+If we `select a+b from table` we only get back 2 rows like normal but the second row is left empty since we are operating with a NULL value.
+Even if we use multiplication or some kind of comparison against null we simply ignore that row since NULL in sqlite3 doesn't mean 0.
+Instead NULL in sqlite3 actually represents something that doesn't exist.
+
+> count will treat NULL as 0 however
+
+This is the only exception to the _ignore NULL_ "rule".
+
+## Aggregation
+
+This section we'll deal with functions similar to `count average min max`.
+We call these functions _aggreagate_ functions because they aggregate multiple data points into one.
+
+> round(integer)
+
+Rounds off the floating point number to some level of precision.
+
+> group by _attr_
+
+This will group attributes to gether in the result of a query
+
+> having(attribute)
+
+Similar to `where` however we only care about group scope in this case.
+
+## Nested Subqueries
+
+Recall that when we perform a query the result is a table. 
+We can leverage this and perform some query to query a resultant table to further our ability to filter results from a table.