1.5 KiB
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.