lec9 for db course

cst363/lec/lec9.md

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+# lec9
+
+## Lab
+
+This lecture has a corresponding lab activity in `lab/`, the instructions are named `views-lab.pdf` and the second one is `contraints-lab.pdf`.
+
+## Views
+
+```
+create view newTabelName as select ... from targetTable;
+```
+
+This will create a `view` which whenever it is queried will pull data from some base table.
+Really the `view` is a kind of "_macro_" which is stored in a `catalog` that normal, non-admin users can use to access a database.
+The catalog is saved in a table somewhere in the database.
+Think of this catalog like a container(_table_) for the other tables in the database.
+
+### Pros & Cons
+
+Problems:
+
+* Computing the view multiple times can be expensive
+* Maintainence
+
+There are two strategies to dealing with the second item: eager and lazy strategies.
+
+1. Eager
+	* If the target table of some view changes the update the view immediately
+2. Lazy
+	* Don't update the view until it is needed(_queried_)
+
+
+## Check Contraint
+
+Checks values when they are inserted to validate their legitimacy.
+
+```
+create table blah(
+	id varchar(8) check (id like "%-%"),
+);
+```
+This is how we can avoid accidently putting in null or downright logically incorrect data into a table.
+
+We can also require entries be unique as well.
+
+```
+create table blah (
+	dept_name varchar(20),
+	...
+	unique(dept_name)
+);
+```
+_KEEP IN MIND HOWEVER_. With `unique()` if we try to check if a new entry is unique it will always fail with NULL since operations with NULL results in false.
+That means we will be able to insert NULL values into the table even if they are not unique.