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