some fixes based on review

Author: louisrli

module2-database-technologies/r1-introduction-to-databases/README.md

@@ -6,5 +6,13 @@ want to persist data for future use. For example, in a web application, when a
 user writes something in a text box, this data will be lost when the page is
 refreshed -- unless this data is persisted in a database.
 
-Databases are a ubiquitous and foundational part of backend technology.
+Databases are a ubiquitous and foundational part of backend technology. One
+would use databases instead of local storage or cookies, for example, when the
+information being persisted needs to be available to any possible client. For
+example, information stored in [local
+storage](https://developer.mozilla.org/en-US/docs/Web/API/Window/localStorage) is not automatically synchronized
+across browsers. For example, if you were to store information about user
+settings in local storage, then those settings would no longer be available on a
+different browser -- but if you store them in a database, the user can access
+them from any client.
 

module2-database-technologies/r2.2-sql-queries/README.md

@@ -7,10 +7,14 @@ examples, we will be using MySQL.
 
 Suppose you have an existing MySQL database. To create the `customer` table (we
 will now lowercase the table names by convention), you would use the `CREATE
-TABLE` statement. Note that every column has a type.
+TABLE` statement. Note that every column has a data type and can also be given additional definition rules.
+
 [`AUTO_INCREMENT`](https://www.guru99.com/auto-increment.html) is used to
-automatically generated the IDs. 
+automatically generated the IDs; when inserting a row, you do not need to
+provide the ID, it is automatically generated by incrementing the previous ID. We will see this in our example queries later. `NOT NULL` means that the user cannot provide a `NULL` value for values in this column. `PRIMARY KEY` is used to denote a column as the primary key field, and it is often used in conjunction with `AUTO INCREMENT` for databases with numeric IDs.
 
+There are many options when creating columns. Refer to the [`CREATE TABLE`
+documentation](https://dev.mysql.com/doc/refman/8.0/en/create-table.html).
 ```sql
 CREATE TABLE IF NOT EXISTS customer (
     task_id INT AUTO_INCREMENT PRIMARY KEY,